Re: [PATCH] io_uring: avoid page allocation warnings

[Matthew Wilcox]

On Tue, Apr 30, 2019 at 02:24:05PM +0100, Mark Rutland wrote:
> In io_sqe_buffer_register() we allocate a number of arrays based on the
> iov_len from the user-provided iov. While we limit iov_len to SZ_1G,
> we can still attempt to allocate arrays exceeding MAX_ORDER.
> 
> On a 64-bit system with 4KiB pages, for an iov where iov_base = 0x10 and
> iov_len = SZ_1G, we'll calculate that nr_pages = 262145. When we try to
> allocate a corresponding array of (16-byte) bio_vecs, requiring 4194320
> bytes, which is greater than 4MiB. This results in SLUB warning that
> we're trying to allocate greater than MAX_ORDER, and failing the
> allocation.
> 
> Avoid this by passing __GFP_NOWARN when allocating arrays for the
> user-provided iov_len. We'll gracefully handle the failed allocation,
> returning -ENOMEM to userspace.
> 
> We should probably consider lowering the limit below SZ_1G, or reworking
> the array allocations.

I'd suggest that kvmalloc is probably our friend here ... we don't really
want to return -ENOMEM to userspace for this case, I don't think.