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From: John Hubbard <jhubbard@nvidia.com>
To: Andrew Morton <akpm@linux-foundation.org>,
	Jason Gunthorpe <jgg@ziepe.ca>, Leon Romanovsky <leon@kernel.org>,
	Christoph Hellwig <hch@infradead.org>
Cc: Ira Weiny <ira.weiny@intel.com>, <linux-rdma@vger.kernel.org>,
	<linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>,
	John Hubbard <jhubbard@nvidia.com>,
	Christoph Hellwig <hch@lst.de>
Subject: [PATCH v2 1/2] mm/gup: allow FOLL_FORCE for get_user_pages_fast()
Date: Wed, 4 Dec 2019 13:36:02 -0800
Message-ID: <20191204213603.464373-2-jhubbard@nvidia.com> (raw)
In-Reply-To: <20191204213603.464373-1-jhubbard@nvidia.com>

Commit 817be129e6f2 ("mm: validate get_user_pages_fast flags") allowed
only FOLL_WRITE and FOLL_LONGTERM to be passed to get_user_pages_fast().
This, combined with the fact that get_user_pages_fast() falls back to
"slow gup", which *does* accept FOLL_FORCE, leads to an odd situation:
if you need FOLL_FORCE, you cannot call get_user_pages_fast().

There does not appear to be any reason for filtering out FOLL_FORCE.
There is nothing in the _fast() implementation that requires that we
avoid writing to the pages. So it appears to have been an oversight.

Fix by allowing FOLL_FORCE to be set for get_user_pages_fast().

Fixes: 817be129e6f2 ("mm: validate get_user_pages_fast flags")
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 mm/gup.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/gup.c b/mm/gup.c
index 7646bf993b25..5244b8090440 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2415,7 +2415,8 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
 	unsigned long addr, len, end;
 	int nr = 0, ret = 0;
 
-	if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM)))
+	if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM |
+				       FOLL_FORCE)))
 		return -EINVAL;
 
 	start = untagged_addr(start) & PAGE_MASK;
-- 
2.24.0


  reply index

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-04 21:36 [PATCH v2 0/2] IB/umem: use get_user_pages_fast() to pin DMA pages John Hubbard
2019-12-04 21:36 ` John Hubbard [this message]
2019-12-09 18:25   ` [PATCH v2 1/2] mm/gup: allow FOLL_FORCE for get_user_pages_fast() Leon Romanovsky
2019-12-09 19:54     ` John Hubbard
2019-12-04 21:36 ` [PATCH v2 2/2] IB/umem: use get_user_pages_fast() to pin DMA pages John Hubbard
2019-12-09 18:25   ` Leon Romanovsky

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