linux-kernel - Re: [PATCH v3 4/9] mm: introduce skip_none

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Message-ID: <5a3428bd-743a-4d51-8b75-163ab560bca7@bytedance.com>
Date: Fri, 15 Nov 2024 22:41:01 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: David Hildenbrand <david@...hat.com>
Cc: jannh@...gle.com, hughd@...gle.com, willy@...radead.org,
 muchun.song@...ux.dev, vbabka@...nel.org, akpm@...ux-foundation.org,
 peterx@...hat.com, mgorman@...e.de, catalin.marinas@....com,
 will@...nel.org, dave.hansen@...ux.intel.com, luto@...nel.org,
 peterz@...radead.org, x86@...nel.org, lorenzo.stoakes@...cle.com,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org, zokeefe@...gle.com,
 rientjes@...gle.com
Subject: Re: [PATCH v3 4/9] mm: introduce skip_none_ptes()



On 2024/11/15 18:22, David Hildenbrand wrote:
>>>> *nr_skip = nr;
>>>>
>>>> and then:
>>>>
>>>> zap_pte_range
>>>> --> nr = do_zap_pte_range(tlb, vma, pte, addr, end, details, &skip_nr,
>>>>                           rss, &force_flush, &force_break);
>>>>        if (can_reclaim_pt) {
>>>>            none_nr += count_pte_none(pte, nr);
>>>>            none_nr += nr_skip;
>>>>        }
>>>>
>>>> Right?
>>>
>>> Yes. I did not look closely at the patch that adds the counting of
>>
>> Got it.
>>
>>> pte_none though (to digest why it is required :) ).
>>
>> Because 'none_nr == PTRS_PER_PTE' is used in patch #7 to detect
>> empty PTE page.
> 
> Okay, so the problem is that "nr" would be "all processed entries" but 
> there are cases where we "process an entry but not zap it".
> 
> What you really only want to know is "was any entry not zapped", which 
> could be a simple input boolean variable passed into do_zap_pte_range?
> 
> Because as soon as any entry was processed but  no zapped, you can 
> immediately give up on reclaiming that table.

Yes, we can set can_reclaim_pt to false when a !pte_none() entry is
found in count_pte_none().

> 
>>
>> Looking forward to your more review feedback on this series.
> 
> Thanks for all your hard work on this, I'm only able to make slow 
> progress because I keep getting distracted by all different kinds of 
> things :(

It's OK, just take your time. :)

Thanks!

>