Andrew Morton <a...@linux-foundation.org> writes: > On Tue, 18 Oct 2016 21:12:45 +0530 "Aneesh Kumar K.V" > <aneesh.ku...@linux.vnet.ibm.com> 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.
This patch workaround the issue reported at https://lkml.kernel.org/r/57ff7bb4.1070...@redhat.com The reason for that OOM was a reserve count accounting issue which happens in the error path of hugetlb_cow. Not this patch avoid us taking the error path and hence we don't have the reported OOM. An actual fix for that issue is being worked on by Mike Kravetz. > > Please be careful about describing end-user visible impacts when fixing > bugs, thanks. -aneesh
