linux-kernel - Re: [RFC PATCH 0/6] Do not call ->writepage[s] from direct reclaim and use a

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Message-ID: <4C19030A.4070406@redhat.com>
Date:	Wed, 16 Jun 2010 12:59:54 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Nick Piggin <npiggin@...e.de>
CC:	Christoph Hellwig <hch@...radead.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mel@....ul.ie>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>
Subject: Re: [RFC PATCH 0/6] Do not call ->writepage[s] from direct reclaim
 and use a_ops->writepages() where possible

On 06/16/2010 03:57 AM, Nick Piggin wrote:
> On Tue, Jun 15, 2010 at 03:13:09PM -0400, Rik van Riel wrote:
>> On 06/15/2010 12:54 PM, Christoph Hellwig wrote:
>>> On Tue, Jun 15, 2010 at 12:49:49PM -0400, Rik van Riel wrote:
>>>> This is already in a filesystem.  Why does ->writepage get
>>>> called a second time?  Shouldn't this have a gfp_mask
>>>> without __GFP_FS set?
>>>
>>> Why would it?  GFP_NOFS is not for all filesystem code, but only for
>>> code where we can't re-enter the filesystem due to deadlock potential.
>>
>> Why?   How about because you know the stack is not big enough
>> to have the XFS call path on it twice? :)
>>
>> Isn't the whole purpose of this patch series to prevent writepage
>> from being called by the VM, when invoked from a deep callstack
>> like xfs writepage?
>>
>> That sounds a lot like simply wanting to not have GFP_FS...
>
> buffered write path uses __GFP_FS by design because huge amounts
> of (dirty) memory can be allocated in doing pagecache writes. If
> would be nasty if that was not allowed to wait for filesystem
> activity.

__GFP_IO can wait for filesystem activity

__GFP_FS can kick off new filesystem activity

At least, that's how I remember it from when I last looked
at that code in detail.  Things may have changed subtly.

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