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+[[!meta copyright="Copyright © 2012, 2013 Free Software Foundation, Inc."]]
+
+[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable
+id="license" text="Permission is granted to copy, distribute and/or modify this
+document under the terms of the GNU Free Documentation License, Version 1.2 or
+any later version published by the Free Software Foundation; with no Invariant
+Sections, no Front-Cover Texts, and no Back-Cover Texts.  A copy of the license
+is included in the section entitled [[GNU Free Documentation
+License|/fdl]]."]]"""]]
+
+[[!tag open_issue_documentation open_issue_hurd open_issue_gnumach]]
+
+[[!toc]]
+
+
+# IRC, freenode, #hurd, 2012-02-14
+
+    <slpz> Open question: what do you think about dropping the memory object
+      model and implementing a simple block-level cache?
+
+[[microkernel/mach/memory_object]].
+
+    <kilobug> slpz: AFAIK the memory object has more purpose than just cache,
+      it's allow used for passing chunk of data between processes, handling
+      swap (which similar to cache, but still slightly different), ...
+    <slpz> kilobug: user processes usually make their way to data with POSIX
+      operations, so memory objects are only needed for mmap'ed files
+    <slpz> kilobug: and swap can be replaced for an in-kernel system or even
+      could still use the memory object
+    <braunr> slpz: memory objects are used for the page cache
+    <kilobug> slpz: translators (especially diskfs based) make heavy use of
+      memory objects, and if "user processes" use POSIX semantics, Hurd process
+      (translators, pagers, ...) shouldn't be bound to POSIX
+    <slpz> braunr: and page cache could be moved to a lower level, near to the
+      devices
+    <braunr> not likely
+    <braunr> well, it could, but then you'd still have the file system overhead
+    <slpz> kilobug: but the use of memory objects it's not compulsory, you can
+      easily write a fs translator without implementing memory objects at all
+      (except to mmap)
+    <braunr> a unified buffer/VM cache as all modern systems have is probably
+      the most efficient approach
+    <slpz> braunr: I agree. I want to look at *BSD/Linux vfs systems to seem
+      how much cache policy depends on the filesystem
+    <slpz> braunr: Are you aware of any good papers on this matter?
+    <braunr> netbsd UVM, the linux virtual memory system
+    <braunr> both a bit old bit still relevant
+    <slpz> braunr: Thanks.
+    <slpz> the problem in our case is that having FS and cache information at
+      different contexts (kernel vs. translator), I find hard to coordinate
+      them.
+    <slpz> that's why I though about a block-level cache that GNU Mach could
+      manage by itself
+    <slpz> I wonder how QNX deals with this
+    <braunr> the point of having a simple page cache is explicitely about not
+      caring if those pages are blocks or files or whatever
+    <braunr> the kernel (at least, mach) normally has all the accounting
+      information it needs to implement its cache policy
+    <braunr> file system translators shouldn't cache much
+    <braunr> the pager interface could be refined, but it looks ok to me as it
+      is
+    <slpz> Mach has the accounting info, but it's not able to purge the cache
+      without coordination with translators
+    <braunr> which is normal
+    <slpz> And this is a big problem when memory pressure increases, as it
+      doesn't know for sure when memory is going to be freed
+    <braunr> Mach flushes its cache when it decides to, and sends back dirty
+      pages if needed by the pager
+    <braunr> that's the case with every paging implementation
+    <braunr> the main difference is security with untrusted pagers
+    <braunr> but that's another issue
+    <slpz> but in a monolithic implementation, the kernel is able for force a
+      chunk of cache memory to be freed without hoping for other process to do
+      the job
+    <braunr> that's not true
+    <braunr> they're not process, they're threads, but the timing issue is the
+      same
+    <braunr> see pdflush on linux
+    <slpz> no, it isn't.
+    <braunr> when memory is scarce, threads that request memory can either wait
+      or immediately fail, and if they wait, they're usually woken by one of
+      the vm threads once flushing is done
+    <slpz> a kernel thread can access all the information in the kernel, and
+      synchronization is pretty easy.
