Re: [PATCH v2 2/2] [PATCH] xfs: Close race between direct IO and xfs_break_layouts()

From: Eric Sandeen <sandeen@redhat.com>
To: Ross Zwisler <zwisler@gmail.com>, dave.jiang@intel.com
Cc: Jan Kara <jack@suse.cz>,
	linux-nvdimm@lists.01.org, darrick.wong@oracle.com,
	Dave Chinner <david@fromorbit.com>,
	linux-xfs <linux-xfs@vger.kernel.org>,
	lczerner@redhat.com,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Theodore Ts'o <tytso@mit.edu>,
	linux-ext4 <linux-ext4@vger.kernel.org>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v2 2/2] [PATCH] xfs: Close race between direct IO and xfs_break_layouts()
Date: Fri, 10 Aug 2018 14:26:42 -0500	[thread overview]
Message-ID: <7930740d-7097-90b7-a4c2-f81d520f411f@redhat.com> (raw)
In-Reply-To: <CAOxpaSWwfd4Wat3v_iH1QymGh51QdcK1=7fsOUeXrdfqui4K=g@mail.gmail.com>

On 8/10/18 2:24 PM, Ross Zwisler wrote:
> On Fri, Aug 10, 2018 at 9:23 AM Dave Jiang <dave.jiang@intel.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
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