linux-kernel - Re: [PATCH v3] mm/gup: Allow real explicit breaking of COW

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Date:   Fri, 21 Aug 2020 05:27:40 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jan Kara <jack@...e.cz>
Cc:     Peter Xu <peterx@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marty Mcfadden <mcfadden8@...l.gov>,
        "Maya B . Gokhale" <gokhale2@...l.gov>,
        Jann Horn <jannh@...gle.com>, Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>,
        Kirill Shutemov <kirill@...temov.name>
Subject: Re: [PATCH v3] mm/gup: Allow real explicit breaking of COW

On Fri, Aug 21, 2020 at 3:13 AM Jan Kara <jack@...e.cz> wrote:
>
> > +             if (page_mapcount(page) != 1 && page_count(page) != 1) {
>
> So this condition looks strange to me... Did you mean:
>
>                 if (page_mapcount(page) != 1 || page_count(page) != 1)

Duh. Yes.

> > -             if (PageKsm(vmf->page)) {
>
> Also I know nothing about KSM but looking at reuse_ksm_page() I can see it
> plays some tricks with page index & mapping even for pages with page_count
> == 1 so you cannot just drop those bits AFAICT.

Yeah, I wasn't really sure what we want to do.

In an optimal world, I was thinking that we'd actually do exactly what
we do at munmap time.

Which is not to get the page lock at all. Just look at what
zap_pte_range() does for an a page when it unmaps it:

                        page_remove_rmap(page, false);

and that's it. No games.

And guess what? That "'page_remove_rmap()" is what wp_page_copy() already does.

So I really think *all* of these games we play are complete garbage
and completely wrong.

Because the zap_page_range() path is a *lot* more common than the WP
path, and triggers for every single page when we do munmap or exit or
whatever.

So why would WP need to do anything else for correctness? Absolutely
no reason I can see.

> Also I'm not sure if dropping this is safe for THP - reuse_swap_page()
> seems to be a misnomer and seems to do also some THP handling.

Again, I think that's a bogus argument.

Because this all is actually not the common path at all, and the thing
is, the common path does none of these odd games.

I really think this COW handling magic is just legacy garbage because
people have carried it along forever and everybody is worried about
it. The fact is, the "copy" case is always safe, because all it does
is basically the same as zap_page_range() does, with just adding a new
page instead.

And it's also possible I'm missing something.

But yes. That '&&' should very much be a '||'. What can I say? Oops.

              Linus