linux-kernel - Re: [PATCH 2/2] mm: mincore: use folio_pte

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Message-ID: <17289428-894a-4397-9d61-c8500d032b28@arm.com>
Date: Wed, 7 May 2025 10:42:14 +0530
From: Dev Jain <dev.jain@....com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
 hughd@...gle.com
Cc: willy@...radead.org, david@...hat.com, 21cnbao@...il.com,
 ryan.roberts@....com, ziy@...dia.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: mincore: use folio_pte_batch() to batch process
 large folios



On 26/03/25 9:08 am, 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 folio_pte_batch() 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			1115us			+81%
> 
> Moreover, I also tested mincore() with disabling mTHP/THP, and did not
> see any obvious regression.
> 
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> ---
>   mm/mincore.c | 27 ++++++++++++++++++++++-----
>   1 file changed, 22 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/mincore.c b/mm/mincore.c
> index 832f29f46767..88be180b5550 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 +120,31 @@ 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)) {
> +			if (pte_batch_hint(ptep, pte) > 1) {
> +				struct folio *folio = vm_normal_folio(vma, addr, pte);
> +
> +				if (folio && folio_test_large(folio)) {
> +					const fpb_t fpb_flags = FPB_IGNORE_DIRTY |
> +								FPB_IGNORE_SOFT_DIRTY;
> +					int max_nr = (end - addr) / PAGE_SIZE;
> +
> +					step = folio_pte_batch(folio, addr, ptep, pte,
> +							max_nr, fpb_flags, NULL, NULL, NULL);
> +				}
> +			}

Can we go ahead with this along with [1], that will help us generalize 
for all arches.

[1] https://lore.kernel.org/all/20250506050056.59250-3-dev.jain@arm.com/
(Please replace PAGE_SIZE with 1)

> +
> +			for (i = 0; i < step; i++)
> +				vec[i] = 1;
> +		} else { /* pte is a swap entry */
>   			swp_entry_t entry = pte_to_swp_entry(pte);
>   
>   			if (non_swap_entry(entry)) {
> @@ -146,7 +163,7 @@ static int mincore_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>   #endif
>   			}
>   		}
> -		vec++;
> +		vec += step;
>   	}
>   	pte_unmap_unlock(ptep - 1, ptl);
>   out: