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Subject: Re: [f2fs-dev] [PATCH] f2fs: fix long latency due to discard during
 umount
From:   Chao Yu <yuchao0@huawei.com>
To:     Sahitya Tummala <stummala@codeaurora.org>,
        Jaegeuk Kim <jaegeuk@kernel.org>,
        <linux-f2fs-devel@lists.sourceforge.net>
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Message-ID: <7804a289-ee55-930c-8b6a-52697d3db679@huawei.com>
Date:   Tue, 24 Mar 2020 17:47:03 +0800
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On 2020/3/24 17:08, Chao Yu wrote:
> On 2020/3/18 12:44, Sahitya Tummala wrote:
>> F2FS already has a default timeout of 5 secs for discards that
>> can be issued during umount, but it can take more than the 5 sec
>> timeout if the underlying UFS device queue is already full and there
>> are no more available free tags to be used. In that case, submit_bio()
>> will wait for the already queued discard requests to complete to get
>> a free tag, which can potentially take way more than 5 sec.
>>
>> Fix this by submitting the discard requests with REQ_NOWAIT
>> flags during umount. This will return -EAGAIN for UFS queue/tag full
>> scenario without waiting in the context of submit_bio(). The FS can
>> then handle these requests by retrying again within the stipulated
>> discard timeout period to avoid long latencies.

BTW, I guess later we can add nowait logic as a sub policy of
discard_policy, then DPOLICY_BG would be more configurable with
this nowait policy support.

Thanks,

>>
>> Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
> 
> Reviewed-by: Chao Yu <yuchao0@huawei.com>
> 
> Thanks,
> 
> 
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