linux-kernel - Re: [PATCH v2 2/3] mm: rmap: support batched checks of the references for large folios

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Message-ID: <121c8ffb-adff-4766-aae9-8be6d37d546a@linux.alibaba.com>
Date: Fri, 19 Dec 2025 08:56:09 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Ryan Roberts <ryan.roberts@....com>, akpm@...ux-foundation.org,
 david@...nel.org, catalin.marinas@....com, will@...nel.org
Cc: lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, vbabka@...e.cz,
 rppt@...nel.org, surenb@...gle.com, mhocko@...e.com, riel@...riel.com,
 harry.yoo@...cle.com, jannh@...gle.com, willy@...radead.org,
 baohua@...nel.org, linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] mm: rmap: support batched checks of the references
 for large folios



On 2025/12/18 20:08, Ryan Roberts wrote:
>>>>    +#define clear_flush_young_ptes clear_flush_young_ptes
>>>> +static inline int clear_flush_young_ptes(struct vm_area_struct *vma,
>>>> +                    unsigned long addr, pte_t *ptep,
>>>> +                    unsigned int nr)
>>>> +{
>>>> +    if (likely(nr == 1))
>>>> +        return __ptep_clear_flush_young(vma, addr, ptep);
>>>
>>> Bug: This is broken if core-mm tries to call this for nr=1 on a pte that is part
>>> of a contpte mapping.
>>>
>>> The similar fastpaths are here to prevent regressing the common small folio case.
>>
>> Thanks for catching this. I had considered this before, but I still missed it.
>>
>>> I guess here the best approach is (note no leading underscores):
>>>
>>>      if (likely(nr == 1))
>>>          return ptep_clear_flush_young(vma, addr, ptep);
>>
>> However, I prefer to use pte_cont() to check it. Later, I plan to clean up the
>> ptep_clear_flush_young().
>>
>>      if (nr == 1 && !pte_cont(__ptep_get(ptep))
>>          return __ptep_clear_flush_young(vma, addr, ptep);
> 
> Sure. That would follow the pattern in clear_young_dirty_ptes(). Please use the
> likely() hint as is done everywhere else:
> 
> 	if (likely(nr == 1 && !pte_cont(__ptep_get(ptep))))

Sure.

> I notice that ptep_test_and_clear_young() and ptep_clear_flush_young() are both
> testing aginst pte_valid_cont(). These could probably be relaxed to pte_cont()
> since it is implicit the the pte must be valid?

Yes, I think so. I can do a cleanup later in a separate patch.