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 ? 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. -- Damien Le Moal Western Digital Research
