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From:   Brian Geffon <bgeffon@google.com>
Date:   Wed, 6 May 2020 18:11:17 -0700
Message-ID: <CADyq12wW-j9vi0oxXB_+sjSyP=FgaNWkRM2Yo9YqkEwGnOb0bA@mail.gmail.com>
Subject: Re: [PATCH] userfaultfd: fix remap event with MREMAP_DONTUNMAP.
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        "Michael S . Tsirkin" <mst@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Sonny Rao <sonnyrao@google.com>,
        Minchan Kim <minchan@kernel.org>,
        "Kirill A . Shutemov" <kirill@shutemov.name>
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> > -       mremap_userfaultfd_complete(&uf, addr, new_addr, old_len);
> > +       mremap_userfaultfd_complete(&uf, addr, ret, old_len);
>
> Not super familiar with this code, but thought I'd ask, does ret
> to be checked for -ENOMEM before calling mremap_userfaultfd_complete?
> Sorry if I missed something.

No, mremap_userfaultfd_complete will do a check similar to
offset_in_page() by checking the page mask.
It does (to & ~PAGE_MASK) to check for a non-aligned "to" value, so we're good.

Additionally, earlier in the process then ctx will be null because we
will have never called mremap_userfaultfd_prep,
and mremap_userfaultfd_complete will check if there is a context
before proceeding.

Brian