• Aneesh Kumar K.V's avatar
    powerpc/mm: Fix Multi hit ERAT cause by recent THP update · c777e2a8
    Aneesh Kumar K.V authored
    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.
    
    Fixes: eef1b3ba ("thp: implement split_huge_pmd()")
    Acked-by: default avatarKirill A. Shutemov <kirill.shutemov@linux.intel.com>
    Signed-off-by: default avatarAneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
    Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    c777e2a8
pgtable.h 9.13 KB