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From: John Hubbard <jhubbard@nvidia.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: "Al Viro" <viro@zeniv.linux.org.uk>,
	"Alex Williamson" <alex.williamson@redhat.com>,
	"Benjamin Herrenschmidt" <benh@kernel.crashing.org>,
	"Björn Töpel" <bjorn.topel@intel.com>,
	"Christoph Hellwig" <hch@infradead.org>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Daniel Vetter" <daniel@ffwll.ch>,
	"Dave Chinner" <david@fromorbit.com>,
	"David Airlie" <airlied@linux.ie>,
	"David S . Miller" <davem@davemloft.net>,
	"Ira Weiny" <ira.weiny@intel.com>, "Jan Kara" <jack@suse.cz>,
	"Jason Gunthorpe" <jgg@ziepe.ca>, "Jens Axboe" <axboe@kernel.dk>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Magnus Karlsson" <magnus.karlsson@intel.com>,
	"Mauro Carvalho Chehab" <mchehab@kernel.org>,
	"Michael Ellerman" <mpe@ellerman.id.au>,
	"Michal Hocko" <mhocko@suse.com>,
	"Mike Kravetz" <mike.kravetz@oracle.com>,
	"Paul Mackerras" <paulus@samba.org>,
	"Shuah Khan" <shuah@kernel.org>,
	"Vlastimil Babka" <vbabka@suse.cz>,
	bpf@vger.kernel.org, dri-devel@lists.freedesktop.org,
	kvm@vger.kernel.org, linux-block@vger.kernel.org,
	linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-kselftest@vger.kernel.org, linux-media@vger.kernel.org,
	linux-rdma@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	netdev@vger.kernel.org, linux-mm@kvack.org,
	LKML <linux-kernel@vger.kernel.org>,
	"John Hubbard" <jhubbard@nvidia.com>,
	"Jason Gunthorpe" <jgg@mellanox.com>
Subject: [PATCH 05/19] mm: fix get_user_pages_remote()'s handling of FOLL_LONGTERM
Date: Sun, 24 Nov 2019 20:19:57 -0800
Message-ID: <20191125042011.3002372-6-jhubbard@nvidia.com> (raw)
In-Reply-To: <20191125042011.3002372-1-jhubbard@nvidia.com>

As it says in the updated comment in gup.c: current FOLL_LONGTERM
behavior is incompatible with FAULT_FLAG_ALLOW_RETRY because of the
FS DAX check requirement on vmas.

However, the corresponding restriction in get_user_pages_remote() was
slightly stricter than is actually required: it forbade all
FOLL_LONGTERM callers, but we can actually allow FOLL_LONGTERM callers
that do not set the "locked" arg.

Update the code and comments to loosen the restriction, allowing
FOLL_LONGTERM in some cases.

Also, copy the DAX check ("if a VMA is DAX, don't allow long term
pinning") from the VFIO call site, all the way into the internals
of get_user_pages_remote() and __gup_longterm_locked(). That is:
get_user_pages_remote() calls __gup_longterm_locked(), which in turn
calls check_dax_vmas(). This check will then be removed from the VFIO
call site in a subsequent patch.

Thanks to Jason Gunthorpe for pointing out a clean way to fix this,
and to Dan Williams for helping clarify the DAX refactoring.

Tested-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 mm/gup.c | 27 ++++++++++++++++++++++-----
 1 file changed, 22 insertions(+), 5 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 3ecce297a47f..c0c56888e7cc 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -29,6 +29,13 @@ struct follow_page_context {
 	unsigned int page_mask;
 };
 
