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

On Fri, Oct 09, 2020 at 09:16:49AM +0200, Christian König wrote:

Am 08.10.20 um 16:12 schrieb Daniel Vetter:

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

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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@amd.com

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@google.com 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.

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With that: Reviewed-by: Daniel Vetter daniel.vetter@ffwll.ch

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

Hm good find, I should have looked at git history myself.

I just noticed this here in the patch because everyone else does not do this. But looking at the mmap_region() code in mmap.c we seem to indeed have this problem for the error path:

unmap_and_free_vma: vma->vm_file = NULL; fput(file);

Note that the success path does things correctly (a bit above):

file = vma->vm_file; out:

So it indeed looks like dma-buf is the only one that does this fully correctly. So maybe we should do a follow-up patch to change the mmap_region exit code to pick up whatever vma->vm_file was set instead, and fput that?

Anyway I correct, r-b: as-is.

Cheers, Daniel

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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