linux-kernel - Re: [f2fs-dev] [PATCH v2] f2fs: fix wrong inflight page stats for directIO

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Message-ID: <YPjNGoFzQojO5Amr@sol.localdomain>
Date:   Wed, 21 Jul 2021 18:42:50 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Chao Yu <chao@...nel.org>
Cc:     jaegeuk@...nel.org, linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, Chao Yu <chao.yu@...ux.dev>
Subject: Re: [f2fs-dev] [PATCH v2] f2fs: fix wrong inflight page stats for
 directIO

On Wed, Jul 21, 2021 at 05:46:26PM -0700, Eric Biggers wrote:
> On Mon, Jul 19, 2021 at 04:45:48PM +0800, Chao Yu wrote:
> > Previously, we use sbi->nr_pages[] to account direct IO, the count should
> > be based on page granularity rather than bio granularity, fix it.
> > 
> > Fixes: 02b16d0a34a1 ("f2fs: add to account direct IO")
> > Signed-off-by: Chao Yu <chao@...nel.org>

Also, do we have to do this, as opposed to just moving F2FS_DIO_WRITE and
F2FS_DIO_READ out of ->nr_pages[] and into separate counters that are clearly
defined to be per-request?  As Christoph pointed out
(https://lkml.kernel.org/r/YPU+3inGclUtcSpJ@infradead.org), these counters are
only used to check whether there is any in-flight direct I/O at all.  (They're
also shown in /sys/kernel/debug/f2fs/status, but that doesn't really matter.)

As Christoph mentioned, there is a way to avoid needing f2fs_dio_submit_bio()
(which would save a memory allocation for every bio, which can fail).  But it
will only work if the counters remain per-request.

Can we leave these as per-request?

- Eric