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Message-ID: <6161e8a8-64a4-c4ea-626d-daac45ccd836@redhat.com>
Date: Mon, 2 Oct 2023 13:04:51 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Mark Rutland <mark.rutland@....com>
Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Jiri Olsa <jolsa@...nel.org>,
Namhyung Kim <namhyung@...nel.org>,
Ian Rogers <irogers@...gle.com>,
Adrian Hunter <adrian.hunter@...el.com>,
Oleg Nesterov <oleg@...hat.com>,
Richard Cochran <richardcochran@...il.com>,
Jason Gunthorpe <jgg@...dia.com>,
John Hubbard <jhubbard@...dia.com>,
Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 3/4] mm/gup: make failure to pin an error if FOLL_NOWAIT
not specified
On 01.10.23 18:00, Lorenzo Stoakes wrote:
> There really should be no circumstances under which a non-FOLL_NOWAIT GUP
> operation fails to return any pages, so make this an error.
>
> To catch the trivial case, simply exit early if nr_pages == 0.
>
> This brings __get_user_pages_locked() in line with the behaviour of its
> nommu variant.
>
> Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
> ---
> mm/gup.c | 11 +++++++++++
> 1 file changed, 11 insertions(+)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index b21b33d1787e..fb2218d74ca5 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1471,6 +1471,9 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm,
> long ret, pages_done;
> bool must_unlock = false;
>
> + if (!nr_pages)
> + return 0;
> +
Probably unlikely() is reasonable. I even wonder if WARN_ON_ONCE() would
be appropriate, but likely there are weird callers that end up calling
this with nr_pages==0 ... probably they should be identified and
changed. Future work.
> /*
> * The internal caller expects GUP to manage the lock internally and the
> * lock must be released when this returns.
> @@ -1595,6 +1598,14 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm,
> mmap_read_unlock(mm);
> *locked = 0;
> }
> +
> + /*
> + * Failing to pin anything implies something has gone wrong except when
> + * FOLL_NOWAIT is specified, so explicitly make this an error.
> + */
> + if (pages_done == 0 && !(flags & FOLL_NOWAIT))
> + return -EFAULT;
> +
But who would be affected by that and why do we care about adding this
check?
This smells like a "if (WARN_ON_ONCE())", correct?
--
Cheers,
David / dhildenb
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