io_uring/dma-buf: allow cloning registered dma-bufs

One problem that was brought up is that the file table size is often a
limiting factor when working with dma-bufs, which is quite an old
problem. With a single io_uring people can work it around by closing
the dma-buf fd after registration, but for more complex cases we need
a way to dynamically register it into another ring. We also need to
deduplicate internal registrations as each creates a new attachment and
related structures, which could involve large allocations and iommu
mappings. Solve it by enabling io_uring buffer cloning.

I made dma-bufs registered for io_uring not cloneable between rings
because the implementation could modify parts of struct io_buf_dma
without any locking and sharing it would've created races. That was an
optimisation that got dropped at some point for simplicity, so we can
safely enable cloning back as long as IORING_SETUP_IOPOLL rejection
is honoured. Kill the uncloneable flag as unused as well.

Reported-by: William Power <william.power@intel.com>
Reported-by: Tushar Gohad <tushar.gohad@intel.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://patch.msgid.link/b76cf21ee0e2362489c864e43781b20d95fd75f7.1792142588.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

0 files changed, 0 insertions, 0 deletions