On Sat, Mar 07, 2015 at 12:31:03PM -0800, Linus Torvalds wrote: > On Sat, Mar 7, 2015 at 7:20 AM, Mel Gorman <mgorman@xxxxxxx> wrote: > > > > if (!prot_numa || !pmd_protnone(*pmd)) { > > - ret = 1; > > entry = pmdp_get_and_clear_notify(mm, addr, pmd); > > entry = pmd_modify(entry, newprot); > > ret = HPAGE_PMD_NR; > > Hmm. I know I acked this already, but the return value - which correct > - is still potentially something we could improve upon. > > In particular, we don't need to flush the TLB's if the old entry was > not present. Sadly, we don't have a helper function for that. > > But the code *could* do something like > > entry = pmdp_get_and_clear_notify(mm, addr, pmd); > ret = pmd_tlb_cacheable(entry) ? HPAGE_PMD_NR : 1; > entry = pmd_modify(entry, newprot); > > where pmd_tlb_cacheable() on x86 would test if _PAGE_PRESENT (bit #0) is set. > I agree with you in principle. pmd_tlb_cacheable looks and sounds very similar to pte_accessible(). > In particular, that would mean that as we change *from* a protnone > (whether NUMA or really protnone) we wouldn't need to flush the TLB. > > In fact, we could make it even more aggressive: it's not just an old > non-present TLB entry that doesn't need flushing - we can avoid the > flushing whenever we strictly increase the access rigths. So we could > have something that takes the old entry _and_ the new protections into > account, and avoids the TLB flush if the new entry is strictly more > permissive. > > This doesn't explain the extra TLB flushes Dave sees, though, because > the old code didn't make those kinds of optimizations either. But > maybe something like this is worth doing. > I think it is worth doing although it'll be after LSF/MM before I do it. I severely doubt this is what Dave is seeing because the vmstats indicated there was no THP activity but it's still a good idea. -- Mel Gorman SUSE Labs -- 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>