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

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Message-ID: <a39a6612-89ac-4255-b737-37c7d16b3185@huaweicloud.com>
Date: Wed, 7 May 2025 15:33:23 +0800
From: Zhang Yi <yi.zhang@...weicloud.com>
To: Christoph Hellwig <hch@....de>
Cc: "Darrick J. Wong" <djwong@...nel.org>, linux-fsdevel@...r.kernel.org,
 linux-ext4@...r.kernel.org, linux-block@...r.kernel.org,
 dm-devel@...ts.linux.dev, linux-nvme@...ts.infradead.org,
 linux-scsi@...r.kernel.org, linux-xfs@...r.kernel.org,
 linux-kernel@...r.kernel.org, tytso@....edu, john.g.garry@...cle.com,
 bmarzins@...hat.com, chaitanyak@...dia.com, shinichiro.kawasaki@....com,
 brauner@...nel.org, yi.zhang@...wei.com, chengzhihao1@...wei.com,
 yukuai3@...wei.com, yangerkun@...wei.com
Subject: Re: [RFC PATCH v4 07/11] fs: statx add write zeroes unmap attribute

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.

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.

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 ?

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.
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.

Thanks,
Yi.