Re: [PATCH] Revert "block: Do not discard buffers under a mounted filesystem"

[Jan Kara <jack@suse.cz>]

On Thu 18-02-21 22:35:41, Damien Le Moal wrote:
> On 2021/02/18 23:07, Jan Kara wrote:
> > On Tue 16-02-21 23:05:57, Damien Le Moal wrote:
> >> On 2021/02/17 2:51, Keith Busch wrote:
> >>> On Tue, Feb 16, 2021 at 04:36:06PM +0000, Christoph Hellwig wrote:
> >>>> On Tue, Feb 16, 2021 at 02:38:49PM +0100, Jan Kara wrote:
> >>>>> Apparently there are several userspace programs that depend on being
> >>>>> able to call BLKDISCARD ioctl without the ability to grab bdev
> >>>>> exclusively - namely FUSE filesystems have the device open without
> >>>>> O_EXCL (the kernel has the bdev open with O_EXCL) so the commit breaks
> >>>>> fstrim(8) for such filesystems. Also LVM when shrinking LV opens PV and
> >>>>> discards ranges released from LV but that PV may be already open
> >>>>> exclusively by someone else (see bugzilla link below for more details).
> >>>>>
> >>>>> This reverts commit 384d87ef2c954fc58e6c5fd8253e4a1984f5fe02.
> >>>>
> >>>> I think that is a bad idea. We fixed the problem for a reason.
> >>>> I think the right fix is to just do nothing if the device hasn't been
> >>>> opened with O_EXCL and can't be reopened with it, just don't do anything
> >>>> but also don't return an error.  After all discard and thus
> >>>> BLKDISCARD is purely advisory.
> >>>
> >>> A discard is advisory, but BLKZEROOUT is not, so something different
> >>> should happen there. We were also planning to send a patch using this
> >>> same pattern for Zone Reset to fix stale page cache issues after the
> >>> reset, but we'll wait to see how this settles before sending that.
> >>
> >> There is also another problem: the truncate_bdev & operation following it
> >> (discard, zeroout or zone reset) are not atomic vs read/write operations to the
> >> bdev. Without mutual exclusion, that page invalidation is best effort only since
> >> reads can snick in between the truncate and discard (or zeroout or zone reset).
> >> With our zone reset stale page problem case, it is reads from udevd that we see
> >> snicking in between the truncate bdev and zone reset and so we still get stale
> >> pages after the zone reset is finished. No solution to propose for solving that,
> >> yet...
> > 
> > Well, at least blkdev_fallocate() does:
> > 
> > 	truncate_bdev_range();
> > 	blkdev_issue_zeroout();
> > 	invalidate_inode_pages2_range();
> > 
> > so racing reads should not result in stale page cache contents AFAICT.
> 
> Yes, but concurrent writes can then get in between the blkdev_issue_zeroout()
> and invalidate_inode_pages2_range() and data discarded before hitting the
> drive... Not very nice either. Granted, that would mean that userland has 2
> concurrent writers that are not synchronized. So weird results are to be
> expected. I guess it is probably safe to ignore that case ?

Yes. IMHO any result that doesn't crash the kernel (or burn the HW) is fine
in that case.

> I guess the same pattern as above for zeroout would work for reset zone too.
> Will try to see if that solves our test problem.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR