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

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Message-ID: <CAHk-=wgmr-BPMTnSuKrAMoHL_A0COV_sZkdcNB9aosYfouA_fw@mail.gmail.com>
Date:   Wed, 9 Oct 2019 13:20:51 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.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 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.

End of story.

> I wanted the pte level to *only* get called for *pre-existing* pte entries.

Again, I told you what the solution was.

But the fact is, it's not what your other code even wants or does.

Seriously. You have two cases you care about in your callbacks

 - an actual hugepmd. This is an ERROR for you and you do a huge
WARN_ON() for it to let people know.

 - regular pages. This is what your other code actually handles.

So for the hugepomd case, you have two choices:

 - handle it by splitting and deal with the regular pages: "return 0;"

 - actually error out: "return -EINVAL".

There simply are no other choices from looking at the users you added.

> Surely those must be able to exist even if we don't split occasional huge pmds in the pagewalk code?

And I told you how to handle that case when you eventually hit it.

But that is immaterial. Because it's not what you actually do right
now. Right now you have a huge WARN_ON().

                Linus