On Tue, 18 Oct 2016 21:12:45 +0530 "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx> wrote: > We cannot use the pte value used in set_pte_at for pte_same comparison, > because archs like ppc64, filter/add new pte flag in set_pte_at. Instead > fetch the pte value inside hugetlb_cow. We are comparing pte value to > make sure the pte didn't change since we dropped the page table lock. > hugetlb_cow get called with page table lock held, and we can take a copy > of the pte value before we drop the page table lock. > > With hugetlbfs, we optimize the MAP_PRIVATE write fault path with no > previous mapping (huge_pte_none entries), by forcing a cow in the fault > path. This avoid take an addition fault to covert a read-only mapping > to read/write. Here we were comparing a recently instantiated pte (via > set_pte_at) to the pte values from linux page table. As explained above > on ppc64 such pte_same check returned wrong result, resulting in us > taking an additional fault on ppc64. >From my reading this is a minor performance improvement and a -stable backport isn't needed. But it is unclear whether the impact warrants a 4.9 merge. Please be careful about describing end-user visible impacts when fixing bugs, thanks. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>