linux-kernel - [PATCH 2/2] drm/shmem-helper: Avoid vm

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Message-Id: <20221129200242.298120-3-robdclark@gmail.com>
Date:   Tue, 29 Nov 2022 12:02:42 -0800
From:   Rob Clark <robdclark@...il.com>
To:     dri-devel@...ts.freedesktop.org
Cc:     Daniel Vetter <daniel@...ll.ch>,
        Rob Clark <robdclark@...omium.org>, stable@...r.kernel.org,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>,
        Eric Anholt <eric@...olt.net>,
        Noralf Trønnes <noralf@...nnes.org>,
        Rob Herring <robh@...nel.org>,
        linux-kernel@...r.kernel.org (open list)
Subject: [PATCH 2/2] drm/shmem-helper: Avoid vm_open error paths

From: Rob Clark <robdclark@...omium.org>

vm_open() is not allowed to fail.  Fortunately we are guaranteed that
the pages are already pinned, and only need to increment the refcnt.  So
just increment it directly.

Fixes: 2194a63a818d ("drm: Add library for shmem backed GEM objects")
Cc: stable@...r.kernel.org
Signed-off-by: Rob Clark <robdclark@...omium.org>
---
 drivers/gpu/drm/drm_gem_shmem_helper.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
index 110a9eac2af8..9885ba64127f 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -571,12 +571,20 @@ static void drm_gem_shmem_vm_open(struct vm_area_struct *vma)
 {
 	struct drm_gem_object *obj = vma->vm_private_data;
 	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
-	int ret;
 
 	WARN_ON(shmem->base.import_attach);
 
-	ret = drm_gem_shmem_get_pages(shmem);
-	WARN_ON_ONCE(ret != 0);
+	mutex_lock(&shmem->pages_lock);
+
+	/*
+	 * We should have already pinned the pages, vm_open() just grabs
+	 * an additional reference for the new mm the vma is getting
+	 * copied into.
+	 */
+	WARN_ON_ONCE(!shmem->pages_use_count);
+
+	shmem->pages_use_count++;
+	mutex_unlock(&shmem->pages_lock);
 
 	drm_gem_vm_open(vma);
 }
-- 
2.38.1