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. -- Jens Axboe
