On Mon, Mar 7, 2011 at 3:50 PM, Michel Lespinasse <walken@xxxxxxxxxx> wrote: > Hi, > > I have been wondering about the following: > > Before the THP work, the if (vma->vm_flags & VM_LOCKED) test in > page_referenced_one() was placed after the page_check_address() call, > but now it is placed above it. Could this be a problem ? > > My understanding is that the page_check_address() check may return > false positives - for example, if an anon page was created before a > process forked, rmap will indicate that the page could be mapped in > both of the processes, even though one of them might have since broken > COW. What would happen if the child process mlocks the corresponding > VMA ? my understanding is that this would break COW, but not cause > rmap to be updated, so the parent's page would still be marked in rmap > as being possibly mapped in the children's VM_LOCKED vma. With the > VM_LOCKED check now placed above the page_check_address() call, this > would cause vmscan to see both the parent's and the child's pages as > being unevictable. I agree. There are two processes called P_A, P_B. P_B is child of P_A. A page "page A" is share between V_A(A's VMA)and V_B(B's VMA) since P_B is created by forking from P_A. When P_B calls mlock the V_B, P_B allocates new page B instead of reusing page A by COW and mapped P_B's page table but rmap of page A still indicates page A is mapped by V_A and V_B. The page_check_address can filter this situation that V_B doesn't include page A any more. So page_check_address should be placed before checking the VM_LOCKED. I think it's valuable to add the comment why we need page_check_address should be placed before the checking VM_LOCKED. -- Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>