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

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Message-ID: <0caffb55-f242-4b01-b60a-c8749aadf661@arm.com>
Date: Thu, 18 Dec 2025 12:08:03 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Baolin Wang <baolin.wang@...ux.alibaba.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

>>>   +#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))))

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?