linux-kernel - Re: [PATCH v3 13/16] mm/gup: disallow follow_page(FOLL

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Message-ID: <08c355b4-fa21-8716-bf05-3331914f150e@suse.cz>
Date:   Thu, 14 Apr 2022 17:18:53 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jann Horn <jannh@...gle.com>, Michal Hocko <mhocko@...nel.org>,
        Nadav Amit <namit@...are.com>, Rik van Riel <riel@...riel.com>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Donald Dutile <ddutile@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>,
        Liang Zhang <zhangliang5@...wei.com>,
        Pedro Gomes <pedrodemargomes@...il.com>,
        Oded Gabbay <oded.gabbay@...il.com>, linux-mm@...ck.org
Subject: Re: [PATCH v3 13/16] mm/gup: disallow follow_page(FOLL_PIN)

On 3/29/22 18:04, David Hildenbrand wrote:
> We want to change the way we handle R/O pins on anonymous pages that
> might be shared: if we detect a possibly shared anonymous page --
> mapped R/O and not !PageAnonExclusive() -- we want to trigger unsharing
> via a page fault, resulting in an exclusive anonymous page that can be
> pinned reliably without getting replaced via COW on the next write
> fault.
> 
> However, the required page fault will be problematic for follow_page():
> in contrast to ordinary GUP, follow_page() doesn't trigger faults
> internally. So we would have to end up failing a R/O pin via
> follow_page(), although there is something mapped R/O into the page
> table, which might be rather surprising.
> 
> We don't seem to have follow_page(FOLL_PIN) users, and it's a purely
> internal MM function. Let's just make our life easier and the semantics of
> follow_page() clearer by just disallowing FOLL_PIN for follow_page()
> completely.
> 
> Signed-off-by: David Hildenbrand <david@...hat.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>