linux-ext4 - Re: [PATCH v2 2/2] [PATCH] xfs: Close race between direct IO and xfs_break

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Date:   Fri, 10 Aug 2018 14:26:42 -0500
From:   Eric Sandeen <sandeen@...hat.com>
To:     Ross Zwisler <zwisler@...il.com>, dave.jiang@...el.com
Cc:     Theodore Ts'o <tytso@....edu>, darrick.wong@...cle.com,
        Jan Kara <jack@...e.cz>, linux-nvdimm@...ts.01.org,
        Dave Chinner <david@...morbit.com>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        lczerner@...hat.com, linux-ext4 <linux-ext4@...r.kernel.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2 2/2] [PATCH] xfs: Close race between direct IO and
 xfs_break_layouts()

On 8/10/18 2:24 PM, Ross Zwisler wrote:
> On Fri, Aug 10, 2018 at 9:23 AM Dave Jiang <dave.jiang@...el.com> wrote:
>> On 08/10/2018 11:31 AM, Eric Sandeen wrote:
>>> On 8/8/18 12:31 PM, Dave Jiang wrote:
>>>> This patch is the duplicate of ross's fix for ext4 for xfs.
>>>>
>>>> If the refcount of a page is lowered between the time that it is returned
>>>> by dax_busy_page() and when the refcount is again checked in
>>>> xfs_break_layouts() => ___wait_var_event(), the waiting function
>>>> xfs_wait_dax_page() will never be called.  This means that
>>>> xfs_break_layouts() will still have 'retry' set to false, so we'll stop
>>>> looping and never check the refcount of other pages in this inode.
>>>>
>>>> Instead, always continue looping as long as dax_layout_busy_page() gives us
>>>> a page which it found with an elevated refcount.
>>>
>>> Hi Dave, does this have a testcase?  Have you seen the issue using Ross's
>>> xfstest generic/503 or is there some other test?  Apologies if I missed
>>> prior discussion on a testcase or race frequency...
>>
>> I do not have a testcase. I know Ross replicated it on ext4. And Jan
>> asked to create the same fix with XFS when he reviewed Ross's fix for ext4.
> 
> In my testing I couldn't get this race to hit with XFS.  I couldn't
> even get a failure with generic/503 when testing XFS before Dan's
> initial patches went in which added xfs_break_layouts() et al.  I
> think that Dan had to manually insert timing delays to get the warning
> to hit for XFS when testing his patches.
> 
> The race we're fixing happens consistently with ext4 and through code
> inspection we can see that the race exists in XFS.

Ok, thanks for the info Dave & Ross!

-Eric