linux-kernel - Re: mm: another VM_BUG_ON

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Date:	Mon, 1 Feb 2016 11:48:27 +0100
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	syzkaller <syzkaller@...glegroups.com>
Cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Vlastimil Babka <vbabka@...e.cz>,
	Doug Gilbert <dgilbert@...erlog.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.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>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: mm: another VM_BUG_ON_PAGE(PageTail(page))

On Fri, Jan 29, 2016 at 1:35 PM, Kirill A. Shutemov
<kirill.shutemov@...ux.intel.com> wrote:
> From 691a961bb401c5815ed741dac63591efbc6027e3 Mon Sep 17 00:00:00 2001
> From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> Date: Fri, 29 Jan 2016 15:06:17 +0300
> Subject: [PATCH 2/2] mempolicy: do not try to queue pages from
>  !vma_migratable()
>
> Maybe I miss some point, but I don't see a reason why we try to queue
> pages from non migratable VMAs.
>
> The only case when we can queue pages from such VMA is MPOL_MF_STRICT
> plus MPOL_MF_MOVE or MPOL_MF_MOVE_ALL for VMA which has pages on LRU,
> but gfp mask is not sutable for migaration (see mapping_gfp_mask() check
> in vma_migratable()). That's looks like a bug to me.
>
> Let's filter out non-migratable vma at start of queue_pages_test_walk()
> and go to queue_pages_pte_range() only if MPOL_MF_MOVE or
> MPOL_MF_MOVE_ALL flag is set.


I've run the fuzzer with these two patches for the weekend and seen no crashes.
I guess we can consider this as fixed.
Thanks!


> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>  mm/mempolicy.c | 14 +++++---------
>  1 file changed, 5 insertions(+), 9 deletions(-)
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 27d135408a22..4c4187c0e1de 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -548,8 +548,7 @@ retry:
>                         goto retry;
>                 }
>
> -               if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL))
> -                       migrate_page_add(page, qp->pagelist, flags);
> +               migrate_page_add(page, qp->pagelist, flags);
>         }
>         pte_unmap_unlock(pte - 1, ptl);
>         cond_resched();
> @@ -625,7 +624,7 @@ static int queue_pages_test_walk(unsigned long start, unsigned long end,
>         unsigned long endvma = vma->vm_end;
>         unsigned long flags = qp->flags;
>
> -       if (vma->vm_flags & VM_PFNMAP)
> +       if (!vma_migratable(vma))
>                 return 1;
>
>         if (endvma > end)
> @@ -644,16 +643,13 @@ static int queue_pages_test_walk(unsigned long start, unsigned long end,
>
>         if (flags & MPOL_MF_LAZY) {
>                 /* Similar to task_numa_work, skip inaccessible VMAs */
> -               if (vma_migratable(vma) &&
> -                       vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE))
> +               if (vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE))
>                         change_prot_numa(vma, start, endvma);
>                 return 1;
>         }
>
> -       if ((flags & MPOL_MF_STRICT) ||
> -           ((flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) &&
> -            vma_migratable(vma)))
> -               /* queue pages from current vma */
> +       /* queue pages from current vma */
> +       if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL))
>                 return 0;
>         return 1;
>  }
> --
> 2.7.0.rc3
>
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