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

From: Nir Soffer
Subject: Re: [Qemu-devel] [PATCH for-3.0] file-posix: Fix write_zeroes with unmap on block devices
Date: Thu, 26 Jul 2018 19:46:54 +0300

On Thu, Jul 26, 2018 at 7:10 PM Eric Blake <address@hidden> wrote:

> 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.

I don't think we should depend on undocumented kernel code.


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