On Thu, Nov 12, 2020 at 01:00:56PM +0100, Jan Kara wrote: > On Thu 12-11-20 12:19:51, Jan Kara wrote: > > [added some relevant people and lists to CC] > > > > On Wed 11-11-20 17:44:05, Maxim Levitsky wrote: > > > On Wed, 2020-11-11 at 17:39 +0200, Maxim Levitsky wrote: > > > > clone of "starship_production" > > > > > > The git-publish destroyed the cover letter: > > > > > > For the reference this is for bz #1872633 > > > > > > The issue is that current kernel code that implements 'fallocate' > > > on kernel block devices roughly works like that: > > > > > > 1. Flush the page cache on the range that is about to be discarded. > > > 2. Issue the discard and wait for it to finish. > > > (as far as I can see the discard doesn't go through the > > > page cache). > > > > > > 3. Check if the page cache is dirty for this range, > > > if it is dirty (meaning that someone wrote to it meanwhile) > > > return -EBUSY. > > > > > > This means that if qemu (or qemu-img) issues a write, and then > > > discard to the area that shares a page, -EBUSY can be returned by > > > the kernel. > > > > Indeed, if you don't submit PAGE_SIZE aligned discards, you can get back > > EBUSY which seems wrong to me. IMO we should handle this gracefully in the > > kernel so we need to fix this. > > > > > On the other hand, for example, the ext4 implementation of discard > > > doesn't seem to be affected. It does take a lock on the inode to avoid > > > concurrent IO and flushes O_DIRECT writers prior to doing discard thought. > > > > Well, filesystem hole punching is somewhat different beast than block device > > discard (at least implementation wise). > > > > > Doing fsync and retrying is seems to resolve this issue, but it might be > > > a too big hammer. Just retrying doesn't work, indicating that maybe the > > > code that flushes the page cache in (1) doesn't do this correctly ? > > > > > > It also can be racy unless special means are done to block IO from happening > > > from qemu during this fsync. > > > > > > This patch series contains two patches: > > > > > > First patch just lets the file-posix ignore the -EBUSY errors, which is > > > technically enough to fail back to plain write in this case, but seems wrong. > > > > > > And the second patch adds an optimization to qemu-img to avoid such a > > > fragmented write/discard in the first place. > > > > > > Both patches make the reproducer work for this particular bugzilla, > > > but I don't think they are enough. > > > > > > What do you think? > > > > So if the EBUSY error happens because something happened to the page cache > > outside of discarded range (like you describe above), that is a kernel bug > > than needs to get fixed. EBUSY should really mean - someone wrote to the > > discarded range while discard was running and userspace app has to deal > > with that depending on what it aims to do... > > So I was looking what it would take to fix this inside the kernel. The > problem is that invalidate_inode_pages2_range() is working on page > granularity and it is non-trivial to extend it to work on byte granularity > since we don't support something like "try to reclaim part of a page". But > I'm also somewhat wondering why we use invalidate_inode_pages2_range() here > instead of truncate_inode_pages_range() again? I mean the EBUSY detection > cannot be reliable anyway and userspace has no way of knowing whether a > write happened before discard or after it so just discarding data is fine > from this point of view. Darrick? Hmmm, I think I overlooked the fact that we can do buffered writes into a block device's pagecache without taking any of the usual locks that have to be held for filesystem files. This is essentially a race between a not-page-aligned fallocate and a buffered write to a different sector that is mapped by a page that caches part of the fallocate range. So yes, Jan is right that we need to use truncate_bdev_range instead of invalidate_inode_pages2_range because the former will zero the sub-page ranges on either end of the fallocate request instead of returning -EBUSY because someone dirtied a part of a page that wasn't involved in the fallocate operation. I /probably/ just copy-pasta'd that invalidation call from directio without thinking hard enough about it, sorry about that. :( --D > > Honza > -- > Jan Kara <jack@xxxxxxxx> > SUSE Labs, CR
