linux-kernel - Re: [PATCH v1] mm/gup: disallow FOLL_FORCE|FOLL

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Message-ID: <Y2JWnNbE9SC/4boX@nvidia.com>
Date:   Wed, 2 Nov 2022 08:38:04 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Peter Xu <peterx@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        syzbot+f0b97304ef90f0d0b1dc@...kaller.appspotmail.com
Subject: Re: [PATCH v1] mm/gup: disallow FOLL_FORCE|FOLL_WRITE on hugetlb
 mappings

On Wed, Nov 02, 2022 at 10:14:34AM +0100, David Hildenbrand wrote:

> RDMA (due to FOLL_FORCE) would now fail (instead of doing something wrong)
> on MAP_PRIVATE hugetlb mappings that are R/O. Do we have any actual examples
> of such RDMA usage? I was able to understand why this case (MAP_PRIVATE,
> PROT_READ) is important for !hugetlb, but I don't immediately see under
> which situations this would apply to hugetlb.

It may be that every one is already using MAP_SHARED for hugetlb,
which would make it fine..

Jason