+static __always_inline long __gup_longterm_locked(struct task_struct *tsk,
+						  struct mm_struct *mm,
+						  unsigned long start,
+						  unsigned long nr_pages,
+						  struct page **pages,
+						  struct vm_area_struct **vmas,
+						  unsigned int flags);
 /*
  * Return the compound head page with ref appropriately incremented,
  * or NULL if that failed.
@@ -1179,13 +1186,23 @@ long get_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
 		struct vm_area_struct **vmas, int *locked)
 {
 	/*
-	 * FIXME: Current FOLL_LONGTERM behavior is incompatible with
+	 * Parts of FOLL_LONGTERM behavior are incompatible with
 	 * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on
-	 * vmas.  As there are no users of this flag in this call we simply
-	 * disallow this option for now.
+	 * vmas. However, this only comes up if locked is set, and there are
+	 * callers that do request FOLL_LONGTERM, but do not set locked. So,
+	 * allow what we can.
 	 */
-	if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM))
-		return -EINVAL;
+	if (gup_flags & FOLL_LONGTERM) {
+		if (WARN_ON_ONCE(locked))
+			return -EINVAL;
+		/*
+		 * This will check the vmas (even if our vmas arg is NULL)
+		 * and return -ENOTSUPP if DAX isn't allowed in this case:
+		 */
+		return __gup_longterm_locked(tsk, mm, start, nr_pages, pages,
+					     vmas, gup_flags | FOLL_TOUCH |
+					     FOLL_REMOTE);
+	}
 
 	return __get_user_pages_locked(tsk, mm, start, nr_pages, pages, vmas,
 				       locked,
-- 
2.24.0


  parent reply index

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-25  4:19 [PATCH 00/19] pin_user_pages(): reduced-risk series for Linux 5.5 John Hubbard
2019-11-25  4:19 ` [PATCH 01/19] mm/gup: factor out duplicate code from four routines John Hubbard
2019-11-25  4:19 ` [PATCH 02/19] mm/gup: move try_get_compound_head() to top, fix minor issues John Hubbard
2019-11-25  4:19 ` [PATCH 03/19] mm: Cleanup __put_devmap_managed_page() vs ->page_free() John Hubbard
2019-11-25  4:19 ` [PATCH 04/19] goldish_pipe: rename local pin_user_pages() routine John Hubbard
2019-11-25  4:19 ` John Hubbard [this message]
2019-11-25  4:19 ` [PATCH 06/19] vfio: fix FOLL_LONGTERM use, simplify get_user_pages_remote() call John Hubbard
2019-11-25  4:19 ` [PATCH 07/19] mm/gup: introduce pin_user_pages*() and FOLL_PIN John Hubbard
2019-11-25  7:47   ` kbuild test robot
2019-11-25  8:44   ` kbuild test robot
2019-11-25 20:13     ` John Hubbard
2019-11-25  4:20 ` [PATCH 08/19] goldish_pipe: convert to pin_user_pages() and put_user_page() John Hubbard
2019-11-25  4:20 ` [PATCH 09/19] IB/{core,hw,umem}: set FOLL_PIN via pin_user_pages*(), fix up ODP John Hubbard
2019-11-25  4:20 ` [PATCH 10/19] mm/process_vm_access: set FOLL_PIN via pin_user_pages_remote() John Hubbard
2019-11-25  4:20 ` [PATCH 11/19] drm/via: set FOLL_PIN via pin_user_pages_fast() John Hubbard
2019-11-25  4:20 ` [PATCH 12/19] fs/io_uring: set FOLL_PIN via pin_user_pages() John Hubbard
2019-11-25  4:20 ` [PATCH 13/19] net/xdp: " John Hubbard
2019-11-25  4:20 ` [PATCH 14/19] media/v4l2-core: set pages dirty upon releasing DMA buffers John Hubbard
2019-11-25  4:20 ` [PATCH 15/19] media/v4l2-core: pin_user_pages (FOLL_PIN) and put_user_page() conversion John Hubbard
2019-11-25  4:20 ` [PATCH 16/19] vfio, mm: " John Hubbard
2019-11-25  4:20 ` [PATCH 17/19] powerpc: book3s64: convert to pin_user_pages() and put_user_page() John Hubbard
2019-11-25  8:59   ` Jan Kara
2019-11-25 20:46     ` John Hubbard
2019-11-25  4:20 ` [PATCH 18/19] mm/gup_benchmark: use proper FOLL_WRITE flags instead of hard-coding "1" John Hubbard
2019-11-25  4:20 ` [PATCH 19/19] mm, tree-wide: rename put_user_page*() to unpin_user_page*() John Hubbard

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