Message-ID: <f79bbfc9-bb4c-4da4-9902-2e73817dd135@redhat.com>
Date: Fri, 9 Aug 2024 18:54:28 +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 07.08.24 05:08, Qi Zheng wrote:
> Hi David,
> 
> On 2024/8/6 22:16, David Hildenbrand wrote:
>> 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?
> 
> If we acuqire the PTL (PTE or PMD lock) after calling
> pte_offset_map_nolock(), it means we may be modifying the corresponding
> pte or pmd entry. In that case, we need to perform a pmd_same() check
> after holding the PTL, just like in pte_offset_map_lock(), to prevent
> the possibility of the PTE page being reclaimed at that time.

Okay, what I thought.

> 
> If we call pte_offset_map_nolock() and do not need to acquire the PTL
> afterwards, it means we are only reading the PTE page. In this case, the
> rcu_read_lock() in pte_offset_map_nolock() will ensure that the PTE page
> cannot be reclaimed.
> 
>>
>> 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?
> 
> Yes.
> 
>>
>> So my thinking is if another function variant can make that clearer.
> 
> OK, how about naming it pte_offset_map_before_lock?

That's the issue with some of the code: for example in 
filemap_fault_recheck_pte_none() we'll call pte_offset_map_nolock() and 
conditionally take the PTL. But we won't be modifying the pages tables.

Maybe something like:

pte_offset_map_readonly_nolock()

and

pte_offset_map_maywrite_nolock()

The latter would require you to pass the PMD pointer such that you have 
to really mess up to ignore what to do with it (check PMD same or not 
check PMD same if you really know what you are douing).

The first would not take a PMD pointer at all, because there is no need to.

-- 
Cheers,

David / dhildenb

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