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Message-ID: <b34ce151-45d5-4074-a869-de93b2bcdef0@kernel.org>
Date: Thu, 18 Dec 2025 08:49:41 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, x86@...nel.org
Cc: akpm@...ux-foundation.org, bp@...en8.de, dave.hansen@...ux.intel.com,
hpa@...or.com, mingo@...hat.com, mjguzik@...il.com, luto@...nel.org,
peterz@...radead.org, tglx@...utronix.de, willy@...radead.org,
raghavendra.kt@....com, chleroy@...nel.org, ioworker0@...il.com,
boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH v10 8/8] mm: folio_zero_user: cache neighbouring pages
On 12/15/25 21:49, Ankur Arora wrote:
> folio_zero_user() does straight zeroing without caring about
> temporal locality for caches.
>
> This replaced commit c6ddfb6c5890 ("mm, clear_huge_page: move order
> algorithm into a separate function") where we cleared a page at a
> time converging to the faulting page from the left and the right.
>
> To retain limited temporal locality, split the clearing in three
> parts: the faulting page and its immediate neighbourhood, and, the
> remaining regions on the left and the right. The local neighbourhood
> will be cleared last.
> Do this only when zeroing small folios (< MAX_ORDER_NR_PAGES) since
> there isn't much expectation of cache locality for large folios.
>
> Performance
> ===
>
> AMD Genoa (EPYC 9J14, cpus=2 sockets * 96 cores * 2 threads,
> memory=2.2 TB, L1d= 16K/thread, L2=512K/thread, L3=2MB/thread)
>
> anon-w-seq (vm-scalability):
> stime utime
>
> page-at-a-time 1654.63 ( +- 3.84% ) 811.00 ( +- 3.84% )
> contiguous clearing 1602.86 ( +- 3.00% ) 970.75 ( +- 4.68% )
> neighbourhood-last 1630.32 ( +- 2.73% ) 886.37 ( +- 5.19% )
>
> Both stime and utime respond in expected ways. stime drops for both
> contiguous clearing (-3.14%) and neighbourhood-last (-1.46%)
> approaches. However, utime increases for both contiguous clearing
> (+19.7%) and neighbourhood-last (+9.28%).
>
> In part this is because anon-w-seq runs with 384 processes zeroing
> anonymously mapped memory which they then access sequentially. As
> such this is likely an uncommon pattern where the memory bandwidth
> is saturated while also being cache limited because we access the
> entire region.
>
> Kernel make workload (make -j 12 bzImage):
>
> stime utime
>
> page-at-a-time 138.16 ( +- 0.31% ) 1015.11 ( +- 0.05% )
> contiguous clearing 133.42 ( +- 0.90% ) 1013.49 ( +- 0.05% )
> neighbourhood-last 131.20 ( +- 0.76% ) 1011.36 ( +- 0.07% )
>
> For make the utime stays relatively flat with an up to 4.9% improvement
> in the stime.
Nice evaluation!
>
> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
> Reviewed-by: Raghavendra K T <raghavendra.kt@....com>
> Tested-by: Raghavendra K T <raghavendra.kt@....com>
> ---
> mm/memory.c | 44 ++++++++++++++++++++++++++++++++++++++++++--
> 1 file changed, 42 insertions(+), 2 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 974c48db6089..d22348b95227 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -7268,13 +7268,53 @@ static void clear_contig_highpages(struct page *page, unsigned long addr,
> * @addr_hint: The address accessed by the user or the base address.
> *
> * Uses architectural support to clear page ranges.
> + *
> + * Clearing of small folios (< MAX_ORDER_NR_PAGES) is split in three parts:
> + * pages in the immediate locality of the faulting page, and its left, right
> + * regions; the local neighbourhood is cleared last in order to keep cache
> + * lines of the faulting region hot.
> + *
> + * For larger folios we assume that there is no expectation of cache locality
> + * and just do a straight zero.
Just wondering: why not do the same thing here as well? Probably
shouldn't hurt and would get rid of some code?
> */
> void folio_zero_user(struct folio *folio, unsigned long addr_hint)
> {
> unsigned long base_addr = ALIGN_DOWN(addr_hint, folio_size(folio));
While at it you could turn that const as well.
> + const long fault_idx = (addr_hint - base_addr) / PAGE_SIZE;
> + const struct range pg = DEFINE_RANGE(0, folio_nr_pages(folio) - 1);
> + const int width = 2; /* number of pages cleared last on either side */
Is "width" really the right terminology? (the way you describe it, it's
more like diameter?)
Wondering whether we should turn that into a define to make it clearer
that we are dealing with a magic value.
Speaking of magic values, why 2 and not 3? :)
> + struct range r[3];
> + int i;
>
> - clear_contig_highpages(folio_page(folio, 0),
> - base_addr, folio_nr_pages(folio));
> + if (folio_nr_pages(folio) > MAX_ORDER_NR_PAGES) {
> + clear_contig_highpages(folio_page(folio, 0),
> + base_addr, folio_nr_pages(folio));
> + return;
> + }
> +
> + /*
> + * Faulting page and its immediate neighbourhood. Cleared at the end to
> + * ensure it sticks around in the cache.
> + */
> + r[2] = DEFINE_RANGE(clamp_t(s64, fault_idx - width, pg.start, pg.end),
> + clamp_t(s64, fault_idx + width, pg.start, pg.end));
> +
> + /* Region to the left of the fault */
> + r[1] = DEFINE_RANGE(pg.start,
> + clamp_t(s64, r[2].start-1, pg.start-1, r[2].start));
"start-1" -> "start - 1" etc.
> +
> + /* Region to the right of the fault: always valid for the common fault_idx=0 case. */
> + r[0] = DEFINE_RANGE(clamp_t(s64, r[2].end+1, r[2].end, pg.end+1),
> + pg.end);
Same here.
> +
> + for (i = 0; i <= 2; i++) {
Can we use ARRAY_SIZE instead of "2" ?
> + unsigned int npages = range_len(&r[i]);
> + struct page *page = folio_page(folio, r[i].start);
> + unsigned long addr = base_addr + folio_page_idx(folio, page) * PAGE_SIZE;
Can't you compute that from r[i].start) instead? The folio_page_idx()
seems avoidable unless I am missing something.
Could make npages and addr const.
const unsigned long addr = base_addr + r[i].start * PAGE_SIZE;
const unsigned int npages = range_len(&r[i]);
struct page *page = folio_page(folio, r[i].start);
> +
> + if (npages > 0)
> + clear_contig_highpages(page, addr, npages);
> + }
> }
>
> static int copy_user_gigantic_page(struct folio *dst, struct folio *src,
--
Cheers
David
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