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Subject: Re: [PATCH 1/4] mm: introduce vma_set_file function v2
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From:   =?UTF-8?Q?Christian_K=c3=b6nig?= <ckoenig.leichtzumerken@gmail.com>
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Date:   Fri, 9 Oct 2020 09:16:49 +0200
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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@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.

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

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
>>