hurd/translator/tmpfs/tmpfs_vs_defpager: New.

1 files changed, 73 insertions, 0 deletions

diff --git a/hurd/translator/tmpfs/tmpfs_vs_defpager.mdwn b/hurd/translator/tmpfs/tmpfs_vs_defpager.mdwn
new file mode 100644
index 00000000..ef041a23
--- /dev/null
+++ b/hurd/translator/tmpfs/tmpfs_vs_defpager.mdwn
@@ -0,0 +1,73 @@
+[[!meta copyright="Copyright © 2010 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]]."]]"""]]
+
+\#hurd, freenode, 2010
+
+    <slpz> humm... why does tmpfs try to use the default pager? that's a bad idea, and probably will never work correctly...
+    * slpz is thinking about old issues
+    <slpz> tmpfs should create its own pagers, just like ext2fs, storeio...
+    <slpz> slopez@slp-hurd:~$ settrans -a tmp /hurd/tmpfs 10M
+    <slpz> slopez@slp-hurd:~$ echo "foo" > tmp/bar
+    <slpz> slopez@slp-hurd:~$ cat tmp/bar
+    <slpz> foo
+    <slpz> slopez@slp-hurd:~$ 
+    <slpz> :-)
+    <pochu> slpz: woo you fixed it?
+    <slpz> pochu: well, it's WIP, but reading/writing works...
+    <slpz> I've replaced the use of default pager for the standard pager creation mechanism
+    <antrik> slpz: err... how is it supposed to use swap space if not using the default pager?
+    <antrik> slpz: or do you mean that it should act as a proxy, just allocating anonymous memory (backed by the default pager) itself?
+    <youpi> antrik: the kernel uses the default pager if the application pager isn't responsive enough
+    <slpz> antrik: it will just create memory objects and provide zerofilled pages when requested by the kernel (after a page fault)
+    <antrik> youpi: that makes sense I guess... but how is that relevant to the question at hand?...
+    <slpz> antrik: memory objects will contain the data by themselves
+    <slpz> antrik: as youpi said, when memory is scarce, GNU Mach will start paging out data from memory objects to the default pager
+    <slpz> antrik: that's the way in which pages will get into swap space
+    <slpz> (if needed)
+    <youpi> the thing being that the tmpfs pager has a chance to select pages he doesn't care any more about
+    <antrik> slpz: well, the point is that instead of writing the pages to a backing store, tmpfs will just keep them in anonymous memory, and let the default pager write them out when there is pressure, right?
+    <antrik> youpi: no idea what you are talking about. apparently I still don't really understand this stuff :-(
+    <youpi> ah, but tmpfs doesn't have pages he doesn't care about, does it?
+    <slpz> antrik: yes, but the term "anonymous memory" could be a bit confusing.
+    <slpz> antrik: in GNU Mach, anonymous memory is backed by a memory object without a pager. In tmpfs, nodes will be allocated in memory objects, and the pager for those memory objects will be tmpfs itself
+    <antrik> slpz: hm... I thought anynymous memory is backed by memory objects created from the default pager?
+    <antrik> yes, I understand that tmpfs is supposed to be the pager for the objects it provides. they are obviously not anonymoust -- they have inodes in the tmpfs name space
+    <antrik> but my understanding so far was that when Mach returns pages to the pager, they end up in anonymous memory allocated to the pager process; and then this pager is responsible for writing them back to the actual backing store
+    <antrik> am I totally off there?...
+    <antrik> (i.e. in my understanding the returned pages do not reside in the actual memory object the pager provides, but in an anonymous memory object)
+    <slpz> antrik: you're right. The trick here is, when does Mach return the pages?
+    <slpz> antrik: if we set the attribute "can_persist" in a memory object, Mach will keep it until object cache is full or memory is scarce
+    <slpz> or we change the attributes so it can no longer persist, of course
+    <slpz> without a backing store, if Mach starts sending us pages to be written, we're in trouble
+    <slpz> so we must do something about it. One option, could be creating another pager and copying the contents between objects.
+    <antrik> another pager? not sure what you mean
+    <antrik> BTW, you didn't really say why we can't use the default pager for tmpfs objects :-)
+    <slpz> well, there're two problems when using the default pager as backing store for translators
+    <slpz> 1) Mach relies on it to do swapping tasks, so meddling with it is not a good idea
+    <slpz> 2) There're problems with seqnos when trying to work with the default pager from tasks other the kernel itself
+    <slpz> (probably, the latter could be fixed)
+    <slpz> antrik: pager's terminology is a bit confusing. One can also say creating another memory object (though the function in libpager is "pager_create")
+    <antrik> not sure why "meddling" with it would be a problem...
+    <antrik> and yeah, I was vaguely aware that there is some seqno problem with tmpfs... though so far I didn't really understand what it was about :-)
+    <antrik> makes sense now
+    <antrik> anyways, AIUI now you are trying to come up with a mechanism where the default pager is not used for tmpfs objects directly, but without making it inefficient?
+    <antrik> slpz: still don't understand what you mean by creating another memory object/pager...
+    <antrik> (and yeat, the terminology is pretty mixed up even in Mach itself)
+    <slpz> antrik: I meant creating another pager, in terms of calling again to libpager's pager_create
+    <antrik> slpz: well, I understand what "create another pager" means... I just don't understand what this other pager would be, when you would create it, and what for...
+    <slpz> antrik: oh, ok, sorry
+    <slpz> antrik: creating another pager it's just a trick to avoid losing information when Mach's objects cache is full, and it decides to purge one of our objects
+    <slpz> anyway, IMHO object caching mechanism is obsolete and should be replaced
+    <slpz> I'm writting a comment to bug #28730 which says something about this
+    <slpz> antrik: just one more thing :-) 
+    <slpz> if you look at the code, for most time of their lives, anonymous memory objects don't have a pager
+    <slpz> not even the default one
+    <slpz> only the pageout thread, when the system is running really low on memory, gives them a reference to the default pager by calling vm_object_pager_create
+    <slpz> this is not really important, but worth noting ;-)