Re: [PATCH v3] mm: mincore: use pte_batch_bint() to batch process large folios

[David Hildenbrand <david@redhat.com>]

On 09.05.25 02:45, Baolin Wang wrote:
> When I tested the mincore() syscall, I observed that it takes longer with
> 64K mTHP enabled on my Arm64 server. The reason is the mincore_pte_range()
> still checks each PTE individually, even when the PTEs are contiguous,
> which is not efficient.
> 
> Thus we can use pte_batch_hint() to get the batch number of the present
> contiguous PTEs, which can improve the performance. I tested the mincore()
> syscall with 1G anonymous memory populated with 64K mTHP, and observed an
> obvious performance improvement:
> 
> w/o patch		w/ patch		changes
> 6022us			549us			+91%
> 
> Moreover, I also tested mincore() with disabling mTHP/THP, and did not
> see any obvious regression for base pages.
> 
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
> Changes from v2:
> - Re-calculate the max_nr, per Barry.
> Changes from v1:
> - Change to use pte_batch_hint() to get the batch number, per Ryan.
> 
> Note: I observed the min_t() can introduce a slight performance regression
> for base pages, so I change to add a batch size check for base pages,
> which can resolve the performance regression issue.
> ---
>   mm/mincore.c | 22 +++++++++++++++++-----
>   1 file changed, 17 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/mincore.c b/mm/mincore.c
> index 832f29f46767..42d6c9c8da86 100644
> --- a/mm/mincore.c
> +++ b/mm/mincore.c
> @@ -21,6 +21,7 @@
>   
>   #include <linux/uaccess.h>
>   #include "swap.h"
> +#include "internal.h"
>   
>   static int mincore_hugetlb(pte_t *pte, unsigned long hmask, unsigned long addr,
>   			unsigned long end, struct mm_walk *walk)
> @@ -105,6 +106,7 @@ static int mincore_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>   	pte_t *ptep;
>   	unsigned char *vec = walk->private;
>   	int nr = (end - addr) >> PAGE_SHIFT;
> +	int step, i;
>   
>   	ptl = pmd_trans_huge_lock(pmd, vma);
>   	if (ptl) {
> @@ -118,16 +120,26 @@ static int mincore_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>   		walk->action = ACTION_AGAIN;
>   		return 0;
>   	}
> -	for (; addr != end; ptep++, addr += PAGE_SIZE) {
> +	for (; addr != end; ptep += step, addr += step * PAGE_SIZE) {
>   		pte_t pte = ptep_get(ptep);
>   
> +		step = 1;
>   		/* We need to do cache lookup too for pte markers */
>   		if (pte_none_mostly(pte))
>   			__mincore_unmapped_range(addr, addr + PAGE_SIZE,
>   						 vma, vec);
> -		else if (pte_present(pte))
> -			*vec = 1;
> -		else { /* pte is a swap entry */
> +		else if (pte_present(pte)) {
> +			unsigned int batch = pte_batch_hint(ptep, pte);
> +
> +			if (batch > 1) {
> +				unsigned int max_nr = (end - addr) >> PAGE_SHIFT;

Nit: probably would have called this max_step to match step.

> +
> +				step = min_t(unsigned int, batch, max_nr);
> +			}
> +
> +			for (i = 0; i < step; i++)
> +				vec[i] = 1;

I'm surprised this micro-optimization matters that much. Probably the compiler
defers the calculation of max_nr. I am not convinced we need that level of
micro-optimization in this code ...


But if we're already micro-optimizing, you could have optimized out the loop as
well for order-0:

	unsigned int batch = pte_batch_hint(ptep, pte);
	
	if (batch > 1) {
		unsigned int max_nr = (end - addr) >> PAGE_SHIFT;

		step = min_t(unsigned int, batch, max_nr);
		for (i = 0; i < step; i++)
			vec[i] = 1;
	} else {
		*vec = 1;
	}


In any case

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb