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

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Date:   Fri, 13 May 2022 17:18:11 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     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>,
        John Hubbard <jhubbard@...dia.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [syzbot] WARNING in follow_hugetlb_page

On Fri, 13 May 2022 16:54:06 -0700 Minchan Kim <minchan@...nel.org> wrote:

> > > >> has been there over a month so I guess it's something else.  Does
> > > >> someone have the time to bisect?
> > > > 
> > > > I can recreate in my 'easy to debug' environment, so I can bisect in
> > > > parallel with other things I need to do today.
> > > > 
> > > 
> > > 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)
> 

argh, I meant to check that change but the grey cell died.

I'll bring it back, with

--- a/include/linux/mm.h~mm-fix-is_pinnable_page-against-on-cma-page-fix
+++ a/include/linux/mm.h
@@ -1635,7 +1635,7 @@ static inline bool is_pinnable_page(stru
 	int __mt = get_pageblock_migratetype(page);
 	int mt = __READ_ONCE(__mt);
 
-	if (mt & (MIGRATE_CMA | MIGRATE_ISOLATE))
+	if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE)
 		return false;
 #endif
 
_