Re: [PATCH 0/2] RFC: Issue with discards on raw block device without O_DIRECT

[Jan Kara]

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?

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