+    <braunr> on mach, synchronization is done with messages, that's even easier
+      than shared kernel locks
+    <slpz> with processes in different spaces, resource coordination becomes
+      really difficult
+    <braunr> and what kind of info would an external pager need when simply
+      asked to take back its dirty pages
+    <braunr> what resources ?
+    <slpz> just take a look at the thread storm problem when GNU Mach needs to
+      clean a bunch of pages
+    <braunr> Mach is big enough to correctly account memory
+    <braunr> there can be thread storms on monolithic systems
+    <braunr> that's a Mach issue, not a microkernel issue
+    <braunr> that's why linux limits the number of pdflush thread instances
+    <slpz> Mach can account memory, but can't assure when be freed by any
+      means, in a lesser degree than a monolithic system
+    <braunr> again i disagree
+    <braunr> no system can guarantee when memory will be freed with paging
+    <slpz> a block level cache can, for most situations
+    <braunr> slpz: why ?
+    <braunr> slpz: or how i mean ?
+    <slpz> braunr: with a block-level page cache, GNU Mach should be able to
+      flush dirty pages directly to the underlaying device without all the
+      complexity and resource cost involved in a m_o_data_return message. It
+      can also throttle the rate at which pages are being cleaned, and do all
+      this while blocking new page allocations to deal with memory exhaustion
+      cases.
+    <slpz> braunr: in the current state, when cleaning dirty pages, GNU Mach
+      sends a bunch on m_o_data_return to the corresponding pagers, hoping they
+      will do their job as soon and as fast as possible.
+    <slpz> memory is not really freed, but transformed from page cache to
+      anonymous memory pertaining to the corresponding translator
+    <slpz> and GNU Mach never knows for sure when this memory is released, if
+      it ever is.
+    <slpz> not being able to flush dirty pages synchronously is a big problem
+      when you need to throttle memory usage
+    <slpz> and needing allocating more memory when you're trying to free (which
+      is the case for the m_o_data_return mechanism) makes the problem even
+      worse
+    <braunr> your idea of a block level cache means in kernel block drivers
+    <braunr> that's not the direction we're taking
+    <braunr> i agree flushing should be a synchronous process, which was one of
+      the proposed improvements in the thread migration papers
+    <braunr> (they didn't achieve it but thought about it for future works, so
+      that the thread at the origin of the fault would handle it itself)
+    <braunr> but it should be possible to have kernel threads similar to
+      pdflush and throttle flush requests too
+    <braunr> again, i really think it's a mach bug, and having a buffer cache
+      would be stepping backward
+    <braunr> the real design issue is allocating memory while trying to free
+      it, yes
+    <slpz> braunr: thread migration doesn't apply to asynchronous IPC, and the
+      entire paging mechanism is implemented this way
+    <slpz> in fact, trying to do a synchronous m_o_data_return will trigger a
+      deadlock for sure
+    <slpz> to achieve synchronous flushing with translators, the entire paging
+      model must be redesigned
+    <slpz> It's true that I'm not very confident of the viability of user space
+      drivers
+    <slpz> at least, not for every device
+    <slpz> I know this is against the current ideas for most ukernel designs,
+      but if we want to achieve real work functionality, I think some
+      sacrifices must be done. Or at least a reasonable compromise.
+    <braunr> slpz: thread migration for paging requests implies synchronous
+      RPC, we don't care much about the IPC layer there
+    <braunr> and it requires large changes of the VM code in addition, yes
+    <braunr> let's not talk about this, we don't have thread migration anyway
+      :p
+    <braunr> except the allocation-on-free-path issue, i really don't see how
+      the current pager interface or the page cache creates problems wrt
+      flushing ..
