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Message-ID: <20250507210310.GG25675@frogsfrogsfrogs> Date: Wed, 7 May 2025 14:03:10 -0700 From: "Darrick J. Wong" <djwong@...nel.org> To: Zhang Yi <yi.zhang@...weicloud.com> Cc: Christoph Hellwig <hch@....de>, 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 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. > > 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. Sounds fine to me... ;) --D > Thanks, > Yi. > >
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