linux-kernel - Re: [PATCH v2] mm: mincore: use pte_batch

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Message-ID: <0a22ab94-f63e-4959-a2cd-8b8a3ae210c3@linux.alibaba.com>
Date: Thu, 8 May 2025 15:57:49 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Barry Song <21cnbao@...il.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, ryan.roberts@....com,
 dev.jain@....com, ziy@...dia.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm: mincore: use pte_batch_bint() to batch process
 large folios



On 2025/5/8 15:42, Barry Song wrote:
> On Thu, May 8, 2025 at 4:09 PM Baolin Wang
> <baolin.wang@...ux.alibaba.com> 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@...ux.alibaba.com>
>> ---
>> 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 | 19 ++++++++++++++-----
>>   1 file changed, 14 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/mincore.c b/mm/mincore.c
>> index 832f29f46767..2e6a9123305e 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 +120,23 @@ 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)
>> +                               step = min_t(unsigned int, batch, nr);
> 
> Not quite sure if nr should be (end - addr) / PAGE_SIZE as nr
> is always the initial value. For example, nr = 50, and we have
> scanned 48 PTEs, then we have 2 ptes left. No?

Ah, you are right. I missed this part when I revised the original 
patch[1]. Thanks for pointing this out.

[1] 
https://lore.kernel.org/all/6a8418ba-dbd1-489f-929b-e31831bea0cf@linux.alibaba.com/