linux-ext4 - Re: dm: use queue_limits

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Message-ID: <Zky951XVqcqI48P3@kernel.org>
Date: Tue, 21 May 2024 11:29:43 -0400
From: Mike Snitzer <snitzer@...nel.org>
To: Christoph Hellwig <hch@....de>, Theodore Ts'o <tytso@....edu>
Cc: dm-devel@...ts.linux.dev, fstests@...r.kernel.org,
	linux-ext4@...r.kernel.org, regressions@...ts.linux.dev,
	linux-block@...r.kernel.org
Subject: Re: dm: use queue_limits_set

On Mon, May 20, 2024 at 08:45:28PM -0400, Mike Snitzer wrote:
> On Mon, May 20, 2024 at 06:03:11PM -0400, Mike Snitzer wrote:
> > On Mon, May 20, 2024 at 10:12:37PM +0200, Christoph Hellwig wrote:
> > > On Mon, May 20, 2024 at 01:17:46PM -0400, Mike Snitzer wrote:
> > > > Doubt there was anything in fstests setting max discard user limit
> > > > (max_user_discard_sectors) in Ted's case. blk_set_stacking_limits()
> > > > sets max_user_discard_sectors to UINT_MAX, so given the use of
> > > > min(lim->max_hw_discard_sectors, lim->max_user_discard_sectors) I
> > > > suspect blk_stack_limits() stacks up max_discard_sectors to match the
> > > > underlying storage's max_hw_discard_sectors.
> > > > 
> > > > And max_hw_discard_sectors exceeds BIO_PRISON_MAX_RANGE, resulting in
> > > > dm_cell_key_has_valid_range() triggering on:
> > > > WARN_ON_ONCE(key->block_end - key->block_begin > BIO_PRISON_MAX_RANGE)
> > > 
> > > Oh, that makes more sense.
> > > 
> > > I think you just want to set the max_hw_discard_sectors limit before
> > > stacking in the lower device limits so that they can only lower it.
> > > 
> > > (and in the long run we should just stop stacking the limits except
> > > for request based dm which really needs it)
> > 
> > This is what I staged, I cannot send a patch out right now.. 
> > 
> > Ted if you need the patch in email (rather than from linux-dm.git) I
> > can send it later tonight.  Please see:
> > 
> > https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-6.10&id=825d8bbd2f32cb229c3b6653bd454832c3c20acb
> 
> From: Mike Snitzer <snitzer@...nel.org>
> Date: Mon, 20 May 2024 13:34:06 -0400
> Subject: [PATCH] dm: always manage discard support in terms of max_hw_discard_sectors
> 
> Commit 4f563a64732d ("block: add a max_user_discard_sectors queue
> limit") changed block core to set max_discard_sectors to:
>  min(lim->max_hw_discard_sectors, lim->max_user_discard_sectors)
> 
> Since commit 1c0e720228ad ("dm: use queue_limits_set") it was reported
> dm-thinp was failing in a few fstests (generic/347 and generic/405)
> with the first WARN_ON_ONCE in dm_cell_key_has_valid_range() being
> reported, e.g.:
> WARNING: CPU: 1 PID: 30 at drivers/md/dm-bio-prison-v1.c:128 dm_cell_key_has_valid_range+0x3d/0x50
> 
> blk_set_stacking_limits() sets max_user_discard_sectors to UINT_MAX,
> so given how block core now sets max_discard_sectors (detailed above)
> it follows that blk_stack_limits() stacks up the underlying device's
> max_hw_discard_sectors and max_discard_sectors is set to match it. If
> max_hw_discard_sectors exceeds dm's BIO_PRISON_MAX_RANGE, then
> dm_cell_key_has_valid_range() will trigger the warning with:
> WARN_ON_ONCE(key->block_end - key->block_begin > BIO_PRISON_MAX_RANGE)
> 
> Aside from this warning, the discard will fail.  Fix this and other DM
> issues by governing discard support in terms of max_hw_discard_sectors
> instead of max_discard_sectors.
> 
> Reported-by: Theodore Ts'o <tytso@....edu>
> Fixes: 1c0e720228ad ("dm: use queue_limits_set")
> Signed-off-by: Mike Snitzer <snitzer@...nel.org>

With this patch applied, I verified xfstests generic/347 and
generic/405 no longer trigger the dm_cell_key_has_valid_range
WARN_ON_ONCE.  I'm sending the fix to Linus now.