Re: [PATCH 2/2] mm: mincore: use folio_pte_batch() to batch process large folios

[Ryan Roberts <ryan.roberts@arm.com>]

On 30/03/2025 15:57, Baolin Wang wrote:
> 
> 
> On 2025/3/27 22:08, Ryan Roberts wrote:
>> On 25/03/2025 23:38, 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@linux.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);
>>> +                }
>>> +            }
>>
>> You could simplify to the following, I think, to avoid needing to grab the folio
>> and call folio_pte_batch():
>>
>>             else if (pte_present(pte)) {
>>                 int max_nr = (end - addr) / PAGE_SIZE;
>>                 step = min(pte_batch_hint(ptep, pte), max_nr);
>>             } ...
>>
>> I expect the regression you are seeing here is all due to calling ptep_get() for
>> every pte in the contpte batch, which will cause 16 memory reads per pte (to
>> gather the access/dirty bits). For small folios its just 1 read per pte.
> 
> Right.
> 
>> pte_batch_hint() will skip forward in blocks of 16 so you now end up with the
>> same number as for the small folio case. You don't need all the fancy extras
>> that folio_pte_batch() gives you here.
> 
> Sounds reasonable. Your suggestion looks simple, but my method can batch the
> whole large folio (such as large folios containing more than 16 contiguous PTEs)
> at once. 

Sure but folio_pte_batch() just implements that with another loop that calls
pte_batch_hint(), so it all amounts to the same thing. In fact there are some
extra checks in folio_pte_batch() that you don't need so it might be a bit slower.

Thanks,
Ryan

> Anyway, let me do some performance measurements for your suggestion.
> Thanks.