Re: [PATCH 1/4] mm: introduce vma_set_file function v2

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Am 08.10.20 um 16:12 schrieb Daniel Vetter:

On Thu, Oct 08, 2020 at 01:23:39PM +0200, Christian König wrote:

Add the new vma_set_file() function to allow changing
vma->vm_file with the necessary refcount dance.

v2: add more users of this.

Signed-off-by: Christian König <christian.koenig@xxxxxxx>
---
  drivers/dma-buf/dma-buf.c                  | 16 +++++-----------
  drivers/gpu/drm/etnaviv/etnaviv_gem.c      |  4 +---
  drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c |  3 +--
  drivers/gpu/drm/i915/gem/i915_gem_mman.c   |  4 ++--
  drivers/gpu/drm/msm/msm_gem.c              |  4 +---
  drivers/gpu/drm/omapdrm/omap_gem.c         |  3 +--
  drivers/gpu/drm/vgem/vgem_drv.c            |  3 +--
  drivers/staging/android/ashmem.c           |  5 ++---
  include/linux/mm.h                         |  2 ++
  mm/mmap.c                                  | 16 ++++++++++++++++
  10 files changed, 32 insertions(+), 28 deletions(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index a6ba4d598f0e..e4316aa7e0f4 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -1163,20 +1163,14 @@ int dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma,
  		return -EINVAL;
/* readjust the vma */ 
-	get_file(dmabuf->file);
-	oldfile = vma->vm_file;
-	vma->vm_file = dmabuf->file;
+	oldfile = vma_set_file(vma, dmabuf->file);
  	vma->vm_pgoff = pgoff;
ret = dmabuf->ops->mmap(dmabuf, vma); 
-	if (ret) {
-		/* restore old parameters on failure */
-		vma->vm_file = oldfile;
-		fput(dmabuf->file);
-	} else {
-		if (oldfile)
-			fput(oldfile);
-	}
+	/* restore old parameters on failure */
+	if (ret)
+		vma_set_file(vma, oldfile);

I think these two lines here are cargo-cult: If this fails, the mmap fails
and therefore the vma structure is kfreed. No point at all in restoring
anything.


This was explicitly added with this patch to fix a problem:

commit 495c10cc1c0c359871d5bef32dd173252fc17995
Author: John Sheu <sheu@xxxxxxxxxx>
Date:   Mon Feb 11 17:50:24 2013 -0800

    CHROMIUM: dma-buf: restore args on failure of dma_buf_mmap

    Callers to dma_buf_mmap expect to fput() the vma struct's vm_file
    themselves on failure.  Not restoring the struct's data on failure
    causes a double-decrement of the vm_file's refcount.


With that: Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>


Can I keep that even with the error handling working? :)

Christian.




