linux-ext4 - Re: [patch] fix up lock order reversal in writeback

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Message-ID: <4CE362B0.6040607@redhat.com>
Date:	Tue, 16 Nov 2010 23:05:52 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Nick Piggin <npiggin@...nel.dk>
CC:	Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-btrfs@...r.kernel.org
Subject: Re: [patch] fix up lock order reversal in writeback

On 11/16/10 10:38 PM, Nick Piggin wrote:
> On Tue, Nov 16, 2010 at 10:30:37PM -0600, Eric Sandeen wrote:
>> On 11/16/10 7:01 AM, Jan Kara wrote:
>>> On Tue 16-11-10 22:00:58, Nick Piggin wrote:
>>>> I saw a lock order warning on ext4 trigger. This should solve it.
>>>> Raciness shouldn't matter much, because writeback can stop just
>>>> after we make the test and return anyway (so the API is racy anyway).
>>>   Hmm, for now the fix is OK. Ultimately, we probably want to call
>>> writeback_inodes_sb() directly from all the callers. They all just want to
>>> reduce uncertainty of delayed allocation reservations by writing delayed
>>> data and actually wait for some of the writeback to happen before they
>>> retry again the allocation.
>>
>> For ext4, at least, it's just best-effort.  We're not actually out of
>> space yet when this starts pushing.  But it helps us avoid enospc:
>>
>> commit c8afb44682fcef6273e8b8eb19fab13ddd05b386
>> Author: Eric Sandeen <sandeen@...hat.com>
>> Date:   Wed Dec 23 07:58:12 2009 -0500
>>
>>     ext4: flush delalloc blocks when space is low
>>     
>>     Creating many small files in rapid succession on a small
>>     filesystem can lead to spurious ENOSPC; on a 104MB filesystem:
>>     
>>     for i in `seq 1 22500`; do
>>         echo -n > $SCRATCH_MNT/$i
>>         echo XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX > $SCRATCH_MNT/$i
>>     done
>>     
>>     leads to ENOSPC even though after a sync, 40% of the fs is free
>>     again.
>>
>>     <snip>
>>
>> We don't need it to be synchronous - in fact I didn't think it was ...
> 
> By synchronous, I just mean that the caller is the one who pushes
> the data into writeout. It _may_ be better if it was done by background
> writeback, with a feedback loop to throttle the caller (preferably
> placed outside any locks it is holding).
> 
> To be pragmatic, I think the thing is fine to actually solve the
> problem at hand. I was just saying that it has a tiny little hackish
> feeling anyway, so a trylock will be right at home there :)
> 
>  
>> ext4 should probably use btrfs's new variant and just get rid of the
>> one I put in, for a very large system/filesystem it could end up doing
>> a rather insane amount of IO when the fs starts to get full.
>>
>> as for the locking problems ... sorry about that!
> 
> That's no problem. So is that an ack? :)
> 

I'd like to test it with the original case it was supposed to solve; will
do that tomorrow.

-Eric
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