FAILED: patch "[PATCH] drm/shmem-helper: Avoid vm_open error paths" failed to apply to 5.4-stable tree

The patch below does not apply to the 5.4-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

Possible dependencies:

09bf649a7457 ("drm/shmem-helper: Avoid vm_open error paths") 526408357318 ("drm/shmem-helpers: Ensure get_pages is not called on imported dma-buf")

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 09bf649a74573cb596e211418a4f8008f265c5a9 Mon Sep 17 00:00:00 2001
From: Rob Clark <[email protected]>
Date: Wed, 30 Nov 2022 10:57:48 -0800
Subject: [PATCH] drm/shmem-helper: Avoid vm_open error paths

vm_open() is not allowed to fail. Fortunately we are guaranteed that the pages are already pinned, thanks to the initial mmap which is now being cloned into a forked process, and only need to increment the refcnt. So just increment it directly. Previously if a signal was delivered at the wrong time to the forking process, the mutex_lock_interruptible() could fail resulting in the pages_use_count not being incremented.

index 3b7b71391a4c..b602cd72a120 100644
@@ -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 when the buffer was first
+	 * mmap'd, vm_open() just grabs an additional reference for the new
+	 * mm the vma is getting copied into (ie. on fork()).
+	 */
+	if (!WARN_ON_ONCE(!shmem->pages_use_count))
+		shmem->pages_use_count++;
+
+	mutex_unlock(&shmem->pages_lock);
 
 	drm_gem_vm_open(vma);
 }