linux-kernel - Re: [RFC PATCH 01/12] khugepaged: Rename hpage_collapse_scan

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Message-ID: <8f1ef412-6f43-4346-bbfd-84431f3a5488@arm.com>
Date: Tue, 17 Dec 2024 19:12:21 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Zi Yan <ziy@...dia.com>, Dev Jain <dev.jain@....com>
Cc: Matthew Wilcox <willy@...radead.org>, akpm@...ux-foundation.org,
 david@...hat.com, kirill.shutemov@...ux.intel.com,
 anshuman.khandual@....com, catalin.marinas@....com, cl@...two.org,
 vbabka@...e.cz, mhocko@...e.com, apopple@...dia.com,
 dave.hansen@...ux.intel.com, will@...nel.org, baohua@...nel.org,
 jack@...e.cz, srivatsa@...il.mit.edu, haowenchao22@...il.com,
 hughd@...gle.com, aneesh.kumar@...nel.org, yang@...amperecomputing.com,
 peterx@...hat.com, ioworker0@...il.com, wangkefeng.wang@...wei.com,
 jglisse@...gle.com, surenb@...gle.com, vishal.moola@...il.com,
 zokeefe@...gle.com, zhengqi.arch@...edance.com, jhubbard@...dia.com,
 21cnbao@...il.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 01/12] khugepaged: Rename hpage_collapse_scan_pmd() ->
 ptes()

On 17/12/2024 18:11, Zi Yan wrote:
> On 17 Dec 2024, at 1:43, Ryan Roberts wrote:
> 
>> On 17/12/2024 04:18, Matthew Wilcox wrote:
>>> On Mon, Dec 16, 2024 at 10:20:54PM +0530, Dev Jain wrote:
>>>> -static int hpage_collapse_scan_pmd(struct mm_struct *mm,
>>>> +static int hpage_collapse_scan_ptes(struct mm_struct *mm,
>>>
>>> i don't think this is necessary at all.  you're scanning a pmd.
>>> you might not be scanning in order to collapse to a pmd, but pmd
>>> is the level you're scanning at.
>>>
>>
>> Sorry Matthew, I don't really understand this statement. Prior to the change we
>> were scanning all PTE entries in a PTE table with the aim of collapsing to a PMD
>> entry. After the change we are scanning some PTE entries in a PTE table with the
>> aim of collapsing to either to a multi-PTE-mapped folio or a single-PMD-mapped
>> folio.
>>
>> So personally I think "scan_pmd" was a misnomer even before the change - we are
>> scanning the ptes.
> 
> But there are still a lot of scan_pmd code in the function, for example,
> VM_BUG_ON(address & ~HPAGE_PMD_MASK), _pte < pte + HPAGE_PMD_NR in the function.
> These need to be changed along with function renaming. If after the change only
> a subset of PTEs are scanned within a PMD, maybe a scan_range parameter can be
> added.

Oh I see; I think your and Matthew's point is that we are scanning a
"PMD-entry's worth of PTEs". Looking at it like that, I guess I understand why
"scan_pmd" makes sense. Fair enough.

> 
> Best Regards,
> Yan, Zi