linux-kernel - Re: [RFC PATCH v2 1/7] mm: pgtable: make pte_offset_map

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Message-ID: <0f467510-a0d0-4a98-8517-43813fa4c131@redhat.com>
Date: Tue, 6 Aug 2024 16:16:25 +0200
From: David Hildenbrand <david@...hat.com>
To: Qi Zheng <zhengqi.arch@...edance.com>
Cc: hughd@...gle.com, willy@...radead.org, mgorman@...e.de,
 muchun.song@...ux.dev, vbabka@...nel.org, akpm@...ux-foundation.org,
 zokeefe@...gle.com, rientjes@...gle.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [RFC PATCH v2 1/7] mm: pgtable: make pte_offset_map_nolock()
 return pmdval

On 06.08.24 04:40, Qi Zheng wrote:
> Hi David,
> 
> On 2024/8/5 22:43, David Hildenbrand wrote:
>> On 05.08.24 14:55, Qi Zheng wrote:
>>> Make pte_offset_map_nolock() return pmdval so that we can recheck the
>>> *pmd once the lock is taken. This is a preparation for freeing empty
>>> PTE pages, no functional changes are expected.
>>
>> Skimming the patches, only patch #4 updates one of the callsites
>> (collapse_pte_mapped_thp).
> 
> In addition, retract_page_tables() and reclaim_pgtables_pmd_entry()
> also used the pmdval returned by pte_offset_map_nolock().

Right, and I am questioning if only touching these two is sufficient, 
and how we can make it clearer when someone actually has to recheck the PMD.

> 
>>
>> Wouldn't we have to recheck if the PMD val changed in more cases after
>> taking the PTL?
>>
>> If not, would it make sense to have a separate function that returns the
>> pmdval and we won't have to update each and every callsite?
> 
> pte_offset_map_nolock() had already obtained the pmdval previously, just
> hadn't returned it. And updating those callsite is simple, so I think
> there may not be a need to add a separate function.

Let me ask this way: why is retract_page_tables() and 
reclaim_pgtables_pmd_entry() different to the other ones, and how would 
someone using pte_offset_map_nolock() know what's to do here?

IIUC, we must check the PMDVAL after taking the PTL in case

(a) we want to modify the page table to turn pte_none() entries to
     !pte_none(). Because it could be that the page table was removed and
     now is all pte_none()

(b) we want to remove the page table ourselves and want to check if it
     has already been removed.

Is that it?

So my thinking is if another function variant can make that clearer.

-- 
Cheers,

David / dhildenb