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

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Message-ID: <35E34233-3435-4873-A82C-A1DFBD0AFC55@nvidia.com>
Date: Tue, 17 Dec 2024 13:11:36 -0500
From: Zi Yan <ziy@...dia.com>
To: Ryan Roberts <ryan.roberts@....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 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.

Best Regards,
Yan, Zi