+    <braunr> monolithic systems also have that problem, with lower impacts
+      though, but still
+    <slpz> braunr: because as it doesn't know when memory is really freed, 1)
+      it just blindly sends a bunch of m_o_data_return to the pagers, usually
+      overloading them (causing thread storms), and 2) it can't properly
+      throttle new page requests to deal with resource exhaustion
+    <braunr> it does know when memory is really freed
+    <braunr> and yes, it blindly sends a bunch of requests, they can and should
+      be trottled
+    <slpz> but dirty pages freed become indistinguishable from common anonymous
+      chunks released, so it doesn't really know if page flushes are really
+      working or not (i.e. doesn't know how fast a device is processing write
+      requests)
+    <braunr> memory is freed when the pager deallocates it
+    <braunr> the speed of the operation is irrelevant
+    <braunr> no system can rely on disk speed to guarantee correct page flushes
+    <braunr> disk or anything else
+    <slpz> requests can't be throttled if Mach doesn't know when they are being
+      processed
+    <braunr> it can easily know it
+    <braunr> they are processed as soon as the request is sent from the kernel
+    <braunr> and processing is done when the pager acknowledges the end of the
+      flush
+    <braunr> memory backing the flushed pages should be released before
+      acknowleding that to avoid starting new requests too soon
+    <slpz> AFAIK pagers doesn't acknowledge the end of the flush
+    <braunr> well that's where the interface should be refined
+    <slpz> Mach just sends the m_o_data_return and continues on its own
+    <braunr> that's why flushing should be synrhconous
+    <braunr> are you sure about that however ?
+    <slpz> so the entire paging system needs a new design... :)
+    <slpz> pretty sure
+    <braunr> not a new design ..
+    <braunr> there is m_o_supply_completed, i don't see how difficult it would
+      be to add m_o_data_return_completed
+    <braunr> it's not a small change, but not a difficult one either
+    <braunr> i'm more worried about the allocation problem
+    <braunr> the default pager should probably be wired in memory
+    <braunr> maybe others
+    <slpz> let's suppose a case in which Mach needs to free memory due to an
+      increase in its pressure. vm_pageout_daemon starts running, clean pages
+      are freed easily, but for each dirty one a m_o_data_return in sent. 1)
+      when should this daemon stop sending m_o_data_return and start waiting
+      for m_o_data_return_completed? 2) what happens if the translator needs to
+      read new blocks to fulfill a write request (pretty common in ext2fs)?
+    <braunr> it should stop after an arbitrary limit is reached
+    <braunr> a reasonable one
+    <braunr> linux limits the number of pdflush threads for that reason as i
+      mentioned (to 8 iirc)
+    <braunr> the problem of reading blocks while flushing is what i'm worried
+      about too, hence the need to wire that code
+    <braunr> well, i'm nto sure it's needed
+    <braunr> again, a reasonable about of free memory should be reserved for
+      that at all times
+    <slpz> but the work for pdflush seems to be a lot easier, as it only deals
+      directly with block devices (if I understood it correctly, I just started
+      looking at it).
+    <braunr> i don't know how other systems compute that, but this is how they
+      seem to do as well
+    <braunr> no, i don't think so
+    <slpz> well, I'll try to invest a few days understanding how pdflush work,
+      to see if some ideas can be borrowed for Hurd
+    <braunr> iirc, freebsd has thresholds in percent for each part of its cache
+      (active, inactive, free, dirty)
+    <slpz> but I still think simple solutions work better, and using the memory
+      object for page cache is tremendously complex.
+    <braunr> the amount of free cache pages is generally sufficient to
+      guarantee much memory can be released at once if needed, without flushing
+      anything
+    <braunr> yes but that's the whole point of the Mach VM
+    <braunr> and its greatest advance ..
+    <slpz> what, memory objects?
+    <braunr> yes
+    <braunr> using physical memory as a cache for anything, not just block
+      buffers
+    <slpz> memory objects work great as a way to provide a shared image of
+      objects between processes, but as page cache they are an overkill (IMHO).
