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Message-ID: <067578f0-9e3a-e2cf-f7c8-ff7eeda2694a@linux.alibaba.com>
Date:   Tue, 20 Jun 2023 11:50:05 +0800
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Subject: Re: [PATCH] fuse: fix return value of inode_inline_reclaim_one_dmap
 in error path
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To:     Vivek Goyal <vgoyal@redhat.com>
Cc:     miklos@szeredi.hu, linux-fsdevel@vger.kernel.org,
        gerry@linux.alibaba.com, linux-kernel@vger.kernel.org,
        German Maglione <gmaglione@redhat.com>
References: <20230424123250.125404-1-jefflexu@linux.alibaba.com>
 <ZHeoIFrp303f0E8d@redhat.com>
 <33fd8e03-7c99-c12d-255d-b7190612379b@linux.alibaba.com>
 <ZHiE2zkFJKBl9GZ+@redhat.com>
From:   Jingbo Xu <jefflexu@linux.alibaba.com>
In-Reply-To: <ZHiE2zkFJKBl9GZ+@redhat.com>
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On 6/1/23 7:45 PM, Vivek Goyal wrote:
> On Thu, Jun 01, 2023 at 09:45:52AM +0800, Jingbo Xu wrote:
>>
>>
>> On 6/1/23 4:03 AM, Vivek Goyal wrote:
>>> On Mon, Apr 24, 2023 at 08:32:50PM +0800, Jingbo Xu wrote:
>>>> When range already got reclaimed by somebody else, return NULL so that
>>>> the caller could retry to allocate or reclaim another range, instead of
>>>> mistakenly returning the range already got reclaimed and reused by
>>>> others.
>>>>
>>>> Reported-by: Liu Jiang <gerry@linux.alibaba.com>
>>>> Fixes: 9a752d18c85a ("virtiofs: add logic to free up a memory range")
>>>> Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
>>>
>>> Hi Jingbo,
>>>
>>> This patch looks correct to me.
>>>
>>> Are you able to reproduce the problem? Or you are fixing it based on
>>> code inspection?
>>
>> It's spotted by Liu Jiang during code review.  Not tested yet.
>>
>>>
>>> How are you testing this? We don't have virtiofsd DAX implementation yet
>>> in rust virtiofsd yet. 
>>>
>>> I am not sure how to test this chagne now. We had out of tree patches
>>> in qemu and now qemu has gotten rid of C version of virtiofsd so these
>>> patches might not even work now.
>>
>> Yeah this exception path may not be so easy to be tested as it is only
>> triggered in the race condition.  I have the old branch (of qemu) with
>> support for DAX, and maybe I could try to reproduce the exception path
>> by configuring limited DAX window and heavy IO workload.
> 
> That would be great. Please test it with really small DAX window size.
> Also put some pr_debug() statements to make sure you are hitting this
> particular path during testing.

I tried to reproduce it but failed.  It seems the race is impossible
theoretically.

In theory, the race occurs when a freeable dmap is found in inode's
interval tree but found it is removed from the interval tree in the
second query.

However the above procedure is protected with
filemap_invalidate_lock(inode->i_mapping) held in
inode_inline_reclaim_one_dmap().  Given the dmap deletion operations
from inode's interval tree are all protected with
filemap_invalidate_lock(inode->i_mapping) held, e.g. inside
inode_inline_reclaim_one_dmap() and lookup_and_reclaim_dmap(), the above
race seems impossible then.

-- 
Thanks,
Jingbo