Re: [RFC PATCH v2 0/2] introduce io_uring_cmd_import_fixed_vec

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

On Thu, Mar 13, 2025 at 08:57:45AM +0000, Pavel Begunkov wrote:
> On 3/12/25 14:23, Sidong Yang wrote:
> > This patche series introduce io_uring_cmd_import_vec. With this function,
> > Multiple fixed buffer could be used in uring cmd. It's vectored version
> > for io_uring_cmd_import_fixed(). Also this patch series includes a usage
> > for new api for encoded read in btrfs by using uring cmd.
> 
> Pretty much same thing, we're still left with 2 allocations in the
> hot path. What I think we can do here is to add caching on the
> io_uring side as we do with rw / net, but that would be invisible
> for cmd drivers. And that cache can be reused for normal iovec imports.
> 
> https://github.com/isilence/linux.git regvec-import-cmd
> (link for convenience)
> https://github.com/isilence/linux/tree/regvec-import-cmd
> 
> Not really target tested, no btrfs, not any other user, just an idea.
> There are 4 patches, but the top 3 are of interest.

Thanks, I justed checked the commits now. I think cache is good to resolve
this without allocation if cache hit. Let me reimpl this idea and test it
for btrfs.

> 
> Another way would be to cache in btrfs, but then btrfs would need to
> care about locking for the cache and some other bits, and we wouldn't
> be able to reuse it for other drivers.

Agreed, it could be better to reuse it for other driver.

Thanks,
Sidong
> 
> -- 
> Pavel Begunkov
>