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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 17 Sep 2020 12:51:49 -0700
Message-ID: <CAHk-=wiY_g+SSjncZi8sO=LrxXmMox0NO7K34-Fs653XVXheGg@mail.gmail.com>
Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     Peter Xu <peterx@redhat.com>, John Hubbard <jhubbard@nvidia.com>,
        Leon Romanovsky <leonro@nvidia.com>,
        Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Maya B . Gokhale" <gokhale2@llnl.gov>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        Marty Mcfadden <mcfadden8@llnl.gov>,
        Kirill Shutemov <kirill@shutemov.name>,
        Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
        Jan Kara <jack@suse.cz>, Kirill Tkhai <ktkhai@virtuozzo.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        Andrew Morton <akpm@linux-foundation.org>
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On Thu, Sep 17, 2020 at 12:38 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> Looking for awhile, this now looks reasonable and
> doable. page_maybe_dma_pinned() was created for exactly this kind of
> case.
>
> I've attached a dumb sketch for the pte level (surely wrong! I have
> never looked at this part of the mm before!) at the end of this
> message.

This looks conceptually fine to me.

But as mentioned, I think I'd be even happier if we added a "thsi vma
has seen a page pin event" flag to the vma flags, and didn't rely
_just_ on the page_maybe_dma_pinned() check, which migth be triggered
by those fork-happy loads.

Side note: I wonder if that COW mapping check could be entirely within
that vm_normal_page() path.

Because how could a non-normal page be a COW page and not already
write-protected?

But that's a separate issue, it's just how your patch makes that odd
case more obvious.

            Linus