linux-kernel - Re: [RFC] mm: page-flags.h: remove the bias against tail pages

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Message-ID: <0cba949e-6c77-491a-bc4e-7f52738e0f36@nvidia.com>
Date: Mon, 25 Mar 2024 20:21:28 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Matthew Wilcox <willy@...radead.org>
CC: Andrew Morton <akpm@...ux-foundation.org>, LKML
	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>, David Hildenbrand
	<david@...hat.com>, Mike Rapoport <rppt@...nel.org>, Theodore Ts'o
	<tytso@....edu>, Vishal Moola <vishal.moola@...il.com>, Peter Collingbourne
	<pcc@...gle.com>
Subject: Re: [RFC] mm: page-flags.h: remove the bias against tail pages

On 3/24/24 10:24 PM, Matthew Wilcox wrote:
..
> It's complicated.  On the one hand, it's "more likely" because there are
> more tail pages than there are head pages or order-0 pages.  On the
> other hand, a _lot_ of the time we call compound_head(), it's done with
> a non-tail page because we tend to pass around head pages (eg,

ah yes, that's true.

> pmd_page() on hugetlbfs, or looking up a folio in the page cache and
> passing &folio->page to some function that's not yet converted.
> 
> On the third hand, does the compiler really do much with the annotation?
> 
> Before your patch:
> 
>      27d6:       a8 01                   test   $0x1,%al
>      27d8:       75 02                   jne    27dc <clear_refs_pte_range+0x9c>

I should have thought to check this. Usually I'll see a change between je/jne
if __builtin_expect is doing its job. Here it is, oddly, missing in action.

Maybe I'll look a little closer into why that is...

>      27da:       eb 59                   jmp    2835 <clear_refs_pte_range+0xf5>
>      27dc:       49 8b 44 24 08          mov    0x8(%r12),%rax
>      27e1:       a8 01                   test   $0x1,%al
>      27e3:       75 6f                   jne    2854 <clear_refs_pte_range+0x114>
>      27e5:       eb 73                   jmp    285a <clear_refs_pte_range+0x11a>
> 
> With your patch:
> 
>      1ee6:       a8 01                   test   $0x1,%al
>      1ee8:       75 02                   jne    1eec <clear_refs_pte_range+0x9c>
>      1eea:       eb 5f                   jmp    1f4b <clear_refs_pte_range+0xfb>
>      1eec:       49 8b 44 24 08          mov    0x8(%r12),%rax
>      1ef1:       a8 01                   test   $0x1,%al
>      1ef3:       75 50                   jne    1f45 <clear_refs_pte_range+0xf5>
>      1ef5:       eb 6c                   jmp    1f63 <clear_refs_pte_range+0x113>
> 
> Looks pretty much the same.  bloat-o-meter says:
> 
> $ ./scripts/bloat-o-meter before.o after.o
> add/remove: 0/0 grow/shrink: 2/4 up/down: 32/-48 (-16)
> Function                                     old     new   delta
> gather_stats.constprop                       730     753     +23
> smaps_hugetlb_range                          635     644      +9
> smaps_page_accumulate                        342     338      -4
> clear_refs_pte_range                         339     328     -11
> pagemap_hugetlb_range                        422     407     -15
> smaps_pte_range                             1406    1388     -18
> Total: Before=20066, After=20050, chg -0.08%
> 
> (I was looking at clear_refs_pte_range above).  This seems marginal.
> The benefits of removing a call to compound_head are much less
> ambiguous:
> 
> $ ./scripts/bloat-o-meter before.o .build/fs/proc/task_mmu.o
> add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-101 (-101)
> Function                                     old     new   delta
> clear_refs_pte_range                         339     238    -101
> Total: Before=20066, After=19965, chg -0.50%
> 
> I'd describe that as replacing four calls to compound_head() with two:
> 
> -               page = pmd_page(*pmd);
> +               folio = page_folio(pmd_page(*pmd));
> 
>                  /* Clear accessed and referenced bits. */
>                  pmdp_test_and_clear_young(vma, addr, pmd);
> -               test_and_clear_page_young(page);
> -               ClearPageReferenced(page);
> +               folio_test_clear_young(folio);
> +               folio_clear_referenced(folio);
> ...
> -               page = vm_normal_page(vma, addr, ptent);
> -               if (!page)
> +               folio = vm_normal_folio(vma, addr, ptent);
> +               if (!folio)
>                          continue;
> 
>                  /* Clear accessed and referenced bits. */
>                  ptep_test_and_clear_young(vma, addr, pte);
> -               test_and_clear_page_young(page);
> -               ClearPageReferenced(page);
> +               folio_test_clear_young(folio);
> +               folio_clear_referenced(folio);
> 
> I'm not saying this patch is necessarily wrong, I just think it's
> "not proven".

I appreciate your looking at this and explaining the analysis steps
you used!


thanks,
-- 
John Hubbard
NVIDIA