Re: [RFC PATCH v5 5/5] btrfs: ioctl: introduce btrfs_uring_import_iovec()

[Pavel Begunkov <asml.silence@gmail.com>]

On 3/20/25 16:19, Sidong Yang wrote:
> On Thu, Mar 20, 2025 at 12:01:42PM +0000, Pavel Begunkov wrote:
>> On 3/19/25 06:12, Sidong Yang wrote:
>>> This patch introduces btrfs_uring_import_iovec(). In encoded read/write
>>> with uring cmd, it uses import_iovec without supporting fixed buffer.
>>> btrfs_using_import_iovec() could use fixed buffer if cmd flags has
>>> IORING_URING_CMD_FIXED.
>>
>> Looks fine to me. The only comment, it appears btrfs silently ignored
>> IORING_URING_CMD_FIXED before, so theoretically it changes the uapi.
>> It should be fine, but maybe we should sneak in and backport a patch
>> refusing the flag for older kernels?
> 
> I think it's okay to leave the old version as it is. Making it to refuse
> the flag could break user application.

Just as this patch breaks it. The cmd is new and quite specific, likely
nobody would notice the change. As it currently stands, the fixed buffer
version of the cmd is going to succeed in 99% of cases on older kernels
because we're still passing an iovec in, but that's only until someone
plays remapping games after a registration and gets bizarre results.

It's up to btrfs folks how they want to handle that, either try to fix
it now, or have a chance someone will be surprised in the future. My
recommendation would be the former one.

-- 
Pavel Begunkov