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Re: [PATCH 02/17] xfs: skip writeback from reclaim context

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 02/17] xfs: skip writeback from reclaim context
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Thu, 3 Jun 2010 02:52:52 -0400
Cc: xfs@xxxxxxxxxxx, linux-mm@xxxxxxxxx
In-reply-to: <20100602230209.GA27325@dastard>
References: <20100531160727.842750532@xxxxxxxxxxxxxxxxxxxxxx> <20100531160859.184576507@xxxxxxxxxxxxxxxxxxxxxx> <20100602043957.GB7011@dastard> <20100602100812.GA25035@xxxxxxxxxxxxx> <20100602230209.GA27325@dastard>
User-agent: Mutt/1.5.19 (2009-01-05)
On Thu, Jun 03, 2010 at 09:02:09AM +1000, Dave Chinner wrote:
> Did you skip it unconditionally, or only when a transaction was
> required?

xfs_vm_releasepage is mostly a no-op if no transaction is required.
If we have neither delalloc nor unwritten buffer we do not actually
enter xfs_page_state_convert, and ->releasepage also doesn't touch
unampped buffers at all.

> The scary part is that I've seen stack traces (i.e. most stack used)
> through this reclaim path for delalloc conversion even for
> allocations that are GFP_NOFS and the only thing saving us from
> deadlocks is th PF_FSTRANS check. Even worse is that
> shrinker_page_list() will call try_to_release_pages() without
> checking whether it's allowed to enter the filesystem or not, so we
> can be doing block allocation in places we've specifically told the
> memory allocation subsystem not to....

s/shrinker_page_list/shrink_page_list/ and
s/try_to_release_pages/try_to_release_page/ above.

shrink_page_list takes the gfp_mask for try_to_release_page from
the scan_control structure passed to it from all the top of the long
callchain.  I can't find anobvious bug, but this could cause a lot
more harm.

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