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

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Date:   Fri, 13 May 2022 17:56:59 -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 17:26, 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 */
		return NULL;

...and now I'm starting to think that this warning might fire even with
the corrected check for MIGRATE_CMA || MIGRATE_ISOLATE. Because
try_grab_folio() didn't always have this early exit and it is starting
to look wrong.

Simply attempting to pin a non-pinnable huge page would hit this
warning. Adding additional reasons that a page is not pinnable (which
the patch does) could make this more likely to fire.

I need to look at this a little more closely, it is making me wonder
whether the is_pinnable_page() check is a problem in this path. The
comment in try_grab_folio() indicates that the early return is a hack
(it assumes that the caller is in the gup fast path), and maybe the hack
is just wrong here--I think we're actually on the slow gup path. Not
good.

Mike, any thoughts here?

thanks,
-- 
John Hubbard
NVIDIA