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

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Message-ID: <20200922184359.GI11679@redhat.com>
Date:   Tue, 22 Sep 2020 20:44:00 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Xu <peterx@...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 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 ;)

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 ;)

Oleg.