+    <slpz> or, at least, in the way we're using them
+    <braunr> probably
+    <braunr> http://lwn.net/Articles/326552/
+    <braunr> this can help udnerstand the problems we may have without better
+      knowledge of the underlying devices, yes
+    <braunr> (e.g. not being able to send multiple requests to pagers that
+      don't share the same disk)
+    <braunr> slpz: actually i'm not sure it's that overkill
+    <braunr> the linux vm uses struct vm_file to represent memory objects iirc
+    <braunr> there are many links between that structure and some vfs related
+      subsystems
+    <braunr> when a system very actively uses the page cache, the kernel has to
+      maintain a lot of objects to accurately describe the cache content
+    <braunr> you could consider this overkill at first too
+    <braunr> the mach way of doing it just implies some ipc messages instead of
+      function calls, it's not that overkill for me
+    <braunr> the main problems are recursion (allocation while freeing,
+      handling page faults in order to handle flushes, that sort of things)
+    <braunr> struct file and struct address_space actually
+    <braunr> slpz: see struct address_space, it contains a set of function
+      pointers that can help understanding the linux pager interface
+    <braunr> they probably sufferred from similar caveats and worked around
+      them, adjusting that interface on the way
+    <slpz> but their strategy makes them able to treat the relationship between
+      the page cache and the block devices in a really simple way, almost as a
+      traditional storage cache.
+    <slpz> meanwhile on Mach+pager scenario, the relationship between a block
+      in a file and its underlying storage becomes really blurry
+    <slpz> this is a huge advantage when flusing out data, specially when
+      resources are scarce
+    <slpz> I think the idea of using abstract objects for page cache, loses a
+      bit the point that we just want to avoid accessing constantly to a slow
+      device
+    <slpz> and breaking the tight relationship between the device and its
+      cache, makes things a lot harder
+    <slpz> this also manifest itself when flushing clean pages, as things like
+      having an static maximum for cached memory objects
+    <slpz> we shouldn't care about the number of objects, we just need to
+      control the number of pages
+    <slpz> but as we need the pager to flush pages, we need to keep alive a lot
+      of control ports to them
+    <mcsim> slpz: When mo_data_return is called, once the memory manager no
+      longer needs supplied data, it should be deallocated using
+      vm_deallocate. So this way pagers acknowledges the end of flush.
+
+
+# IRC, freenode, #hurd, 2013-08-26
+
+    < Spyro> Ok, so
+    < Spyro> idiot question:  in a nutshell, what is a memory object?
+    < Spyro> and how is swapping/paging handled?
+    < braunr> Spyro: a memory object is how the virtual memory system views a
+      file
+    < braunr> so it's a sequence of bytes with a length
+    < braunr> "swapping" is just a special case of paging that applies to
+      anonymous objects
+    < braunr> (which are named so because they're not associated with a file
+      and have no name)
+    < Spyro> Who creates a memory object, and when?
+    < braunr> pagers create memory objects when needed, e.g. when you open a
+      file
+    < Spyro> and this applies both to mmap opens as well as regular I/O opens
+      as in for read() and write()?
+    < braunr> basically, all file systems capable of handling mmap requests
+      and/or caching in physical memory are pagers
+    < braunr> yes
+    < braunr> read/write will go through the page cache when possible
+    < Spyro> and who owns the page cache?
+    < Spyro> also, who decides what pages ot evict to swap/file if physical
+      memory gets tight?
+    < braunr> the kernel
+    < braunr> that's one of the things that make mach a hybrid
+    < Spyro> so the kernel owns the page cage?
+    < Spyro> ...fml
+    < Spyro> cache!
+    < braunr> yes
+
+
+## IRC, freenode, #hurd, 2013-08-27
+
+    < Spyro> so braunr:  So, who creates the memory object, and how does it get
+      populated?
+    < Spyro> and how does a process accessing a file get hooked up to the
+      memory object?
+    < braunr> Spyro: i told you, pagers create memory objects
+    < braunr> memory objects are how the VM system views files, so they're
+      populated from the content of files
+    < braunr> either true files or virtual files such as in /proc
+    < braunr> Spyro: processes don't directly access memory objects unless
+      memory mapping them with vm_map()
+    < braunr> pagers (basically = file systems) do
+    <Spyro> ok, so how is a pager/fs involved in handling a fault?
+
+
+## IRC, freenode, #hurd, 2013-08-28
+
+    <braunr> Spyro: each object is linked to a pager
+    <braunr> Spyro: when a fault occurs, the kernel looks up the VM map (kernel
+      or a user one), and the address in this map, then the map entry, checks
+      access and lots of other details
+    <Spyro> ok, so it's pager -> object -> vmem
+    <Spyro> ?
