Messages in this thread | | | Date | Mon, 27 Mar 2023 09:23:04 +0800 | Subject | Re: [PATCH v4 35/36] mm: Convert do_set_pte() to set_pte_range() | From | Yin Fengwei <> |
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On 3/25/23 01:23, Will Deacon wrote: > On Fri, Mar 24, 2023 at 03:11:00PM +0000, Matthew Wilcox wrote: >> On Fri, Mar 24, 2023 at 02:58:29PM +0000, Will Deacon wrote: >>> Yes, please don't fault everything in as young as it has caused horrible >>> vmscan behaviour leading to app-startup slowdown in the past: >>> >>> https://lore.kernel.org/all/20210111140149.GB7642@willie-the-truck/ >>> >>> If we have to use the same value for all the ptes, then just base them >>> all on arch_wants_old_prefaulted_pte() as iirc hardware AF was pretty >>> cheap in practice for us. >> >> I think that's wrong, because this is a different scenario. >> >> Before: >> >> We faulted in N single-page folios. Each page/folio is tracked >> independently. That's N entries on whatever LRU list it ends up on. >> The prefaulted ones _should_ be marked old -- they haven't been >> accessed; we've just decided to put them in the page tables to >> speed up faultaround. The unaccessed pages need to fall off the LRU >> list as quickly as possible; keeping them around only hurts if the >> workload has no locality of reference. >> >> After: >> >> We fault in N folios, some possibly consisting of multiple pages. >> Each folio is tracked separately, but individual pages in the folio >> are not tracked; they belong to their folio. In this scenario, if >> the other PTEs for pages in the same folio are marked as young or old >> doesn't matter; the entire folio will be tracked as young, because we >> referenced one of the pages in this folio. Marking the other PTEs as >> young actually helps because we don't take pagefaults on them (whether >> we have a HW or SW accessed bit). >> >> (can i just say that i dislike how we mix up our old/young accessed/not >> terminology here?) >> >> We should still mark the PTEs referencing unaccessed folios as old. >> No argument there, and this patch does that. But it's fine for all the >> PTEs referencing the accessed folio to have the young bit, at least as >> far as I can tell. > > Ok, thanks for the explanation. So as long as > arch_wants_old_prefaulted_pte() is taken into account for the unaccessed > folios, then I think we should be good? Unconditionally marking those > PTEs as old probably hurts x86. Yes. We do only mark PTEs old for arch_wants_old_prefaulted_pte() system. Thanks.
Regards Yin, Fengwei
> > Will
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