Re: [PATCH] thp: close race between split and zap huge pages

[Sasha Levin <sasha.levin@oracle.com>]

On 04/15/2014 05:48 PM, Kirill A. Shutemov wrote:
> Sasha Levin has reported two THP BUGs[1][2]. I believe both of them have
> the same root cause. Let's look to them one by one.
> 
> The first bug[1] is "kernel BUG at mm/huge_memory.c:1829!".
> It's BUG_ON(mapcount != page_mapcount(page)) in __split_huge_page().
> From my testing I see that page_mapcount() is higher than mapcount here.
> 
> I think it happens due to race between zap_huge_pmd() and
> page_check_address_pmd(). page_check_address_pmd() misses PMD
> which is under zap:
> 
> 	CPU0						CPU1
> 						zap_huge_pmd()
> 						  pmdp_get_and_clear()
> __split_huge_page()
>   anon_vma_interval_tree_foreach()
>     __split_huge_page_splitting()
>       page_check_address_pmd()
>         mm_find_pmd()
> 	  /*
> 	   * We check if PMD present without taking ptl: no
> 	   * serialization against zap_huge_pmd(). We miss this PMD,
> 	   * it's not accounted to 'mapcount' in __split_huge_page().
> 	   */
> 	  pmd_present(pmd) == 0
> 
>   BUG_ON(mapcount != page_mapcount(page)) // CRASH!!!
> 
> 						  page_remove_rmap(page)
> 						    atomic_add_negative(-1, &page->_mapcount)
> 
> The second bug[2] is "kernel BUG at mm/huge_memory.c:1371!".
> It's VM_BUG_ON_PAGE(!PageHead(page), page) in zap_huge_pmd().
> 
> This happens in similar way:
> 
> 	CPU0						CPU1
> 						zap_huge_pmd()
> 						  pmdp_get_and_clear()
> 						  page_remove_rmap(page)
> 						    atomic_add_negative(-1, &page->_mapcount)
> __split_huge_page()
>   anon_vma_interval_tree_foreach()
>     __split_huge_page_splitting()
>       page_check_address_pmd()
>         mm_find_pmd()
> 	  pmd_present(pmd) == 0	/* The same comment as above */
>   /*
>    * No crash this time since we already decremented page->_mapcount in
>    * zap_huge_pmd().
>    */
>   BUG_ON(mapcount != page_mapcount(page))
> 
>   /*
>    * We split the compound page here into small pages without
>    * serialization against zap_huge_pmd()
>    */
>   __split_huge_page_refcount()
> 						VM_BUG_ON_PAGE(!PageHead(page), page); // CRASH!!!
> 
> So my understanding the problem is pmd_present() check in mm_find_pmd()
> without taking page table lock.
> 
> The bug was introduced by me commit with commit 117b0791ac42. Sorry for
> that. :(
> 
> Let's open code mm_find_pmd() in page_check_address_pmd() and do the
> check under page table lock.
> 
> Note that __page_check_address() does the same for PTE entires
> if sync != 0.
> 
> I've stress tested split and zap code paths for 36+ hours by now and
> don't see crashes with the patch applied. Before it took <20 min to
> trigger the first bug and few hours for second one (if we ignore
> first).
> 
> [1] https://lkml.kernel.org/g/<53440991.9090001@oracle.com>
> [2] https://lkml.kernel.org/g/<5310C56C.60709@oracle.com>
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Reported-by: Sasha Levin <sasha.levin@oracle.com>
> Cc: <stable@vger.kernel.org> #3.13+

Seems to work for me, thanks!


Thanks,
Sasha

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