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

[Hannes Frederic Sowa]

Hi!

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.

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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