linux-kernel - Re: [PATCH v7 1/3] mm/madvise: introduce clear_young_dirty

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Date: Wed, 17 Apr 2024 11:53:09 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Lance Yang <ioworker0@...il.com>, David Hildenbrand <david@...hat.com>
Cc: 21cnbao@...il.com, akpm@...ux-foundation.org, fengwei.yin@...el.com,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, mhocko@...e.com,
 minchan@...nel.org, peterx@...hat.com, shy828301@...il.com,
 songmuchun@...edance.com, wangkefeng.wang@...wei.com, xiehuan09@...il.com,
 zokeefe@...gle.com
Subject: Re: [PATCH v7 1/3] mm/madvise: introduce clear_young_dirty_ptes()
 batch helper

On 17/04/2024 10:01, Lance Yang wrote:
> On Wed, Apr 17, 2024 at 4:19 PM David Hildenbrand <david@...hat.com> wrote:
>>
>> On 17.04.24 07:04, Lance Yang wrote:
>>> Hey David, Ryan,
>>>
>>> How about this change?
>>>
>>> static inline void clear_young_dirty_ptes(struct vm_area_struct *vma,
>>>                                         unsigned long addr, pte_t *ptep,
>>>                                         unsigned int nr, cydp_t flags)
>>> {
>>>       if (flags == CYDP_CLEAR_YOUNG) {
>>>               for (;;) {
>>>                       ptep_test_and_clear_young(vma, addr, ptep);
>>>                       if (--nr == 0)
>>>                               break;
>>>                       ptep++;
>>>                       addr += PAGE_SIZE;
>>>               }
>>>               return;
>>>       }
>>>
>>>       pte_t pte;
>>>
>>>       for (;;) {
>>>               pte = ptep_get_and_clear(vma->vm_mm, addr, ptep);
>>>
>>>               if (flags & CYDP_CLEAR_YOUNG)
>>>                       pte = pte_mkold(pte);
>>>               if (flags & CYDP_CLEAR_DIRTY)
>>>                       pte = pte_mkclean(pte);
>>>
>>>               if (--nr == 0)
>>>                       break;
>>>               ptep++;
>>>               addr += PAGE_SIZE;
>>>       }
>>> }
>>
>> Likely it might be best to just KIS for now and leave it as is. The
>> compiler should optimize out based on flags already, that's what I ignored.
> 
> Got it. Let's keep it as is for now :)

Yep agreed; you're passing the flags as constants so the compiler should be
completely removing one half of the conditional. And if the input isn't a
constant, I'd still expect the compiler to hoist the conditional out of the loop.

> 
> Thanks,
> Lance
> 
>>
>> --
>> Cheers,
>>
>> David / dhildenb
>>