+
  	return ret;
} 
diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gem.c b/drivers/gpu/drm/etnaviv/etnaviv_gem.c
index 312e9d58d5a7..10ce267c0947 100644
--- a/drivers/gpu/drm/etnaviv/etnaviv_gem.c
+++ b/drivers/gpu/drm/etnaviv/etnaviv_gem.c
@@ -145,10 +145,8 @@ static int etnaviv_gem_mmap_obj(struct etnaviv_gem_object *etnaviv_obj,
  		 * address_space (so unmap_mapping_range does what we want,
  		 * in particular in the case of mmap'd dmabufs)
  		 */
-		fput(vma->vm_file);
-		get_file(etnaviv_obj->base.filp);
  		vma->vm_pgoff = 0;
-		vma->vm_file  = etnaviv_obj->base.filp;
+		vma_set_file(vma, etnaviv_obj->base.filp);
vma->vm_page_prot = vm_page_prot; 
  	}
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
index fec0e1e3dc3e..8ce4c9e28b87 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
@@ -119,8 +119,7 @@ static int i915_gem_dmabuf_mmap(struct dma_buf *dma_buf, struct vm_area_struct *
  	if (ret)
  		return ret;
- fput(vma->vm_file); 
-	vma->vm_file = get_file(obj->base.filp);
+	vma_set_file(vma, obj->base.filp);
return 0; 
  }
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_mman.c b/drivers/gpu/drm/i915/gem/i915_gem_mman.c
index 3d69e51f3e4d..c9d5f1a38af3 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_mman.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_mman.c
@@ -893,8 +893,8 @@ int i915_gem_mmap(struct file *filp, struct vm_area_struct *vma)
  	 * requires avoiding extraneous references to their filp, hence why
  	 * we prefer to use an anonymous file for their mmaps.
  	 */
-	fput(vma->vm_file);
-	vma->vm_file = anon;
+	vma_set_file(vma, anon);
+	fput(anon);
switch (mmo->mmap_type) { 
  	case I915_MMAP_TYPE_WC:
diff --git a/drivers/gpu/drm/msm/msm_gem.c b/drivers/gpu/drm/msm/msm_gem.c
index de915ff6f4b4..a71f42870d5e 100644
--- a/drivers/gpu/drm/msm/msm_gem.c
+++ b/drivers/gpu/drm/msm/msm_gem.c
@@ -223,10 +223,8 @@ int msm_gem_mmap_obj(struct drm_gem_object *obj,
  		 * address_space (so unmap_mapping_range does what we want,
  		 * in particular in the case of mmap'd dmabufs)
  		 */
-		fput(vma->vm_file);
-		get_file(obj->filp);
  		vma->vm_pgoff = 0;
-		vma->vm_file  = obj->filp;
+		vma_set_file(vma, obj->filp);
vma->vm_page_prot = vm_get_page_prot(vma->vm_flags); 
  	}
diff --git a/drivers/gpu/drm/omapdrm/omap_gem.c b/drivers/gpu/drm/omapdrm/omap_gem.c
index 979d53a93c2b..0d4542ff1d7d 100644
--- a/drivers/gpu/drm/omapdrm/omap_gem.c
+++ b/drivers/gpu/drm/omapdrm/omap_gem.c
@@ -564,9 +564,8 @@ int omap_gem_mmap_obj(struct drm_gem_object *obj,
  		 * address_space (so unmap_mapping_range does what we want,
  		 * in particular in the case of mmap'd dmabufs)
  		 */
-		fput(vma->vm_file);
  		vma->vm_pgoff = 0;
-		vma->vm_file  = get_file(obj->filp);
+		vma_set_file(vma, obj->filp);
vma->vm_page_prot = vm_get_page_prot(vma->vm_flags); 
  	}
diff --git a/drivers/gpu/drm/vgem/vgem_drv.c b/drivers/gpu/drm/vgem/vgem_drv.c
index fa54a6d1403d..ea0eecae5153 100644
--- a/drivers/gpu/drm/vgem/vgem_drv.c
+++ b/drivers/gpu/drm/vgem/vgem_drv.c
@@ -397,8 +397,7 @@ static int vgem_prime_mmap(struct drm_gem_object *obj,
  	if (ret)
  		return ret;
- fput(vma->vm_file); 
-	vma->vm_file = get_file(obj->filp);
+	vma_set_file(vma, obj->filp);
  	vma->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP;
  	vma->vm_page_prot = pgprot_writecombine(vm_get_page_prot(vma->vm_flags));
diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c 
index 10b4be1f3e78..a51dc089896e 100644
--- a/drivers/staging/android/ashmem.c
+++ b/drivers/staging/android/ashmem.c
@@ -450,9 +450,8 @@ static int ashmem_mmap(struct file *file, struct vm_area_struct *vma)
  		vma_set_anonymous(vma);
  	}
- if (vma->vm_file) 
-		fput(vma->vm_file);
-	vma->vm_file = asma->file;
+	vma_set_file(vma, asma->file);
+	fput(asma->file);
out: 
  	mutex_unlock(&ashmem_mutex);
diff --git a/include/linux/mm.h b/include/linux/mm.h
index ca6e6a81576b..a558602afe1b 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2693,6 +2693,8 @@ static inline void vma_set_page_prot(struct vm_area_struct *vma)
  }
  #endif
+struct file *vma_set_file(struct vm_area_struct *vma, struct file *file); 
+
  #ifdef CONFIG_NUMA_BALANCING
  unsigned long change_prot_numa(struct vm_area_struct *vma,
  			unsigned long start, unsigned long end);
diff --git a/mm/mmap.c b/mm/mmap.c
index 40248d84ad5f..d3c3c510f643 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -136,6 +136,22 @@ void vma_set_page_prot(struct vm_area_struct *vma)
  	WRITE_ONCE(vma->vm_page_prot, vm_page_prot);
  }
+/* 
+ * Change backing file, only valid to use during initial VMA setup.
+ */
+struct file *vma_set_file(struct vm_area_struct *vma, struct file *file)
+{
+	if (file)
+	        get_file(file);
+
+	swap(vma->vm_file, file);
+
+	if (file)
+		fput(file);
+
+	return file;
+}
+
  /*
   * Requires inode->i_mapping->i_mmap_rwsem
   */
--
2.17.1
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