linux-kernel - Re: [PATCH] io_uring: avoid page allocation warnings

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Date:   Tue, 30 Apr 2019 10:21:03 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Mark Rutland <mark.rutland@....com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: [PATCH] io_uring: avoid page allocation warnings

On 4/30/19 8:59 AM, Mark Rutland wrote:
> 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.

kvmalloc() is fine, the io_uring code doesn't care about the layout of
the memory, it just uses it as an index.

-- 
Jens Axboe