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

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Message-ID: <20200923010332.GP19098@xz-x1>
Date:   Tue, 22 Sep 2020 21:03:32 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.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 08:44:00PM +0200, Oleg Nesterov wrote:
> On 09/22, Peter Xu wrote:
> >
> > Or I can also do it in inverted order if you think better:
> >
> >         if (unlikely(copy_ret == COPY_MM_BREAK_COW)) {
> >                 WARN_ON_ONCE(!data.cow_new_page);
> >                 ...
> >         }
> 
> Peter, let me say this again. I don't understand this code enough, you
> can safely ignore me ;)

Why? I appreciate every single comment from you! :)

> 
> However. Personally I strongly prefer the above. Personally I really
> dislike this part of 4/5:
> 
> 	 again:
> 	+	/* We don't reset this for COPY_MM_BREAK_COW */
> 	+	memset(&data, 0, sizeof(data));
> 	+
> 	+again_break_cow:
> 
> 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 :).

Thanks,

-- 
Peter Xu