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From:   Dmitry Monakhov <dmtrmonakhov@yandex-team.ru>
To:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Cc:     "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "axboe@kernel.dk" <axboe@kernel.dk>
In-Reply-To: <20210319052859.3220-1-dmtrmonakhov@yandex-team.ru>
References: <20210319052859.3220-1-dmtrmonakhov@yandex-team.ru>
Subject: Re: [PATCH] io_uring: Try to merge io requests only for regular files
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Date:   Fri, 19 Mar 2021 10:45:46 +0300
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- stable@


19.03.2021, 08:29, "Dmitry Monakhov" <dmtrmonakhov@yandex-team.ru>:
> Otherwise we may endup blocking on pipe or socket.
>
> Fixes: 6d5d5ac ("io_uring: extend async work merge")
> Testcase: https://github.com/dmonakhov/liburing/commit/16d171b6ef9d68e6db66650a83d98c5c721d01f6
> Signed-off-by: Dmitry Monakhov <dmtrmonakhov@yandex-team.ru>
> ---
>  fs/io_uring.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 478df7e..848657c 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -2183,6 +2183,9 @@ static int __io_submit_sqe(struct io_ring_ctx *ctx, struct io_kiocb *req,
>  static struct async_list *io_async_list_from_req(struct io_ring_ctx *ctx,
>                                                   struct io_kiocb *req)
>  {
> + if (!(req->flags & REQ_F_ISREG))
> + return NULL;
> +
IMHO it is reasonable to completely disable  io_should_merge logic because
even with the this fix it still affected by latency spikes like follows:

->submit_read: req1[slow_hdd, sector=z]
->submit_read: req2[nvme, sector=X]
-> wait(req2)  -> fast

->submit_read: req3[nvme, sector=Y] 
--> wait(req3)  ->slow  
  if completes if X and Y belongs to same page merge logic will wait for req1 to complete


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