Re: [PATCHv3 5/5] io_uring: cache nodes and mapped buffers

[Keith Busch <kbusch@kernel.org>]

On Fri, Feb 14, 2025 at 06:22:09PM -0800, Caleb Sander Mateos wrote:
> > +static struct io_mapped_ubuf *io_alloc_imu(struct io_ring_ctx *ctx,
> > +                                          int nr_bvecs)
> > +{
> > +       if (nr_bvecs <= IO_CACHED_BVECS_SEGS)
> > +               return io_cache_alloc(&ctx->buf_table.imu_cache, GFP_KERNEL);
> 
> If there is no entry available in the cache, this will heap-allocate
> one with enough space for all IO_CACHED_BVECS_SEGS bvecs. 

The cache can only have fixed size objects in them, so we have to pick
some kind of trade off. The cache starts off empty and fills up on
demand, so whatever we allocate needs to be of that cache's element
size.

> Consider
> using io_alloc_cache_get() instead of io_cache_alloc(), so the
> heap-allocated fallback uses the minimal size.

We wouldn't be able to fill the cache with the new dynamic object if we
did that.

> Also, where are these allocations returned to the imu_cache? Looks
> like kvfree(imu) in io_buffer_unmap() and io_sqe_buffer_register()
> needs to try io_alloc_cache_put() first.

Oops. I kind of rushed this series last Friday as I needed to shut down
very early in the day.

Thanks for the comments. Will take my time for the next version.