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

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Message-ID: <6fc62631-fb6f-4207-badb-1964b20fa89a@huaweicloud.com>
Date: Wed, 7 May 2025 16:23:06 +0800
From: Zhang Yi <yi.zhang@...weicloud.com>
To: "Darrick J. Wong" <djwong@...nel.org>, Christoph Hellwig <hch@....de>
Cc: dhowells@...hat.com, brauner@...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,
 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 23:55, Darrick J. Wong wrote:
> On Tue, May 06, 2025 at 02:10:12PM +0200, Christoph Hellwig wrote:
>> On Tue, May 06, 2025 at 07:25:06PM +0800, Zhang Yi wrote:
>>> +       if (request_mask & STATX_WRITE_ZEROES_UNMAP &&
>>> +           bdev_write_zeroes_unmap(bdev))
>>> +               stat->result_mask |= STATX_WRITE_ZEROES_UNMAP;
>>
>> That would be my expectation.  But then again this area seems to
>> confuse me a lot, so maybe we'll get Christian or Dave to chim in.
> 
> Um... does STATX_WRITE_ZEROES_UNMAP protect a field somewhere?
> It might be nice to expose the request alignment granularity/max
> size/etc.

I think that simply returning the support state is sufficient at the
moment. __blkdev_issue_write_zeroes() will send write zeroes through
multiple iterations, and there are no specific restrictions on the
parameters provided by users.

> Or does this flag exist solely to support discovering that
> FALLOC_FL_WRITE_ZEROES is supported?  In which case, why not discover
> its existence by calling fallocate(fd, WRITE_ZEROES, 0, 0) like the
> other modes?
> 

Current STATX_WRITE_ZEROES_UNMAP and FALLOC_FL_WRITE_ZEROES are
inconsistent, we allow users to call fallocate(FALLOC_FL_WRITE_ZEROES) on
files that STATX_WRITE_ZEROES_UNMAP is not set. Users can check whether
the device supports unmap write zeroes through STATX_WRITE_ZEROES_UNMAP
and then decide to call fallocate(FALLOC_FL_WRITE_ZEROES) if it is
supported. Please see this explanation for details.

  https://lore.kernel.org/linux-fsdevel/20250421021509.2366003-1-yi.zhang@huaweicloud.com/T/#mc1618822bc27d486296216fc1643d5531fee03e1

However, removing STATX_WRITE_ZEROES_UNMAP also seems good to me(Perhaps
it would be better.).It means we do not allow to call
fallocate(FALLOC_FL_WRITE_ZEROES) if the device does not explicitly
support unmap write zeroes.

Thanks,
Yi.