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 <2023101201-grasp-smartly-2085@gregkh> <2023101241-old-germinate-7a05@gregkh>
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From:   Alexander Potapenko <glider@google.com>
Date:   Thu, 12 Oct 2023 13:14:39 +0200
Message-ID: <CAG_fn=W0cRThcNzE3G7bu5VqwOddTp_uqup3k7tZSMqM2te0_w@mail.gmail.com>
Subject: Re: [PATCH] lib/test_meminit: fix off-by-one error in test_pages()
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     linux-kernel@vger.kernel.org, Andrew Donnellan <ajd@linux.ibm.com>,
        Xiaoke Wang <xkernel.wang@foxmail.com>, stable@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
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On Thu, Oct 12, 2023 at 12:37=E2=80=AFPM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> On Thu, Oct 12, 2023 at 12:26:58PM +0200, Greg Kroah-Hartman wrote:
> > On Thu, Oct 12, 2023 at 10:40:14AM +0200, Alexander Potapenko wrote:
> > > On Thu, Oct 12, 2023 at 10:17=E2=80=AFAM Greg Kroah-Hartman
> > > <gregkh@linuxfoundation.org> wrote:
> > > >
> > > > In commit efb78fa86e95 ("lib/test_meminit: allocate pages up to ord=
er
> > > > MAX_ORDER"), the loop for testing pages is set to "<=3D MAX_ORDER" =
which
> > > > causes crashes in systems when run.  Fix this to "< MAX_ORDER" to f=
ix
> > > > the test to work properly.
> > >
> > > What are the crashes you are seeing? Are those OOMs?
> >
> > They are WARN_ON() triggers.  They are burried in the Android build
> > system, let me see if I can uncover them.
> >
> > > IIUC it should be valid to allocate with MAX_ORDER.
> >
> > "should", but I no longer get runtime warnings with this patch applied,
> > so something is wrong :)

I think I know what's going on. In March 2023 Kirill changed the
semantics of MAX_ORDER to be inclusive: now alloc_pages() accepts
0..MAX_ORDER, whereas previously it was 0..MAX_ORDER-1:
https://github.com/torvalds/linux/commit/23baf831a32c04f9a968812511540b1b3e=
648bf5
Older kernel versions had an explicit check for order >=3D MAX_ORDER,
which is what you're seeing on Android.