Re: [Qemu-devel] [PULL 5/7] file-posix: Support BDRV_REQ_NO_FALLBACK for zero writes

[Kevin Wolf]

Am 15.08.2019 um 04:44 hat Eric Blake geschrieben:
> On 3/26/19 10:51 AM, Kevin Wolf wrote:
> > We know that the kernel implements a slow fallback code path for
> > BLKZEROOUT, so if BDRV_REQ_NO_FALLBACK is given, we shouldn't call it.
> > The other operations we call in the context of .bdrv_co_pwrite_zeroes
> > should usually be quick, so no modification should be needed for them.
> > If we ever notice that there are additional problematic cases, we can
> > still make these conditional as well.
> 
> Are there cases where fallocate(FALLOC_FL_ZERO_RANGE) falls back to slow
> writes?  It may be fast on some file systems, but when used on a block
> device, that may equally trigger slow fallbacks.  The man page is not
> clear on that fact; I suspect that there may be cases in there that need
> to be made conditional (it would be awesome if the kernel folks would
> give us another FALLOC_ flag when we want to guarantee no fallback).

The NO_FALLBACK changes were based on the Linux code rather than
documentation because no interface is explicitly documented to forbid
fallbacks.

I think for file systems, we can generally assume that we don't get
fallbacks because for file systems, just deallocating blocks is the
easiest way to implement the function anyway. (Hm, or is it when we
don't punch holes...?)

And for block devices, we don't try FALLOC_FL_ZERO_RANGE because it also
involves the same slow fallback as BLKZEROOUT. In other words,
bdrv_co_pwrite_zeroes() with NO_FALLBACK, but without MAY_UNMAP, always
fails on Linux block devices, and we fall back to emulation in user
space.

We would need a kernel interface that calls blkdev_issue_zeroout() with
BLKDEV_ZERO_NOUNMAP | BLKDEV_ZERO_NOFALLBACK, but no such interface
exists.

When I talked to some file system people, they insisted that "efficient"
or "fast" wasn't well-defined enough for them or something, so if we
want to get a kernel change, maybe a new block device ioctl would be the
most realistic thing.

We do use FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE for MAY_UNMAP,
which works for both file systems (I assume - each file system has a
separate implementation) and block devices without slow fallbacks.

qemu-img create sets MAY_UNMAP, so the case we are most interested in is
covered with a fast implementation.

> By the way, is there an easy setup to prove (maybe some qemu-img convert
> command on a specially-prepared source image) whether the no fallback
> flag makes a difference?  I'm about to cross-post a series of patches to
> nbd/qemu/nbdkit/libnbd that adds a new NBD_CMD_FLAG_FAST_ZERO which fits
> the bill of BDRV_REQ_NO_FALLBACK, but would like to include some
> benchmark numbers in my cover letter if I can reproduce a setup where it
> matters.

Hm, the original case came from Nir, maybe he can suggest something.

You'll definitely need a block device that doesn't support
FALLOC_FL_PUNCH_HOLE, otherwise you can't trigger the fallback. My
first though was a loop device, but this actually does support the
operation and passes it through to the underlying file system. So maybe
if you know a file system that doesn't support it. Or if you have an old
hard disk handy.

Or... Actually there is an easily available block device that doesn't
suppport zero writes: The in-kernel NBD client! :-)

And I think I remember now how I tested this back then:

1. qemu-nbd exports an image with very slow throttling enabled
   (throttling affects writes, but not write_zeroes)

2. Attach the NBD device to /dev/nbd0

3. Convert to there (use a second NBD connection to test the fix)

> And this patch has a bug:
> 
> > +++ b/block/file-posix.c
> > @@ -652,7 +652,7 @@ static int raw_open_common(BlockDriverState *bs, QDict 
> > *options,
> >      }
> >  #endif
> >  
> > -    bs->supported_zero_flags = BDRV_REQ_MAY_UNMAP;
> > +    bs->supported_zero_flags = BDRV_REQ_MAY_UNMAP | BDRV_REQ_NO_FALLBACK;
> >      ret = 0;
> >  fail:
> >      if (filename && (bdrv_flags & BDRV_O_TEMPORARY)) {
> > @@ -1500,14 +1500,19 @@ static ssize_t 
> > handle_aiocb_write_zeroes_block(RawPosixAIOData *aiocb)
> {
>     int ret = -ENOTSUP;
>     BDRVRawState *s = aiocb->bs->opaque;
> 
>     if (!s->has_write_zeroes) {
>         return -ENOTSUP;
> >      }
> 
> At this point, ret is -ENOTSUP.
> 
> >  
> >  #ifdef BLKZEROOUT
> > -    do {
> > -        uint64_t range[2] = { aiocb->aio_offset, aiocb->aio_nbytes };
> > -        if (ioctl(aiocb->aio_fildes, BLKZEROOUT, range) == 0) {
> > -            return 0;
> > -        }
> > -    } while (errno == EINTR);
> > +    /* The BLKZEROOUT implementation in the kernel doesn't set
> > +     * BLKDEV_ZERO_NOFALLBACK, so we can't call this if we have to avoid 
> > slow
> > +     * fallbacks. */
> > +    if (!(aiocb->aio_type & QEMU_AIO_NO_FALLBACK)) {
> > +        do {
> > +            uint64_t range[2] = { aiocb->aio_offset, aiocb->aio_nbytes };
> > +            if (ioctl(aiocb->aio_fildes, BLKZEROOUT, range) == 0) {
> > +                return 0;
> > +            }
> > +        } while (errno == EINTR);
> >  
> > -    ret = translate_err(-errno);
> > +        ret = translate_err(-errno);
> > +    }
> 
> If the very first call to this function is with NO_FALLBACK, then this
> 'if' is skipped,
> 
> >  #endif
> >  
> >      if (ret == -ENOTSUP) {
>         s->has_write_zeroes = false;
>     }
> 
> and we set s->has_write_zeroes to false, permanently disabling any
> BLKZEROOUT attempts in future calls, even if the future calls no longer
> pass the NO_FALLBACK flag.

Right, this should be moved inside the if (!NO_FALLBACK) block.

Kevin

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