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

[Zhang Yi <yi.zhang@huaweicloud.com>]

On 2025/5/6 13:47, Christoph Hellwig wrote:
> On Mon, May 05, 2025 at 10:36:54PM -0700, Darrick J. Wong wrote:
>> I think STATX_* (i.e. not STATX_ATTR_*) flags have two purposes: 1) to
>> declare that specific fields in struct statx actually have meaning, most
>> notably in scenarios where zeroes are valid field contents; and 2) if
>> filling out the field is expensive, userspace can elect not to have it
>> filled by leaving the bit unset.  I don't know how userspace is supposed
>> to figure out which fields are expensive.
> 
> Yes.
> 

IIUC, it seems I was misled by STATX_ATTR_WRITE_ATOMIC, adding this
STATX_ATTR_WRITE_ZEROES_UNMAP attribute flag is incorrect. The right
approach should be to add STATX_WRITE_ZEROES_UNMAP, setting it in the
result_mask if the request_mask includes this flag and
bdev_write_zeroes_unmap(bdev) returns true. Something like below. Is
my understanding right?

diff --git a/block/bdev.c b/block/bdev.c
index 4ba48b8735e7..e1367f30dbce 100644
--- a/block/bdev.c
+++ b/block/bdev.c
@@ -1303,9 +1303,9 @@ void bdev_statx(const struct path *path, struct kstat *stat, u32 request_mask)
                        queue_atomic_write_unit_max_bytes(bd_queue));
        }

+       if (request_mask & STATX_WRITE_ZEROES_UNMAP &&
+           bdev_write_zeroes_unmap(bdev))
+               stat->result_mask |= STATX_WRITE_ZEROES_UNMAP;

        stat->blksize = bdev_io_min(bdev);

Thanks,
Yi.