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

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Message-ID: <CAG48ez0o+yBpYdzR_-bU3A0nrpzXyM+c+Yk=ZtOZ92qe5x0izA@mail.gmail.com>
Date:   Mon, 21 Sep 2020 23:43:38 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        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>,
        John Hubbard <jhubbard@...dia.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned

On Mon, Sep 21, 2020 at 11:17 PM Peter Xu <peterx@...hat.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?