linux-kernel - Re: [PATCH v4 35/36] mm: Convert do_set_pte() to set_pte

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Date:   Fri, 24 Mar 2023 17:23:20 +0000
From:   Will Deacon <will@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     "Yin, Fengwei" <fengwei.yin@...el.com>,
        Ryan Roberts <ryan.roberts@....com>,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 35/36] mm: Convert do_set_pte() to set_pte_range()

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.

Will