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

[Sidong Yang <sidong.yang@furiosa.ai>]

On Fri, Mar 21, 2025 at 05:17:15AM -0600, Jens Axboe wrote:
> On 3/21/25 4:28 AM, Pavel Begunkov wrote:
> > 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.
> 
> I'd strongly recommend that the btrfs side check for valid flags and
> error it. It's a new enough addition that this should not be a concern,
> and silently ignoring (currently) unsupported flags rather than erroring
> them is a mistake.
> 
> Sidong, please do send a patch for that so it can go into 6.13 stable
> and 6.14 to avoid any confusion in this area in the future.

Agreed, It could be seen as a bug that the flag is dismissed silently.
I'll write a patch for this.

Thanks,
Sidong

> 
> -- 
> Jens Axboe