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Date:   Mon, 24 Sep 2018 15:55:04 +0000
From:   Christopher Lameter <cl@linux.com>
To:     Dmitry Vyukov <dvyukov@google.com>
cc:     Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        syzbot+87829a10073277282ad1@syzkaller.appspotmail.com,
        Pekka Enberg <penberg@kernel.org>,
        "linux-input@vger.kernel.org" <linux-input@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Henrik Rydberg <rydberg@bitmath.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Linux-MM <linux-mm@kvack.org>
Subject: Re: WARNING: kmalloc bug in input_mt_init_slots
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On Mon, 24 Sep 2018, Dmitry Vyukov wrote:

> On Mon, Sep 24, 2018 at 5:08 PM, Christopher Lameter <cl@linux.com> wrote:
> > On Sun, 23 Sep 2018, Dmitry Vyukov wrote:
> >
> >> What was the motivation behind that WARNING about large allocations in
> >> kmalloc? Why do we want to know about them? Is the general policy that
> >> kmalloc calls with potentially large size requests need to use NOWARN?
> >> If this WARNING still considered useful? Or we should change it to
> >> pr_err?
> >
> > In general large allocs should be satisfied by the page allocator. The
> > slab allocators are used for allocating and managing small objects. The
> > page allocator has mechanisms to deal with large objects (compound pages,
> > multiple page sized allocs etc).
>
> I am asking more about the status of this warning. If it fires in
> input_mt_init_slots(), does it mean that input_mt_init_slots() needs
> to be fixed? If not, then we need to change this warning to something
> else.

Hmmm.. kmalloc falls back to the page allocator already?

See

static __always_inline void *kmalloc(size_t size, gfp_t flags)
{
        if (__builtin_constant_p(size)) {
                if (size > KMALLOC_MAX_CACHE_SIZE)
                        return kmalloc_large(size, flags);


Note that this uses KMALLOC_MAX_CACHE_SIZE which should be smaller than
KMALLOC_MAX_SIZE.


How large is the allocation? AFACIT nRequests larger than KMALLOC_MAX_SIZE
are larger than the maximum allowed by the page allocator. Thus the warning
and the NULL return.