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

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Message-ID: <20200928171321.GA59869@xz-x1>
Date:   Mon, 28 Sep 2020 13:13:21 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Leon Romanovsky <leonro@...dia.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        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>, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned

On Sun, Sep 27, 2020 at 11:16:34AM -0700, Linus Torvalds wrote:
>  - split up the "copy page" into its own function kind of like I had
> done for my minimal patch

I didn't do that majorly because of the wrprotect() (of the fast-gup race) that
can potentially be kept if it's a normal cow.  Maybe we'd add one more comment
above the caller of copy_present_page() (even if we have a "NOTE!" section
already inside the helper) since it'll change *src_pte and it's hard to notice
from the function name "copy_present_page()".  Not that important, though.

Thanks for doing the changes.  I went over the whole patch and it looks indeed
cleaner than before (I also didn't spot anything wrong either).

Regarding the other even rarer "hardware race on dirty/access bits" - maybe we
can simply set all these two bits always when page copy happens and we want to
recover the ptes?  I also agree it's trivial enough, so that maybe we even
don't need to care about that.

-- 
Peter Xu