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@xxxxxxxx> SUSE Labs, CR
