Re: [PATCH v2] base: dma-mapping: Postpone cpu addr translation on mmap()

[Christoph Hellwig]

Thanks,

applied to the dma-mapping tree for 4.17.
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[PATCH v2] base: dma-mapping: Postpone cpu addr translation on mmap()

[Jacopo Mondi]

Postpone calling virt_to_page() translation on memory locations not
guaranteed to be backed by a struct page. Try first to map memory from
device's coherent memory pool, then perform translation if that fails.

On some architectures, specifically SH when configured with SPARSEMEM
memory model, assuming a struct page is always assigned to a memory
address lead to unexpected hangs during the virtual to page address
translation. This patch fixes that specific issue but applies in the
general case too.

Suggested-by: Laurent Pinchart 
Signed-off-by: Jacopo Mondi 

---

It has now been clarified this patch does not resolve the issue, but only
mitigate it on platforms where dma_mmap_from_dev_coherent() succeeds and
delay page_to_pfn() faulty conversion.

A suggested proper solution would be not relying on dma_common_mmap() but
require all platforms to implement an mmap methods known to work, as noted
by Christoph in v1 review.

v1 -> v2:
- Save the 'pfn' temp variable performing the page_to_pfn() conversion in the
  remap_pfn_range() function call as suggested by Christoph.

---
 drivers/base/dma-mapping.c | 6 ++
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/drivers/base/dma-mapping.c b/drivers/base/dma-mapping.c
index 3b11835..d82566d 100644
--- a/drivers/base/dma-mapping.c
+++ b/drivers/base/dma-mapping.c
@@ -226,7 +226,6 @@ int dma_common_mmap(struct device *dev, struct 
vm_area_struct *vma,
 #ifndef CONFIG_ARCH_NO_COHERENT_DMA_MMAP
unsigned long user_count = vma_pages(vma);
unsigned long count = PAGE_ALIGN(size) >> PAGE_SHIFT;
-   unsigned long pfn = page_to_pfn(virt_to_page(cpu_addr));
unsigned long off = vma->vm_pgoff;

vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
@@ -234,12 +233,11 @@ int dma_common_mmap(struct device *dev, struct 
vm_area_struct *vma,
if (dma_mmap_from_dev_coherent(dev, vma, cpu_addr, size, ))
return ret;

-   if (off < count && user_count <= (count - off)) {
+   if (off < count && user_count <= (count - off))
ret = remap_pfn_range(vma, vma->vm_start,
- pfn + off,
+ page_to_pfn(virt_to_page(cpu_addr)) + off,
  user_count << PAGE_SHIFT,
  vma->vm_page_prot);
-   }
 #endif /* !CONFIG_ARCH_NO_COHERENT_DMA_MMAP */

return ret;
--
2.7.4

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Re: [PATCH v2] base: dma-mapping: Postpone cpu addr translation on mmap()

[Robin Murphy]

On 13/04/18 18:25, Jacopo Mondi wrote:

Postpone calling virt_to_page() translation on memory locations not
guaranteed to be backed by a struct page. Try first to map memory from
device's coherent memory pool, then perform translation if that fails.

On some architectures, specifically SH when configured with SPARSEMEM
memory model, assuming a struct page is always assigned to a memory
address lead to unexpected hangs during the virtual to page address
translation. This patch fixes that specific issue but applies in the
general case too.


Reviewed-by: Robin Murphy 


Suggested-by: Laurent Pinchart 
Signed-off-by: Jacopo Mondi 

---

It has now been clarified this patch does not resolve the issue, but only
mitigate it on platforms where dma_mmap_from_dev_coherent() succeeds and
delay page_to_pfn() faulty conversion.

A suggested proper solution would be not relying on dma_common_mmap() but
require all platforms to implement an mmap methods known to work, as noted
by Christoph in v1 review.


Note that that "proper solution" should still involve having 
dma_common_mmap() since we certainly don't want an explosion of code 
duplication. It just means that architectures that do use it should be 
defining their dma_map_ops with an explicit ".mmap = dma_common_mmap" 
instead of relying on dma_mmap_attrs() calling it by default. Thus the 
more architectures this implementation *is* definitely safe for, the 
better :)


Robin.


v1 -> v2:
- Save the 'pfn' temp variable performing the page_to_pfn() conversion in the
   remap_pfn_range() function call as suggested by Christoph.

---
  drivers/base/dma-mapping.c | 6 ++
  1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/drivers/base/dma-mapping.c b/drivers/base/dma-mapping.c
index 3b11835..d82566d 100644
--- a/drivers/base/dma-mapping.c
+++ b/drivers/base/dma-mapping.c
@@ -226,7 +226,6 @@ int dma_common_mmap(struct device *dev, struct 
vm_area_struct *vma,
  #ifndef CONFIG_ARCH_NO_COHERENT_DMA_MMAP
unsigned long user_count = vma_pages(vma);
unsigned long count = PAGE_ALIGN(size) >> PAGE_SHIFT;
-   unsigned long pfn = page_to_pfn(virt_to_page(cpu_addr));
unsigned long off = vma->vm_pgoff;

vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
@@ -234,12 +233,11 @@ int dma_common_mmap(struct device *dev, struct 
vm_area_struct *vma,
if (dma_mmap_from_dev_coherent(dev, vma, cpu_addr, size, ))
return ret;

-   if (off < count && user_count <= (count - off)) {
+   if (off < count && user_count <= (count - off))
ret = remap_pfn_range(vma, vma->vm_start,
- pfn + off,
+ page_to_pfn(virt_to_page(cpu_addr)) + off,
  user_count << PAGE_SHIFT,
  vma->vm_page_prot);
-   }
  #endif/* !CONFIG_ARCH_NO_COHERENT_DMA_MMAP */

return ret;
--
2.7.4


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