linux-kernel - Re: [RFC PATCH 0/3] asynchronously scan and free empty user PTE pages

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Date: Tue, 18 Jun 2024 11:40:51 +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, akpm@...ux-foundation.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] asynchronously scan and free empty user PTE pages

On 18.06.24 09:51, Qi Zheng wrote:
> Hi David,
> 
> On 2024/6/18 01:49, David Hildenbrand wrote:
>>
>>>>
>>>> No strong opinion, something synchronous sounds to me like the
>>>> low-hanging fruit, that could add the infrastructure to be used by
>>>> something more advanced/synchronously :)
>>>
>>> Got it, I will try to do the following in the next version.
>>>
>>> a. for MADV_DONTNEED case, try synchronous reclaim as you said
>>>
>>
>> I think that really is the low hanging fruit that would cover quite some
>> cases already: (1) reclaim when MADV_DONTNEED spans the complete page
>> table.
> 
> I will check and free the PTE page in the zap_pte_range() if the
> (end - addr >= PMD_SIZE) condition is met.
> 
>>
>> Then, there is (2) reclaim when MADV_DONTNEED spans only part of the
>> page table (e.g., single PTE), but my best guess is that it's better to
>> scan for that asynchronously than making possibly each MADV_DONTNEED
>> sycall invocation slower.
> 
> Maybe just mark the vma, and then scan it in the system reclaim path.
> 
> I also plan to do this in the MADV_FREE case, instead of adding an
> asynchronous madvise option first.
> 
>>
>> (1) would already help a lot and showcase how the locking/machinery
>> would work.
>>
>>
>>> b. for MADV_FREE case:
>>>
>>>      - add a madvise option for synchronous reclaim
>>>
>>>      - add another madvise option to mark the vma, then add its
>>>              corresponding mm to a global list, and then traverse
>>>              the list and reclaim it when the memory is tight and
>>>              enters the system reclaim path.
>>>              (maybe there is an option to unmark)
>>>
>>> c. for s390 case you mentioned, create a CONFIG_FREE_PT first, and
>>>       then s390 will not select this config until the problem is solved.
>>>
>>> d. for lockless scan, try using disabling IRQ or (mmap read lock +
>>> pte_offset_map_nolock).
>>
>> Although d) really only is desired when scanning asynchronously I think.
>> During (1) above, we know that the table will be very likely empty
>> (unless weird race).
> 
> Agree.

Again, thanks for working on this. Let me know (can also do privately) 
if you run into any issues or think I can be of help. :)

-- 
Cheers,

David / dhildenb