linux-kernel - Re: [PATCH] mm/khugepaged: fix collapse_pte_mapped_thp() to allow anon

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Date:   Mon, 23 Jan 2023 12:09:22 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Zach O'Keefe <zokeefe@...gle.com>
Cc:     Yang Shi <shy828301@...il.com>, Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>, Song Liu <songliubraving@...com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm/khugepaged: fix collapse_pte_mapped_thp() to allow
 anon_vma

On 18.01.23 00:00, Zach O'Keefe wrote:
> On Mon, Jan 9, 2023 at 12:50 AM David Hildenbrand <david@...hat.com> wrote:
>>
>>>>>>>>
>>>>>>>> Side note: set_huge_pmd() wins the award of "ugliest mm function of early
>>>>>>>> 2023". I was briefly concerned how do_set_pmd() decides whether the PMD can be
>>>>>>>> writable or not. Turns out it's communicated via vm_fault->flags. Just
>>>>>>>> horrible.
>>>
> 
> Hey David,
> 
> Sorry for the late response here.
> 
>>> My first Linux award! :) At least it's not "worst mm security issue of
>>> early 2023". I'll take it!
>>
>> Good that you're not taking my words the wrong way.
>>
>> MADV_COLLAPSE is a very useful feature (especially also for THP tests
>> [1]). I wish I could have looked at some of the patches earlier. But we
>> cannot wait forever to get something merged, otherwise we'd never get
>> bigger changes upstream.
>>
>> ... so there is plenty of time left in 2023 to cleanup khugepaged.c :P
>>
>>
>> [1] https://lkml.kernel.org/r/20230104144905.460075-1-david@redhat.com
> 
> Yes, thank you for these tests. I have them open in another tab along
> with a mental TODO to take a closer look at them, and response
> half-written. In-place collapse of anonymous memory *is* something
> that I was interested in exploring later (I have a use-case for it;
> hugepage-aware malloc() implementations). I'm taking so long on it
> (sorry) b/c I need to review your point (2) (all PTE's mapping
> exclusively). Hopefully I can get to it shortly.

Cool, please let me know if you have any questions. Being able to 
in-place collapse a PTE-mapped anon THP again would be nice.

-- 
Thanks,

David / dhildenb