on only triggers in the places that must already be covered by pmd_none_or_trans_huge_or_clear_bad() so the fix is localized there but this bug is not related to THP. NOTE: this can trigger on x86 32bit systems with PAE enabled with more than 4G of ram, otherwise the high part of the pmd will never risk to be truncated because it would be zero at all times, in turn so hiding the SMP race. This bug was discovered and fully debugged by Ulrich, quote: ---- [..] pmd_none_or_trans_huge_or_clear_bad() loads the content of edx and eax. 496 static inline int pmd_none_or_trans_huge_or_clear_bad(pmd_t *pmd) 497 { 498 /* depend on compiler for an atomic pmd read */ 499 pmd_t pmdval = *pmd; // edi = pmd pointer 0xc0507a74 : mov 0x8(%esp),%edi ... // edx = PTE page table high address 0xc0507a84 : mov 0x4(%edi),%edx ... // eax = PTE page table low address 0xc0507a8e : mov (%edi),%eax [..] Please note that the PMD is not read atomically. These are two "mov" instructions where the high order bits of the PMD entry are fetched first. Hence, the above machine code is prone to the following race. - The PMD entry {high|low} is 0x0000000000000000. The "mov" at 0xc0507a84 loads 0x00000000 into edx. - A page fault (on another CPU) sneaks in between the two "mov" instructions and instantiates the PMD. - The PMD entry {high|low} is now 0x00000003fda38067. The "mov" at 0xc0507a8e loads 0xfda38067 into eax. ---- Reported-by: Ulrich Obergfell Signed-off-by: Andrea Arcangeli Cc: Mel Gorman Cc: Hugh Dickins Cc: Larry Woodman Cc: Petr Matousek Cc: Rik van Riel Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds PÈbo–`x