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From:   Jann Horn <jannh@google.com>
Date:   Mon, 21 Sep 2020 23:43:38 +0200
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Subject: Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned
To:     Peter Xu <peterx@redhat.com>
Cc:     Linux-MM <linux-mm@kvack.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jan Kara <jack@suse.cz>, Michal Hocko <mhocko@suse.com>,
        Kirill Tkhai <ktkhai@virtuozzo.com>,
        Kirill Shutemov <kirill@shutemov.name>,
        Hugh Dickins <hughd@google.com>,
        Christoph Hellwig <hch@lst.de>,
        Andrea Arcangeli <aarcange@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Leon Romanovsky <leonro@nvidia.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
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On Mon, Sep 21, 2020 at 11:17 PM Peter Xu <peterx@redhat.com> wrote:
>
> (Commit message collected from Jason Gunthorpe)
>
> Reduce the chance of false positive from page_maybe_dma_pinned() by keeping
> track if the mm_struct has ever been used with pin_user_pages(). mm_structs
> that have never been passed to pin_user_pages() cannot have a positive
> page_maybe_dma_pinned() by definition.

There are some caveats here, right? E.g. this isn't necessarily true
for pagecache pages, I think?