linux-kernel - Re: [PATCH v4 3/9] mm: pagewalk: Don't split transhuge pmds when a pmd

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Date:   Thu, 10 Oct 2019 01:50:49 +0200
From:   Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Thomas Hellstrom <thellstrom@...are.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Matthew Wilcox <willy@...radead.org>,
        Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rik van Riel <riel@...riel.com>,
        Minchan Kim <minchan@...nel.org>,
        Michal Hocko <mhocko@...e.com>,
        Huang Ying <ying.huang@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>
Subject: Re: [PATCH v4 3/9] mm: pagewalk: Don't split transhuge pmds when a
 pmd_entry is present

On 10/10/19 12:30 AM, Thomas Hellström (VMware) wrote:
> On 10/9/19 10:20 PM, Linus Torvalds wrote:
>> On Wed, Oct 9, 2019 at 1:06 PM Thomas Hellström (VMware)
>> <thomas_os@...pmail.org> wrote:
>>> On 10/9/19 9:20 PM, Linus Torvalds wrote:
>>>> Don't you get it? There *is* no PTE level if you didn't split.
>>> Hmm, This paragraph makes me think we have very different 
>>> perceptions about what I'm trying to achieve.
>> It's not about what you're trying to achieve.
>>
>> It's about the actual code.
>>
>> You cannot do that
>>
>>> - split_huge_pmd(walk->vma, pmd, addr);
>>> +               if (!ops->pmd_entry)
>>> +                       split_huge_pmd(walk->vma, pmd, addr);
>> it's insane.
>>
>> You *have* to call split_huge_pmd() if you're doing to call the
>> pte_entry() function.
>>
>> I don't understand why you are arguing. This is not about "feelings"
>> and "intentions" or about "trying to achieve".
>>
>> This is about cold hard "you can't do that", and this is now the third
>> time I tell you _why_ you can't do that: you can't walk the last level
>> if you don't _have_ a last level. You have to split the pmd to do so.
> It's not so much arguing but rather trying to understand your concerns 
> and your perception of what the final code should look like.
>>
>> End of story.
>
> So is it that you want pte_entry() to be strictly called for *each* 
> virtual address, even if we have a pmd_entry()?
> In that case I completely follow your arguments, meaning we skip this 
> patch completely?

Or if you're still OK with your original patch

https://lore.kernel.org/lkml/CAHk-=wj5NiFPouYd6zUgY4K7VovOAxQT-xhDRjD6j5hifBWi_g@mail.gmail.com/

I'd happily use that instead.

Thanks,

Thomas