+    <braunr> Spyro: then finds the object mapped at that address (similar to
+      how a file is mapped with mmap)
+    <braunr> from the object, it finds the pager
+    <Spyro> ok
+    <braunr> and asks the pager about the data at the appropriate offset
+    <Spyro> so how does a user process do normal file I/O?  is faulting just a
+      special case of it?
+    <braunr> it's completely separate
+    <Spyro> eww
+    <braunr> normal I/O is done with message passing
+    <braunr> the hurd io interface
+    <Spyro> ok
+    <Spyro> so who talks to who on a file I/O?
+    <braunr> a client (e.g. cat) talks to a file system server (e.g. ext2fs)
+    <Spyro> ok so
+    <Spyro> it's client to the pager for regular file I/O?
+    <braunr> Spyro: i don't understand the question
+    <braunr> Spyro: it's client to server, the server might not be a pager
+    <Spyro> ok
+    <Spyro> just trying to figure out the difference between paging/faulting
+      and regular I/O
+    <braunr> regular I/O is just message passing
+    <braunr> page fault handling is dealt with by pagers
+    <Spyro> and I have a hunch that the fs/pager is involved somehow in both,
+      because the server is the source of the data
+    <Spyro> I'm getting a headache
+    <braunr> nalaginrut: a server like ext2fs is both a file server and a pager
+    <Spyro> oh!
+    <Spyro> oh btw, does a file server make use of memory objects for caching?
+    <braunr> Spyro: yes
+    <Spyro> or rather, can it?
+    <Spyro> does it have to?
+    <braunr> memory objects are for caching, and thus for page faults
+    <braunr> Spyro: for caching, it's a requirement
+    <braunr> for I/O, it's not
+    <braunr> you could have I/O without memory objects
+    <Spyro> ok
+    <Spyro> so how does the pager/fileserver use memory objects for caching?
+    <Spyro> does it just map and write to them?
+    <braunr> basically yes but there is a complete protocol with the kernel for
+      that
+    <braunr>
+      http://www.gnu.org/software/hurd/gnumach-doc/External-Memory-Management.html#External-Memory-Management
+    <Spyro> heh, lucky guess
+    <Spyro> ty
+    <Spyro> I am in way over my head here btw
+    <Spyro> zero experience with micro kernels in practice
+    <braunr> it's not trivial
+    <braunr> that's not a microkernel thing at all
+    <braunr> that's how it works in monolithic kernels too
+    <braunr> i recommend netbsd uvm thesis
+    <braunr> there are nice pictures describing the vm system
+    <Spyro> derrr...preacious?
+    <Spyro> wow
+    <braunr> just ignore the anonymous memory handling part which is specific
+      to uvm
+    <Spyro> @_@
+    <braunr> the rest is common to practically all VM systems out there
+    <Spyro> I know about the linux page cache
+    <braunr> well it's almost the same
+    <Spyro> with memory objects being the same thing as files in a page cache?
+    <braunr> memory objects are linux "address spaces"
+    <braunr> and address spaces are how the linux mm views a file, yes
+    <Spyro> derp
+    <Spyro> ...
+    <Spyro> um...
+    <braunr> struvt vm_page == struct page
+    * Spyro first must learn what an address_space is
+    <braunr> struct vm_map == struct mm_struct
+    <braunr> struct vm_map_entry == struct vm_area_struct
+    * Spyro isn't a linux kernel vm expert either
+    <braunr> struct vm_object == struct address_space
+    <braunr> roughly
+    <braunr> details vary a lot
+    <Spyro> and what's an address_space ?
+    <braunr> 11:41 < braunr> and address spaces are how the linux mm views a
+      file, yes
+    <Spyro> ok
+    <braunr> see include/linux/fs.h
+    <braunr> struct address_space_operations is the pager interface
+    * Spyro should look at the linux kernel sources perhaps, unless you have an
+        easier reference
+    <Spyro> embarrassingly, RVR hired me as an editor for the linux-mm wiki
+    <Spyro> I should know this stuff
+    <braunr> see
+      http://darnassus.sceen.net/~rbraun/design_and_implementation_of_the_uvm_virtual_memory_system.pdf
+    <braunr> page 42
+    <braunr> page 66 for another nice view
+    <braunr> i wouldn't recommend using linux source as refernece
+    <braunr> it's very complicated, filled with a lot of code dealing with
+      details
+    <Spyro> lmao
+    <braunr> and linux guys have a habit of choosing crappy names
+    <Spyro> I was only going to 
+    <Spyro> stoppit
+    <braunr> except for "linux" and "git"
+    <Spyro> ...make me laugh any more and I'll need rib surgery
+    <braunr> laugh ?
