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

From: Daniel Borkmann <[email-masked]>
Date: Wed, 12 Feb 2014 13:02:54 +0100

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 <[email-masked]>
>> 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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