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Re: [RFC PATCH 2/4] xfs: create function to scan and clear EOFBLOCKS ino

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [RFC PATCH 2/4] xfs: create function to scan and clear EOFBLOCKS inodes
From: Brian Foster <bfoster@xxxxxxxxxx>
Date: Wed, 05 Sep 2012 08:22:00 -0400
Cc: xfs@xxxxxxxxxxx
In-reply-to: <20120905064202.GG15292@dastard>
References: <1346097111-4476-1-git-send-email-bfoster@xxxxxxxxxx> <1346097111-4476-3-git-send-email-bfoster@xxxxxxxxxx> <20120903050612.GR15292@dastard> <50460BB8.1060701@xxxxxxxxxx> <20120905064202.GG15292@dastard>
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On 09/05/2012 02:42 AM, Dave Chinner wrote:
> On Tue, Sep 04, 2012 at 10:10:00AM -0400, Brian Foster wrote:
>> On 09/03/2012 01:06 AM, Dave Chinner wrote:
>>> On Mon, Aug 27, 2012 at 03:51:49PM -0400, Brian Foster wrote:
>> ...
>>>> +/*
>>>> + * Handle an EOFBLOCKS tagged inode. If this is a forced scan, we wait on 
>>>> the
>>>> + * iolock ourselves rather than rely on the trylock in 
>>>> xfs_free_eofblocks(). 
>>>> + *
>>>> + * We rely on the output parameter from xfs_free_eofblocks() to determine
>>>> + * whether we should clear the tag because in the trylock case, it could 
>>>> have
>>>> + * skipped the inode due to lock contention.
>>>> + */
>>>> +STATIC int
>>>> +xfs_inode_free_eofblocks(
>>>> +  struct xfs_inode        *ip,
>>>> +  int                     flags)
>>>> +{
>>>> +  int ret = 0;
>>>> +  bool freed = false;
>>>> +  bool wait_iolock = (flags & EOFBLOCKS_WAIT) ? true : false;
>>>> +
>>>> +  if (wait_iolock)
>>>> +          xfs_ilock(ip, XFS_IOLOCK_EXCL);
>>>
>>> Why do we need the IO lock here? xfs_free_eofblocks() does all the
>>> necessary locking....
>>>
>>
>> This was for the wait case (e.g., xfs_free_eofblocks() does a trylock
>> on the IO lock and we want to wait for the lock in this case).
> 
> Does it really matter if we can't get the iolock and skip the inode?
> 

Well I thought that was the point of the force/wait scan as opposed to
the best effort scan. But...

> Note that xfs_inode_ag_walk() will keep a skipped count if you
> return EAGAIN, and re-run the walk on the AG if any inodes are
> skipped. This is done to prevent blocking on locks slowing down the
> scan - instead of blocking we move on to the next inode, and revist
> the blocked inodes again in a later pass.
> 
> IOWs, if you convert the code to use the generic walk and make
> xfs_free_eofblocks() return EAGAIN when it can't get the io lock,
> you get the retry for free...
> 

This sounds more efficient. ;) I'll move it to this approach. Thanks.

Brian

> Cheers,
> 
> Dave.
> 

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