Re: [Qemu-devel] [PATCH for-3.0] file-posix: Fix write_zeroes with unmap on block devices

[Eric Blake]

On 07/26/2018 10:33 AM, Kevin Wolf wrote:

> As far as I know, the comment you quoted is accurate for BLKDISCARD and
> BLKZEROOUT, but not for the fallocate() flags.

I sure wish the man pages were more explicit on what guarantees each 
flag offers.

> > Hmm - that thread also mentions FALLOC_FL_NO_HIDE_STALE, which is a new flag
> > not present/documented on Fedora 28. I wonder if it helps, too.
> >
> > > FALLOC_FL_ZERO_RANGE in contrast implements write_zeroes without unmap.
> >
> > I thought the opposite: FALLOC_FL_ZERO_RANGE guarantees that you read back
> > 0, using whatever is most efficient under the hood (in the case of block
> > devices, unmapping that reliably reads back as zero is favored).
>
> See the code I quoted above, FALLOC_FL_ZERO_RANGE calls
> blkdev_issue_zeroout() with BLKDEV_ZERO_NOUNMAP internally.

Having to resort to reading the kernel code to learn what the guarantees 
are is annoying (it's nice that GPL guarantees that we CAN do that, but 
it means the man pages could use some TLC so that we don't have to). 
Especially since the kernel code has changed over time.

But your paste of current kernel code is rather hard to argue against.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org