Date:	Tue, 27 Apr 2010 22:03:18 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Theodore Tso <tytso@....EDU>
Cc:	kosaki.motohiro@...fujitsu.com, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-btrfs@...r.kernel.org,
	Hugh Dickins <hugh.dickins@...cali.co.uk>
Subject: Re: No one seems to be using AOP_WRITEPAGE_ACTIVATE?

> 
> On Apr 26, 2010, at 6:18 AM, KOSAK
> > AOP_WRITEPAGE_ACTIVATE was introduced for ramdisk and tmpfs thing
> > (and later rd choosed to use another way).
> > Then, It assume writepage refusing aren't happen on majority pages.
> > IOW, the VM assume other many pages can writeout although the page can't.
> > Then, the VM only make page activation if AOP_WRITEPAGE_ACTIVATE is returned.
> > but now ext4 and btrfs refuse all writepage(). (right?)
> 
> No, not exactly.   Btrfs refuses the writepage() in the direct reclaim cases (i.e., if PF_MEMALLOC is set), but will do writepage() in the case of zone scanning.  I don't want to speak for Chris, but I assume it's due to stack depth concerns --- if it was just due to worrying about fs recursion issues, i assume all of the btrfs allocations could be done GFP_NOFS.
> 
> Ext4 is slightly different; it refuses writepages() if the inode blocks for the page haven't yet been allocated.  (Regardless of whether it's happening for direct reclaim or zone scanning.)  However, if the on-disk block has been assigned (i.e., this isn't a delalloc case), ext4 will honor the writepage().   (i.e., if this is an mmap of an already existing file, or if the space has been pre-allocated using fallocate()).    The reason for ext4's concern is lock ordering, although I'm investigating whether I can fix this.   If we call set_page_writeback() to set PG_writeback (plus set the various bits of magic fs accounting), and then drop the page_lock, does that protect us from random changes happening to the page (i.e., from vmtruncate, etc.)?
> 
> > 
> > IOW, I don't think such documentation suppose delayed allocation issue ;)
> > 
> > The point is, Our dirty page accounting only account per-system-memory
> > dirty ratio and per-task dirty pages. but It doesn't account per-numa-node
> > nor per-zone dirty ratio. and then, to refuse write page and fake numa
> > abusing can make confusing our vm easily. if _all_ pages in our VM LRU
> > list (it's per-zone), page activation doesn't help. It also lead to OOM.
> > 
> > And I'm sorry. I have to say now all vm developers fake numa is not
> > production level quority yet. afaik, nobody have seriously tested our
> > vm code on such environment. (linux/arch/x86/Kconfig says "This is only 
> > useful for debugging".)
> 
> So I'm sorry I mentioned the fake numa bit, since I think this is a bit of a red herring.   That code is in production here, and we've made all sorts of changes so ti can be used for more than just debugging.  So please ignore it, it's our local hack, and if it breaks that's our problem.    More importantly, just two weeks ago I talked to soeone in the financial sector, who was testing out ext4 on an upstream kernel, and not using our hacks that force 128MB zones, and he ran into the ext4/OOM problem while using an upstream kernel.  It involved Oracle pinning down 3G worth of pages, and him trying to do a huge streaming backup (which of course wasn't using fallocate or direct I/O) under ext4, and he had the same issue --- an OOM, that I'm pretty sure was caused by the fact that ext4_writepage() was refusing the writepage() and most of the pages weren't nailed down by Oracle were delalloc.    The same test scenario using ext3 worked just fine, of course.
> 
> Under normal cases it's not a problem since statistically there should be enough other pages in the system compared to the number of pages that are subject to delalloc, such that pages can usually get pushed out until the writeback code can get around to writing out the pages.   But in cases where the zones have been made artificially small, or you have a big program like Oracle pinning down a large number of pages, then of course we have problems. 
> 
> I'm trying to fix things from the file system side, which means trying to understand magic flags like AOP_WRITEPAGE_ACTIVATE, which is described in Documentation/filesystems/Locking as something which MUST be used if writepage() is going refuse a page.  And then I discovered no one is actually using it.   So that's why I was asking with respect whether the Locking documentation file was out of date, or whether all of the file systems are doing it wrong.
> 
> On a related example of how file system code isnt' necessarily following what is required/recommended by the Locking documentation, ext2 and ext3 are both NOT using set_page_writeback()/end_page_writeback(), but are rather keeping the page locked until after they call block_write_full_page(), because of concerns of truncate coming in and screwing things up.   But now looking at Locking, it appears that set_page_writeback() is as good as page_lock() for preventing the truncate code from coming in and screwing everything up?   It's not clear to me exactly what locking guarantees are provided against truncate by set_page_writeback().   And suppose we are writing out a whole cluster of pages, say 4MB worth of pages; do we need to call set_page_writeback() on every single page in the cluster before we do the I/O to make sure things don't change out from under us?  (I'm pretty sure at least some of the other filesystems that are submitting huge numbers of pages using bio instead of 4k at a time like ext2/3/4 aren't calling set_page_writeback() on all of the pages first.)
> 
> Part of the problem is that the writeback Locking semantics aren't well documented, and where they are documented, it's not clear they are up to date --- and all of the file systems that are doing delayed allocation writeback are doing things slightly differently, or in some cases very differently.    (And even without delalloc, as I've pointed out ext2/3 don't use set_page_writeback() --- if this is a MUST USE as implied by the Locking file, why did whoever added this requirement didn't go in and modify common filesystems like ext2 and ext3 to use the set_page_writeback/end_page_writeback calls?)
> 
> I'm happy to change things in ext4; in fact I'm pretty sure ext4 probably isn't completely right here.   But it's not clear what "right" actually is, and when I look to see what protects writepage() racing with vmtruncate(), it's enough to give me a headache.  :-(    

Umm.. sorry, I'm not good person to answer your question. 
probably Nick has best knowledge in this area.

afaics, vmtruncate call graph is here.

vmtruncate
 -> truncate_pagecache
    -> truncate_inode_pages
       -> truncate_inode_pages_range
            lock_page(page);
            wait_on_page_writeback(page);
            truncate_inode_page(mapping, page);
             -> truncate_complete_page
                -> remove_from_page_cache
             ....
            unlock_page(page);

Then, PG_lock and/or PG_writeback protect against remove_from_page_cache().

But..
Now I'm afraid it can't solve ext4 delalloc issue. I'm pretty sure  
you have done above easy grepping. I guess you are suffering from more
difficult issue. I hope to ask you, why ext4 couing logic of number of
delalloc pages can't take page lock?

and, today's my grep result is, 

ext2_writepage
  block_write_full_page
    block_write_full_page_endio
      __block_write_full_page
        set_page_writeback

end_buffer_async_write
  end_page_writeback


ext3 seems to have similar logic of ext2. Am I missing something?




> 
> Hence my question about wouldn't it be simpler if we simply added more high-level locking to prevent truncate from racing against writepage/writeback.  
> 
> -- Ted
> 



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