linux-ext4 - Re: potential regression in ext[34] call to __page

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Message-ID: <20081029024048.GB3766@mit.edu>
Date:	Tue, 28 Oct 2008 22:40:48 -0400
From:	Theodore Tso <tytso@....edu>
To:	Mike Snitzer <snitzer@...il.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org, Nick Piggin <npiggin@...e.de>,
	Kirill Korotaev <dev@...nvz.org>
Subject: Re: potential regression in ext[34] call to __page_symlink()?

On Tue, Oct 28, 2008 at 08:11:48PM -0400, Mike Snitzer wrote:
> The gfp_mask that is passed to __page_symlink() is being completely
> dropped on the floor.  Historically this mask was at least used by
> ext3 and ext4 to avoid recursing back into the FS from within a
> journal transaction; Kirill fixed that issue with this commit:
> 0adb25d2e71ab047423d6fc63d5d184590d0a66f
> 
> I'm quite naive when it comes to Nick's relatively new (>= 2.6.24) AOP
> pagecache_write_{begin,end} code that motivated __page_symlink to
> change with this commit:
> afddba49d18f346e5cc2938b6ed7c512db18ca68
> 
> Nick's change clearly did away with using the explicitly passed
> gfp_mask in __page_symlink().
> So at a minimum it would seem __page_symlink() now has an unused
> parameter that should be removed.
> 
> But a more serious concern would be: have ext[34]_symlink() regressed
> to being susceptible to the bug that Kirill fixed some time ago?

Yeah, I think this would be a potential problem for ext3/4.  Looks
like pagemap_write_begin() should take a gfp_mask argument, and then
pass it down through to __grab_cache_page(), which should then call
__page_cache_alloc() instead of _page_cache_alloc().  Then
__page_symlink() can actually pass in its gfp_mask to
pagemap_write_begin().

Nick, do you agree?

						- Ted
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