Re: [BUG] at include/linux/page-flags.h:415 (PageTransHuge)

[Daniel Borkmann]

Hi Vlastimil,

On 02/07/2014 07:58 PM, Hannes Frederic Sowa wrote:
> On Fri, Jan 31, 2014 at 03:40:38PM +0100, Vlastimil Babka wrote:
> > From: Vlastimil Babka <vbabka@xxxxxxx>
> > Date: Fri, 31 Jan 2014 11:50:21 +0100
> > Subject: [PATCH] mm: include VM_MIXEDMAP flag in the VM_SPECIAL list to avoid
> >   m(un)locking
> >
> > Daniel Borkmann reported a bug with VM_BUG_ON assertions failing where
> > munlock_vma_pages_range() thinks it's unexpectedly in the middle of a THP page.
> > This can be reproduced in tools/testing/selftests/net/ by running make and
> > then ./psock_tpacket.
> >
> > The problem is that an order=2 compound page (allocated by
> > alloc_one_pg_vec_page() is part of the munlocked VM_MIXEDMAP vma (mapped by
> > packet_mmap()) and mistaken for a THP page and assumed to be order=9.
> >
> > The checks for THP in munlock came with commit ff6a6da60b89 ("mm: accelerate
> > munlock() treatment of THP pages"), i.e. since 3.9, but did not trigger a bug.
> > It just makes munlock_vma_pages_range() skip such compound pages until the next
> > 512-pages-aligned page, when it encounters a head page. This is however not a
> > problem for vma's where mlocking has no effect anyway, but it can distort the
> > accounting.
> > Since commit 7225522bb ("mm: munlock: batch non-THP page isolation and
> > munlock+putback using pagevec") this can trigger a VM_BUG_ON in PageTransHuge()
> > check.
> >
> > This patch fixes the issue by adding VM_MIXEDMAP flag to VM_SPECIAL - a list of
> > flags that make vma's non-mlockable and non-mergeable. The reasoning is that
> > VM_MIXEDMAP vma's are similar to VM_PFNMAP, which is already on the VM_SPECIAL
> > list, and both are intended for non-LRU pages where mlocking makes no sense
> > anyway.

Thanks a lot for your efforts.

Is your patch queued up somewhere for mainline and stable?

> I also ran into this problem and wanted to ask what the status of this
> patch is? Does it need further testing? I can surely help with that. ;)
>
> Thanks,
>
>    Hannes

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