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

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Date:   Fri, 13 May 2022 18:16:46 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Minchan Kim <minchan@...nel.org>
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 5/13/22 17:56, John Hubbard wrote:
> On 5/13/22 17:26, Minchan Kim wrote:
>> 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?
> 
> Well, yes. First of all, the comments above the warning that fired have
> gone a little bit stale: they claim that we can only hit the warning if
> the page refcount overflows. However, we almost certainly got here via:
> 
> try_grab_folio()
>      /*
>       * Can't do FOLL_LONGTERM + FOLL_PIN gup fast path if not in a
>       * right zone, so fail and let the caller fall back to the slow
>       * path.
>       */
>      if (unlikely((flags & FOLL_LONGTERM) &&
>               !is_pinnable_page(page))) /* which we just changed */

Specifically, the recent patch effectively acted as an error injection
test, by forcing is_pinnable_page() to always return true (if CONFIG_CMA
is defined). Because: MIGRATE_CMA|MIGRATE_ISOLATE == 7, which will match
any of the MIGRATE_* enums when checked with bitwise AND.

I suspect this particular error path has not been exercised much, or if
it has, not reported here anyway. Until now.


thanks,
-- 
John Hubbard
NVIDIA