Re: [V3] powerpc/mm: Fix Multi hit ERAT cause by recent THP update

[Michael Ellerman <mpe@ellerman.id.au>]

On Tue, 2016-09-02 at 01:20:31 UTC, "Aneesh Kumar K.V" wrote:
> With ppc64 we use the deposited pgtable_t to store the hash pte slot
> information. We should not withdraw the deposited pgtable_t without
> marking the pmd none. This ensure that low level hash fault handling
> will skip this huge pte and we will handle them at upper levels.
> 
> Recent change to pmd splitting changed the above in order to handle the
> race between pmd split and exit_mmap. The race is explained below.
> 
> Consider following race:
> 
> 		CPU0				CPU1
> shrink_page_list()
>   add_to_swap()
>     split_huge_page_to_list()
>       __split_huge_pmd_locked()
>         pmdp_huge_clear_flush_notify()
> 	// pmd_none() == true
> 					exit_mmap()
> 					  unmap_vmas()
> 					    zap_pmd_range()
> 					      // no action on pmd since pmd_none() == true
> 	pmd_populate()
> 
> As result the THP will not be freed. The leak is detected by check_mm():
> 
> 	BUG: Bad rss-counter state mm:ffff880058d2e580 idx:1 val:512
> 
> The above required us to not mark pmd none during a pmd split.
> 
> The fix for ppc is to clear the huge pte of _PAGE_USER, so that low
> level fault handling code skip this pte. At higher level we do take ptl
> lock. That should serialze us against the pmd split. Once the lock is
> acquired we do check the pmd again using pmd_same. That should always
> return false for us and hence we should retry the access. We do the
> pmd_same check in all case after taking plt with
> THP (do_huge_pmd_wp_page, do_huge_pmd_numa_page and
> huge_pmd_set_accessed)
> 
> Also make sure we wait for irq disable section in other cpus to finish
> before flipping a huge pte entry with a regular pmd entry. Code paths
> like find_linux_pte_or_hugepte depend on irq disable to get
> a stable pte_t pointer. A parallel thp split need to make sure we
> don't convert a pmd pte to a regular pmd entry without waiting for the
> irq disable section to finish.
> 
> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>

Applied to powerpc fixes, thanks.

https://git.kernel.org/powerpc/c/9db4cd6c21535a4846b38808f3

cheers

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