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From: Miklos Szeredi <miklos@szeredi.hu>
To: linux-kernel@vger.kernel.org, linux-mm@kvack.org
Cc: akpm@linux-foundation.org
Subject: dirty balancing deadlock
Date: Sun, 18 Feb 2007 19:28:18 +0100	[thread overview]
Message-ID: <E1HIqlm-0004iZ-00@dorka.pomaz.szeredi.hu> (raw)

I was testing the new fuse shared writable mmap support, and finding
that bash-shared-mapping deadlocks (which isn't so strange ;).  What
is more strange is that this is not an OOM situation at all, with
plenty of free and cached pages.

A little more investigation shows that a similar deadlock happens
reliably with bash-shared-mapping on a loopback mount, even if only
half the total memory is used.

The cause is slightly different in the two cases:

  - loopback mount: allocation by the underlying filesystem is stalled
    on throttle_vm_writeout()

  - fuse-loop: page dirtying on the underlying filesystem is stalled on
    balance_dirty_pages()

In both cases the underlying fs is totally innocent, with no
dirty/writback pages, yet it's waiting for the global dirty+writeback
to go below the threshold, which obviously won't, until the
allocation/dirtying succeeds.

I'm not quite sure what the solution is, and asking for thoughts.

Ideas:

  - per filesystem dirty counters.  If filesystem is clean (or dirty
    is below some minimum), then balance_dirty_pages() should no wait
    any more

  - throttle_vm_writeout() was meant to throttle swapping, no?  So in
    that case there should be a separate swap-writback counter

Thanks,
Miklos

             reply	other threads:[~2007-02-18 18:28 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2007-02-18 18:28 Miklos Szeredi [this message]
2007-02-18 20:53 ` Andrew Morton
2007-02-18 21:25   ` Rik van Riel
2007-02-18 22:54     ` Miklos Szeredi
2007-02-18 22:50   ` Miklos Szeredi
2007-02-18 22:59     ` Andrew Morton
2007-02-18 23:22       ` Miklos Szeredi
2007-02-18 23:59         ` Andrew Morton
2007-02-19  0:25           ` Miklos Szeredi
2007-02-19  0:30             ` Miklos Szeredi
2007-02-19  0:45             ` Miklos Szeredi
2007-02-19  0:45             ` Chris Mason
2007-02-19  0:54               ` Miklos Szeredi
2007-02-19  1:01                 ` Chris Mason
2007-02-19  1:14                   ` Miklos Szeredi
2007-02-20  0:16                     ` Chris Mason
2007-02-20  8:53                       ` Miklos Szeredi
2007-02-19 17:11           ` Miklos Szeredi
2007-02-19 23:12             ` Miklos Szeredi
2007-02-20  0:13             ` Chris Mason
2007-02-20  8:47               ` Miklos Szeredi
2007-02-20 11:30                 ` Chris Mason
2007-02-21 21:36             ` Andrew Morton
2007-02-22  7:42               ` Miklos Szeredi
2007-02-22  7:55                 ` Andrew Morton
2007-02-22  8:02                   ` Miklos Szeredi

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