+    <Spyro> complicated and crappy
+    <braunr> seriously, "address space" for a file is very very confusing
+    <Spyro> oh I agree with that
+    <braunr> yes, names are crappy
+    <braunr> and the code is very complicated
+    <braunr> it took me half an hour to find where readahead is done once
+    <braunr> and i'm still not sure it was the right code
+    <Spyro> so in linkern, there is an address_space for each cached file?
+    <braunr> takes me 30 seconds in netbsd ..
+    <braunr> yes
+    <Spyro> eww
+    <Spyro> yeah, BAD name
+    <Spyro> but thanks for the explanation
+    <Spyro> now I finally know what an address space is
+    <braunr> many linux core developers admit they don't care much about names
+    <Spyro> so, in hurd, a memory object is to hurd, what an address_space is
+      to linux?
+    <braunr> yes
+    <braunr> notto hurd
+    <Spyro> ok
+    <braunr> to mach
+    <Spyro> you know what I mean
+    <Spyro> :P
+    <Spyro> easier than for linux I can tell you that much
+    <braunr> and the bsd vm system is a stripped version of the mach vm
+    <Spyro> ok
+    <braunr> that's why i think it's important to note it
+    <Spyro> good, I learned something abou tthe linux vm...from the mach guys
+    <Spyro> this is funny
+    <braunr> linux did too
+    <braunr> there is a paper about linux page eviction that directly borrows
+      the mach algorithm and improves it
+    <braunr> mach is the historic motivation behind mmap on posix
+    <Spyro> oh nice!
+    <Spyro> but yes, linux picked a shitty name
+    <braunr> is all that clearer to you ?
+    <Spyro> I think that address_space connection was a magic bolt of
+      understanding
+    <braunr> and do you see how I/O and paging are mostly unrelated ?
+    <Spyro> almost
+    <Spyro> but how does a file I/O take advantage of caching by a memory
+      object?
+    <Spyro> does the file server just nudge the core for a hint?
+    <braunr> the file system copies from the memory object
+    * Spyro noddles
+    <Spyro> I think I understand a bit better now
+    <braunr> it's message passing
+    <Spyro> but I havfe too much to digest already
+    <braunr> memory copying
+    <braunr> if the memory is already there, good, if not, the kernel will ask
+      the file system to bring the data
+    <braunr> if message passing uses zero copy, data retrieval can be deferred
+      until the client actually accesses it
+    <Spyro> which is a fancy way of saying demand paging? :P
+    <braunr> it's always demand paging
+    <braunr> what i mean is that the file system won't fetch data as soon as it
+      copies memory
+    <braunr> but when this data is actually needed by the client
+    <Spyro> uh...
+    <Spyro> whta's a precious page?
+    <braunr> let me check quickly
+    <braunr> If precious is FALSE, the kernel treats the data as a temporary
+      and may throw it away if it hasn't been changed. If the precious value is
+      TRUE, the kernel treats its copy as a data repository and promises to
+      return it to the manager
+    <braunr> basically, it's used when you want the kernel to keep cached data
+      in memory
+    <braunr> the cache becomes a lossless container for such pages
+    <braunr> the kernel may flush them, but not evict them
+    <Spyro> what's the difference?
+    <braunr> imagine a ramfs
+    <Spyro> point made
+    <braunr> ok
+    <Spyro> would be pretty hard to flush something that doesn't have a backing
+      store
+    <braunr> that was quick :)
+    <braunr> well
+    <braunr> the normal backing store for anonymous memory is the default pager
+    <braunr> aka swap
+    <Spyro> eww
+    <braunr> but if you want your data *either* in swap or in memory and never
+      in both
+    <braunr> it may be useful