Re: [PATCH v10 8/8] mm: folio_zero_user: cache neighbouring pages

["David Hildenbrand (Red Hat)" <david@kernel.org>]

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@oracle.com>
> Reviewed-by: Raghavendra K T <raghavendra.kt@amd.com>
> Tested-by: Raghavendra K T <raghavendra.kt@amd.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