On Tue, 23 Feb 2016 19:49:10 +0100 Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote: > pmd_trans_unstable()/pmd_none_or_trans_huge_or_clear_bad() were > introduced to locklessy (but atomically) detect when a pmd is a > regular (stable) pmd or when the pmd is unstable and can infinitely > transition from pmd_none() and pmd_trans_huge() from under us, while > only holding the mmap_sem for reading (for writing not). > > While holding the mmap_sem only for reading, MADV_DONTNEED can run > from under us and so before we can assume the pmd to be a regular > stable pmd we need to compare it against pmd_none() and > pmd_trans_huge() in an atomic way, with pmd_trans_unstable(). The old > pmd_trans_huge() left a tiny window for a race. > > Useful applications are unlikely to notice the difference as doing > MADV_DONTNEED concurrently with a page fault would lead to undefined > behavior. Thanks. I put a cc:stable on this as it appears to be applicable to 4.4 and perhaps earlier. It generates a reject against 4.4 because of the recently-added pmd_devmap() test. It's easily fixed but I don't have a process to handle -stable rejects. This means that when Greg hits the reject he'll ask us for a fixed up version. -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html