From 4b1f7411dd9c3b3e20a766cbef071d0010ab842d Mon Sep 17 00:00:00 2001 From: Thomas Schwinge Date: Sat, 24 Jul 2010 10:39:20 +0200 Subject: hurd/translator/tmpfs/tmpfs_vs_defpager: New. --- hurd/translator/tmpfs/tmpfs_vs_defpager.mdwn | 73 ++++++++++++++++++++++++++++ 1 file changed, 73 insertions(+) create mode 100644 hurd/translator/tmpfs/tmpfs_vs_defpager.mdwn (limited to 'hurd/translator') 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 + + 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 + tmpfs should create its own pagers, just like ext2fs, storeio... + slopez@slp-hurd:~$ settrans -a tmp /hurd/tmpfs 10M + slopez@slp-hurd:~$ echo "foo" > tmp/bar + slopez@slp-hurd:~$ cat tmp/bar + foo + slopez@slp-hurd:~$ + :-) + slpz: woo you fixed it? + pochu: well, it's WIP, but reading/writing works... + I've replaced the use of default pager for the standard pager creation mechanism + slpz: err... how is it supposed to use swap space if not using the default pager? + slpz: or do you mean that it should act as a proxy, just allocating anonymous memory (backed by the default pager) itself? + antrik: the kernel uses the default pager if the application pager isn't responsive enough + antrik: it will just create memory objects and provide zerofilled pages when requested by the kernel (after a page fault) + youpi: that makes sense I guess... but how is that relevant to the question at hand?... + antrik: memory objects will contain the data by themselves + antrik: as youpi said, when memory is scarce, GNU Mach will start paging out data from memory objects to the default pager + antrik: that's the way in which pages will get into swap space + (if needed) + the thing being that the tmpfs pager has a chance to select pages he doesn't care any more about + 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? + youpi: no idea what you are talking about. apparently I still don't really understand this stuff :-( + ah, but tmpfs doesn't have pages he doesn't care about, does it? + antrik: yes, but the term "anonymous memory" could be a bit confusing. + 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 + slpz: hm... I thought anynymous memory is backed by memory objects created from the default pager? + 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 + 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 + am I totally off there?... + (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) + antrik: you're right. The trick here is, when does Mach return the pages? + 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 + or we change the attributes so it can no longer persist, of course + without a backing store, if Mach starts sending us pages to be written, we're in trouble + so we must do something about it. One option, could be creating another pager and copying the contents between objects. + another pager? not sure what you mean + BTW, you didn't really say why we can't use the default pager for tmpfs objects :-) + well, there're two problems when using the default pager as backing store for translators + 1) Mach relies on it to do swapping tasks, so meddling with it is not a good idea + 2) There're problems with seqnos when trying to work with the default pager from tasks other the kernel itself + (probably, the latter could be fixed) + antrik: pager's terminology is a bit confusing. One can also say creating another memory object (though the function in libpager is "pager_create") + not sure why "meddling" with it would be a problem... + 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 :-) + makes sense now + 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? + slpz: still don't understand what you mean by creating another memory object/pager... + (and yeat, the terminology is pretty mixed up even in Mach itself) + antrik: I meant creating another pager, in terms of calling again to libpager's pager_create + 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... + antrik: oh, ok, sorry + 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 + anyway, IMHO object caching mechanism is obsolete and should be replaced + I'm writting a comment to bug #28730 which says something about this + antrik: just one more thing :-) + if you look at the code, for most time of their lives, anonymous memory objects don't have a pager + not even the default one + 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 + this is not really important, but worth noting ;-) -- cgit v1.2.3