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

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Message-ID: <d17b69a1-2f22-4a8d-8260-ddea38ebc7b0@redhat.com>
Date: Tue, 1 Apr 2025 15:04:40 +0200
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
 hughd@...gle.com
Cc: willy@...radead.org, 21cnbao@...il.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 01.04.25 12:45, Ryan Roberts wrote:
> 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@...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);
>>>> +                }
>>>> +            }
>>>
>>> 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.

I don't enjoy open-coding the batching, especially if only cont-pte 
users will benefit from it. But I also don't enjoy the open-coded 
pte_batch_hint() :)

But we really don't need the folio here, so I assume the short variant 
you (Ryan) suggest is alright to just avoid the ptep_get().

As Oscar says, these details might soon be hidden inside a new page 
table walker API (even though it will likely end up using 
folio_pte_batch() internally, TBD).

-- 
Cheers,

David / dhildenb