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

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Message-ID: <c054849e-1e24-6b27-6a54-740ea9d17054@shipmail.org>
Date:   Wed, 9 Oct 2019 22:06:32 +0200
From:   Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Hellstrom <thellstrom@...are.com>
Cc:     "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 10/9/19 9:20 PM, Linus Torvalds wrote:
>
> No. Your logic is garbage. The above code is completely broken.
>
> YOU CAN NOT AVOID TRHE SPLIT AND THEN GO ON AT THE PTE LEVEL.
>
> 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.

I wanted the pte level to *only* get called for *pre-existing* pte entries.
Surely those must be able to exist even if we don't split occasional huge pmds in the pagewalk code?

>
> So what you should do is to just always return 0 in your pmd_entry().
> Boom, done. The only reason for the pmd_entry existing at all is to
> get the warning. Then, if you don't want to split it, you make that
> warning just return an error (or a positive value) instead and say
> "ok, that was bad, we don't handle it at all".
>
> And in some _future_ life, if anybody wants to actually say "yeah,
> let's not split it", make it have some "yeah I handled it" case.

Well yes, this is exactly what I want. Because any huge pmd we encounter 
should be read-only.

/Thomas