linux-kernel - Re: [PATCH 1/5] mm: Introduce mm_struct.has

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Date:   Fri, 25 Sep 2020 18:15:37 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Peter Xu <peterx@...hat.com>, John Hubbard <jhubbard@...dia.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Michal Hocko <mhocko@...e.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Kirill Shutemov <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>,
        Christoph Hellwig <hch@....de>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned

On Fri, Sep 25, 2020 at 5:41 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
>
> I don't completely grok the consequences of the anon_vma check. We
> can exclude file backed mappings as they are broken for pinning
> anyhow, so what is left that could be MAP_PRIVATE of a non-anon_vma?

It really shouldn't ever happen.

The only way a COW vma can have a NULL anon_vma should be if it has no
pages mapped at all.

Technically I think it could happen if you only ever mapped the
special zero page in there (but that shouldn't then get to the
"vm_normal_page()").

> Otherwise, I do prefer Peter's version because of the GFP_KERNEL. To
> touch on your other email..

Yeah, no, I just hadn't seen a new version, so I started getting antsy
and that's when I decided to see what a minimal patch looks like.

I think that over the weekend I'll do Peter's version but with the
"page_mapcount() == 1"  check, because I'm starting to like that
better than the mm->has_pinned.

Comments on that plan?

              Linus