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

[Kevin Wolf <kwolf@redhat.com>]

Am 26.07.2018 um 16:28 hat Eric Blake geschrieben:
> On 07/26/2018 06:33 AM, Kevin Wolf wrote:
> > The BLKDISCARD ioctl doesn't guarantee that the discarded blocks read as
> > all-zero afterwards, so don't try to abuse it for zero writing. We try
> > to only use this if BLKDISCARDZEROES tells us that it is safe, but this
> > is unreliable on older kernels and a constant 0 in newer kernels. In
> 
> For my own curiosity, which kernel commit switched to constant 0, so I can
> read the rationale in that commit message?

I think 48920ff2a5a9 is the commit that actually changes the value.

Essentially the kernel abused discard for zero writes previously (so it
needed something like BLKDISCARDZEROES) and then switched to a separate
write_zeroes operation like we do in QEMU. Once this was done, second
guessing the behaviour of discard wasn't necessary any more because you
just tell what you really want to do (discard or zero out).

> > other words, this code path is never actually used with newer kernels,
> > so we don't even try to unmap while writing zeros.
> > 
> > This patch removes the abuse of discard for writing zeroes from
> > file-posix and instead adds a new function that uses interfaces that are
> > actually meant to deallocate and zero out at the same time. Only if
> > those fail, it falls back to zeroing out without unmap. We never fall
> > back to a discard operation any more that may or may not result in
> > zeros.
> 
> Makes sense.
> 
> > 
> > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > ---
> >   block/file-posix.c | 62 +++++++++++++++++++++++++++++++++++++++++-------------
> >   1 file changed, 47 insertions(+), 15 deletions(-)
> > 
> > diff --git a/block/file-posix.c b/block/file-posix.c
> > index 60af4b3d51..9c66cd87d1 100644
> > --- a/block/file-posix.c
> > +++ b/block/file-posix.c
> > @@ -648,7 +648,7 @@ static int raw_open_common(BlockDriverState *bs, QDict *options,
> >       }
> >   #endif
> > -    bs->supported_zero_flags = s->discard_zeroes ? BDRV_REQ_MAY_UNMAP : 0;
> > +    bs->supported_zero_flags = BDRV_REQ_MAY_UNMAP;
> >       ret = 0;
> >   fail:
> >       if (filename && (bdrv_flags & BDRV_O_TEMPORARY)) {
> > @@ -1487,6 +1487,38 @@ static ssize_t handle_aiocb_write_zeroes(RawPosixAIOData *aiocb)
> >       return -ENOTSUP;
> >   }
> > +static ssize_t handle_aiocb_write_zeroes_unmap(RawPosixAIOData *aiocb)
> > +{
> > +    BDRVRawState *s = aiocb->bs->opaque;
> > +    int ret;
> > +
> > +    /* First try to write zeros and unmap at the same time */
> > +
> > +#ifdef CONFIG_FALLOCATE_PUNCH_HOLE
> > +    ret = do_fallocate(s->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
> > +                       aiocb->aio_offset, aiocb->aio_nbytes);
> 
> Umm, doesn't this have to use FALLOC_FL_ZERO_RANGE? FALLOC_FL_PUNCH_HOLE
> deallocs, but is not required to write zeroes.

Yes, it is. See the man page:

    Specifying the FALLOC_FL_PUNCH_HOLE flag (available since Linux
    2.6.38) in mode deallocates space (i.e., creates a hole) in the byte
    range starting at offset and continuing for len bytes. Within the
    specified range, partial filesystem blocks are zeroed, and whole
    filesystem blocks are removed from the file. After a successful
    call, subsequent reads from this range will return zeroes.

FALLOC_FL_ZERO_RANGE in contrast implements write_zeroes without unmap.

> > +    if (ret != -ENOTSUP) {
> > +        return ret;
> > +    }
> > +#endif
> > +
> > +#ifdef CONFIG_XFS
> > +    if (s->is_xfs) {
> > +        /* xfs_discard() guarantees that the discarded area reads as all-zero
> > +         * afterwards, so we can use it here. */
> > +        return xfs_discard(s, aiocb->aio_offset, aiocb->aio_nbytes);
> > +    }
> > +#endif
> > +
> > +    /* Make the compiler happy if neither of the above is compiled in */
> > +    (void) s;
> 
> Could also be done in fewer lines by use of:
> 
>    BDRVRawState *s G_GNUC_UNUSED = aiocb->bs->opaque;
> 
> since that attribute means "might be unused, don't warn if it is actually
> unused" (and not the stricter "must be unused, warn if it got used anyway")

Thanks, I wasn't aware of G_GNUC_UNUSED (and didn't want to add a
__attribute__ here without a macro that wraps it).

Kevin