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Message-ID: <6a8418ba-dbd1-489f-929b-e31831bea0cf@linux.alibaba.com>
Date: Wed, 7 May 2025 17:48:40 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Dev Jain <dev.jain@....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 2025/5/7 13:12, Dev Jain wrote:
>
>
> 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)
As discussed with Ryan, we don’t need to call folio_pte_batch()
(something like the code below), so your patch seems unnecessarily
complicated. However, David is unhappy about the open-coded
pte_batch_hint().
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 +120,21 @@ 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 max_nr = (end - addr) / PAGE_SIZE;
+
+ step = min(pte_batch_hint(ptep, pte), max_nr);
+ 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 +153,7 @@ 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:
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