linux-kernel - Re: [PATCH 4/5] mm: Do early cow for pinned pages during fork() for ptes

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Date:   Wed, 23 Sep 2020 13:25:52 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     Oleg Nesterov <oleg@...hat.com>, Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>,
        Kirill Shutemov <kirill@...temov.name>,
        Jann Horn <jannh@...gle.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Hugh Dickins <hughd@...gle.com>,
        Leon Romanovsky <leonro@...dia.com>, Jan Kara <jack@...e.cz>,
        John Hubbard <jhubbard@...dia.com>,
        Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH 4/5] mm: Do early cow for pinned pages during fork() for ptes

On Tue, Sep 22, 2020 at 6:03 PM Peter Xu <peterx@...hat.com> wrote:
>
> > If we rely on "copy_ret == COPY_MM_BREAK_COW" we can unify "again" and
> > "again_break_cow", we don't need to clear ->cow_new_page, this makes the
> > logic more understandable. To me at least ;)
>
> I see your point.  I'll definitely try it out.  I think I'll at least use what
> you preferred above since it's actually the same as before, logically.  Then
> I'll consider drop the again_break_cow, as long as I'm still as confident after
> I do the change on not leaking anything :).

So the two patches I sent out to re-organize copy_one_pte() were
literally meant to make all this mess go away.

IOW, the third patch would be something (COMPLETELY UNTESTED) like the attached.

I think the logic for the preallocation is fairly obvious, but it
might be better to allocate a batch of pages for all I know. That
said, I can't really make myself care about the performance of a
fork() after you've pinned pages in it, so..

                 Linus

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