linux-kernel - Re: [PATCH] fs: avoid locking sb_lock in grab_super

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Message-ID: <20150221023754.GT29656@ZenIV.linux.org.uk>
Date:	Sat, 21 Feb 2015 02:37:55 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs: avoid locking sb_lock in grab_super_passive()

On Thu, Feb 19, 2015 at 08:19:35PM +0300, Konstantin Khlebnikov wrote:
> I've noticed significant locking contention in memory reclaimer around
> sb_lock inside grab_super_passive(). Grab_super_passive() is called from
> two places: in icache/dcache shrinkers (function super_cache_scan) and
> from writeback (function __writeback_inodes_wb). Both are required for
> progress in memory reclaimer.
> 
> Also this lock isn't irq-safe. And I've seen suspicious livelock under
> serious memory pressure where reclaimer was called from interrupt which
> have happened right in place where sb_lock is held in normal context,
> so all other cpus were stuck on that lock too.

Excuse me, but this part is BS - its call is immediately preceded by
        if (!(sc->gfp_mask & __GFP_FS))
                return SHRINK_STOP;
and if we *ever* hit GFP_FS allocation from interrupt, we are really
screwed.  If nothing else, both prune_dcache_sb() and prune_icache_sb()
can wait for all kinds of IO; you really don't want that called in an
interrupt context.  The same goes for writeback_sb_inodes(), while we
are at it.

If you ever see that in an interrupt context, you have a very bad problem
on hands.

Said that, not bothering with sb_lock (and ->s_count) in those two callers
makes sense.  Applied, with name changed to trylock_super().
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