linux-kernel - Re: [syzbot] WARNING in follow_hugetlb

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Date:   Fri, 13 May 2022 17:26:07 -0700
From:   Minchan Kim <minchan@...nel.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        syzbot <syzbot+acf65ca584991f3cc447@...kaller.appspotmail.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        llvm@...ts.linux.dev, nathan@...nel.org, ndesaulniers@...gle.com,
        syzkaller-bugs@...glegroups.com, trix@...hat.com,
        Matthew Wilcox <willy@...radead.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [syzbot] WARNING in follow_hugetlb_page

On Fri, May 13, 2022 at 05:09:11PM -0700, John Hubbard wrote:
> On 5/13/22 16:54, Minchan Kim wrote:
> > > > I isolated this to Minchan Kim's "mm: fix is_pinnable_page against on cma
> > > > page".  Yes, the fat finger fix is in next-20220513.
> > > > 
> > > > I don't have time to analyze right now, but can confirm that in the
> > > > reproducer is_pinnable_page is returning false after this change when it
> > > > previously returned true.
> > > 
> > > OK, thanks, I dropped mm-fix-is_pinnable_page-against-on-cma-page.patch
> > 
> > Seems like bug of the patch v5 due to change of this
> > 
> >      if (mt & (MIGRATE_CMA | MIGRATE_ISOLATE))
> > 
> > The migration type is not bit type so it shold be
> > 
> > if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE)
> > 
> 
> Sorry for leading you astray by recommending the bitwise OR, Minchan.
> I overlooked that point even though it was right in front of me.

No worry, John.

Anything else further can we get insight from the warning?

For example, pin_user_pages going on against a hugetlb page
which are concurrently running alloc_contig_range(it's
exported function so anyone can call randomly) so
alloc_contig_range changes pageblock type as MIGRATE_ISOLATE
under us so the hit at the warning?