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

[Maxim Levitsky]

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.

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.

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?

Best regards,
        Maxim Levitsky