linux-kernel - Re: [PATCH] lib/test_meminit: fix off-by-one error in test

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Date:   Thu, 12 Oct 2023 13:24:01 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     linux-kernel@...r.kernel.org, Andrew Donnellan <ajd@...ux.ibm.com>,
        Xiaoke Wang <xkernel.wang@...mail.com>, stable@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH] lib/test_meminit: fix off-by-one error in test_pages()

On Thu, Oct 12, 2023 at 01:14:39PM +0200, Alexander Potapenko wrote:
> On Thu, Oct 12, 2023 at 12:37 PM Greg Kroah-Hartman
> <gregkh@...uxfoundation.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 AM Greg Kroah-Hartman
> > > > <gregkh@...uxfoundation.org> wrote:
> > > > >
> > > > > In commit efb78fa86e95 ("lib/test_meminit: allocate pages up to order
> > > > > MAX_ORDER"), the loop for testing pages is set to "<= MAX_ORDER" which
> > > > > causes crashes in systems when run.  Fix this to "< MAX_ORDER" to fix
> > > > > 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/23baf831a32c04f9a968812511540b1b3e648bf5
> Older kernel versions had an explicit check for order >= MAX_ORDER,
> which is what you're seeing on Android.

Thanks for digging this up.  I'll just queue this patch up for 6.1 and
older stable kernels then as the commit above you found is in 6.4 so
nothing newer than that will need this fix.

Thanks for the quick response!

greg k-h