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-[[!meta copyright="Copyright © 2012 Free Software Foundation, Inc."]]
+[[!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
@@ -10,6 +10,8 @@ License|/fdl]]."]]"""]]
[[!tag open_issue_documentation open_issue_hurd open_issue_gnumach]]
# IRC, freenode, #hurd, 2012-02-14
@@ -271,3 +273,242 @@ License|/fdl]]."]]"""]]
<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>
+ <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
+ <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