Re: [RFC PATCH v4 07/11] fs: statx add write zeroes unmap attribute

[Christoph Hellwig <hch@lst.de>]

On Wed, May 07, 2025 at 03:33:23PM +0800, Zhang Yi wrote:
> On 2025/5/6 20:11, Christoph Hellwig wrote:
> > On Tue, May 06, 2025 at 07:16:56PM +0800, Zhang Yi wrote:
> >> Sorry, but I don't understand your suggestion. The
> >> STATX_ATTR_WRITE_ZEROES_UNMAP attribute only indicate whether the bdev
> >> and the block device that under the specified file support unmap write
> >> zeroes commoand. It does not reflect whether the bdev and the
> >> filesystems support FALLOC_FL_WRITE_ZEROES. The implementation of
> >> FALLOC_FL_WRITE_ZEROES doesn't fully rely on the unmap write zeroes
> >> commoand now, users simply refer to this attribute flag to determine
> >> whether to use FALLOC_FL_WRITE_ZEROES when preallocating a file.
> >> So, STATX_ATTR_WRITE_ZEROES_UNMAP and FALLOC_FL_WRITE_ZEROES doesn't
> >> have strong relations, why do you suggested to put this into the ext4
> >> and bdev patches that adding FALLOC_FL_WRITE_ZEROES?
> > 
> > So what is the point of STATX_ATTR_WRITE_ZEROES_UNMAP?
> 
> My idea is not to strictly limiting the use of FALLOC_FL_WRITE_ZEROES to
> only bdev or files where bdev_unmap_write_zeroes() returns true. In
> other words, STATX_ATTR_WRITE_ZEROES_UNMAP and FALLOC_FL_WRITE_ZEROES
> are not consistent, they are two independent features. Even if some
> devices STATX_ATTR_WRITE_ZEROES_UNMAP are not set, users should still be
> allowed to call fallcoate(FALLOC_FL_WRITE_ZEROES). This is because some
> devices and drivers currently cannot reliably ascertain whether they
> support the unmap write zero command; however, certain devices, such as
> specific cloud storage devices, do support it. Users of these devices
> may also wish to use FALLOC_FL_WRITE_ZEROES to expedite the zeroing
> process.

What are those "cloud storage devices" where you set it reliably,
i.e.g what drivers?

> Therefore, I think that the current point of
> STATX_ATTR_WRITE_ZEROES_UNMAP (possibly STATX_WRITE_ZEROES_UNMAP) should
> be to just indicate whether a bdev or file supports the unmap write zero
> command (i.e., whether bdev_unmap_write_zeroes() returns true). If we
> use standard SCSI and NVMe storage devices, and the
> STATX_ATTR_WRITE_ZEROES_UNMAP attribute is set, users can be assured
> that FALLOC_FL_WRITE_ZEROES is fast and can choose to use
> fallocate(FALLOC_FL_WRITE_ZEROES) immediately.

That's breaking the abstracton again.  An attribute must say something
about the specific file, not about some underlying semi-related feature.

> Would you prefer to make STATX_ATTR_WRITE_ZEROES_UNMAP and
> FALLOC_FL_WRITE_ZEROES consistent, which means
> fallcoate(FALLOC_FL_WRITE_ZEROES) will return -EOPNOTSUPP if the block
> device doesn't set STATX_ATTR_WRITE_ZEROES_UNMAP ?

Not sure where the block device comes from here, both of these operate
on a file.

> If so, I'd suggested we need to:
> 1) Remove STATX_ATTR_WRITE_ZEROES_UNMAP since users can check the
>    existence by calling fallocate(FALLOC_FL_WRITE_ZEROES) directly, this
>    statx flag seems useless.

Yes, that was my inital thought.

> 2) Make the BLK_FEAT_WRITE_ZEROES_UNMAP sysfs interface to RW, allowing
>    users to adjust the block device's support state according to the
>    real situation.

No, it's a feature and not a flag.