linux-kernel - Re: mm: VM_BUG_ON_PAGE(PageTail(page)) in mbind

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Date:	Tue, 26 Jan 2016 12:48:23 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Dmitry Vyukov <dvyukov@...gle.com>,
	Doug Gilbert <dgilbert@...erlog.com>,
	David Rientjes <rientjes@...gle.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Shiraz Hashim <shashim@...eaurora.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Hugh Dickins <hughd@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	linux-scsi@...r.kernel.org
Subject: Re: mm: VM_BUG_ON_PAGE(PageTail(page)) in mbind

On Tue, 26 Jan 2016 22:28:29 +0200 "Kirill A. Shutemov" <kirill@...temov.name> wrote:

> The patch below fixes the issue for me, but this bug makes me wounder how
> many bugs like this we have in kernel... :-/
> 
> Looks like we are too permissive about which VMA is migratable:
> vma_migratable() filters out VMA by VM_IO and VM_PFNMAP.
> I think VM_DONTEXPAND also correlate with VMA which cannot be migrated.
> 
> $ git grep VM_DONTEXPAND drivers | grep -v '\(VM_IO\|VM_PFNMAN\)' | wc -l 
> 33
> 
> Hm.. :-|
> 
> It worth looking on them closely... And I wouldn't be surprised if some
> VMAs without all of these flags are not migratable too.
> 
> Sigh.. Any thoughts?

Sigh indeed.  I think that both VM_DONTEXPAND and VM_DONTDUMP are
pretty good signs that mbind() should not be mucking with this vma.  If
such a policy sometimes results in mbind failing to set a policy then
that's not a huge loss - something runs a bit slower maybe.

I mean, we only really expect mbind() to operate against regular old
anon/pagecache memory, yes?