Re: [PATCH] io_uring: avoid page allocation warnings

[Mark Rutland <mark.rutland@arm.com>]

On Tue, Apr 30, 2019 at 07:18:10AM -0700, Matthew Wilcox wrote:
> 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.

Sure. I'll go verify that the uring code doesn't assume this memory is
physically contiguous.

I also guess we should be passing GFP_KERNEL_ACCOUNT rateh than a plain
GFP_KERNEL.

Thanks,
Mark.