linux-kernel - Re: v4.2-rc dcache regression, probably 75a6f82a0d10

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Message-ID: <20150731180040.GA19719@fieldses.org>
Date:	Fri, 31 Jul 2015 14:00:40 -0400
From:	bfields@...ldses.org (J. Bruce Fields)
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: v4.2-rc dcache regression, probably 75a6f82a0d10

On Fri, Jul 31, 2015 at 10:46:51AM -0700, Hugh Dickins wrote:
> I think there's something not quite right with the fs/dcache.c
> commit 75a6f82a0d10 ("freeing unlinked file indefinitely delayed").
> 
> When running my old tmpfs swapping load (two repetitive make -j20
> kernel builds, one on tmpfs, one on ext4 over loop over tmpfs file,
> in limited memory with plenty of swapping; rm -rf of both trees
> in between the builds): one of the builds, always the ext4 so far,
> fails after several hours, one or another header file "No such file
> or directory", but the file's there when I check the tree afterwards.

That's weird.

In theory, if you're not running knfsd, and if nobody's doing
open_by_handle_at() calls, then you shouldn't have any
DCACHE_DISCONNECTED dentries on your system, so this patch shouldn't
affect behavior.

--b.

> 
> Sounds like a dcache problem, and 75a6f82a0d10 seemed the only
> likely candidate, so I experimented with reverting it yesterday,
> and ran successfully for 24 hours.  That's a little too soon to
> be sure (I've set another run going this morning), but I'd say
> 90% certain that is to blame, and thought I'd better alert you
> sooner than later - you'll probably guess what's the matter
> long before I get back to check today's run.
> 
> (I saw exactly the same symptom two months ago; but that was just
> before you put in 2159184ea01e "d_walk() might skip too much",
> which fixed it back then.)
> 
> Hugh
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