Re: [PATCH 29/31] parisc: handle page-less SG entries

[Dan Williams]

On Thu, Aug 13, 2015 at 9:11 PM, David Miller da...@davemloft.net wrote:
 From: James Bottomley james.bottom...@hansenpartnership.com
 At least on some PA architectures, you have to be very careful.
 Improperly managed, multiple aliases will cause the system to crash
 (actually a machine check in the cache chequerboard). For the most
 temperamental systems, we need the cache line flushed and the alias
 mapping ejected from the TLB cache before we access the same page at an
 inequivalent alias.

 Also, I want to mention that on sparc64 we manage the cache aliasing
 state in the page struct.

 Until a page is mapped into userspace, we just record the most recent
 cpu to store into that page with kernel side mappings.  Once the page
 ends up being mapped or the cpu doing kernel side stores changes, we
 actually perform the cache flush.

 Generally speaking, I think that all actual physical memory the kernel
 operates on should have a struct page backing it.  So this whole
 discussion of operating on physical memory in scatter lists without
 backing page structs feels really foreign to me.

So the only way for page-less pfns to enter the system is through the
-direct_access() method provided by a pmem device's struct
block_device_operations.  Architectures that require struct page for
cache management to must disable -direct_access() in this case.

If an arch still wants to support pmem+DAX then it needs something
like this patchset (feedback welcome) to map pmem pfns:

https://lkml.org/lkml/2015/8/12/970

Effectively this would disable -direct_access() on /dev/pmem0, but
permit -direct_access() on /dev/pmem0m.
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[PATCH 29/31] parisc: handle page-less SG entries

[Christoph Hellwig]

Make all cache invalidation conditional on sg_has_page() and use
sg_phys to get the physical address directly.

Signed-off-by: Christoph Hellwig h...@lst.de
---
 arch/parisc/kernel/pci-dma.c | 29 ++---
 1 file changed, 18 insertions(+), 11 deletions(-)

diff --git a/arch/parisc/kernel/pci-dma.c b/arch/parisc/kernel/pci-dma.c
index b9402c9..6cad0e0 100644
--- a/arch/parisc/kernel/pci-dma.c
+++ b/arch/parisc/kernel/pci-dma.c
@@ -483,11 +483,13 @@ static int pa11_dma_map_sg(struct device *dev, struct 
scatterlist *sglist, int n
BUG_ON(direction == DMA_NONE);
 
for_each_sg(sglist, sg, nents, i) {
-   unsigned long vaddr = (unsigned long)sg_virt(sg);
-
-   sg_dma_address(sg) = (dma_addr_t) virt_to_phys(vaddr);
+   sg_dma_address(sg) = sg_phys(sg);
sg_dma_len(sg) = sg-length;
-   flush_kernel_dcache_range(vaddr, sg-length);
+
+   if (sg_has_page(sg)) {
+   flush_kernel_dcache_range((unsigned long)sg_virt(sg),
+ sg-length);
+   }
}
return nents;
 }
@@ -504,9 +506,10 @@ static void pa11_dma_unmap_sg(struct device *dev, struct 
scatterlist *sglist, in
 
/* once we do combining we'll need to use 
phys_to_virt(sg_dma_address(sglist)) */
 
-   for_each_sg(sglist, sg, nents, i)
-   flush_kernel_vmap_range(sg_virt(sg), sg-length);
-   return;
+   for_each_sg(sglist, sg, nents, i) {
+   if (sg_has_page(sg))
+   flush_kernel_vmap_range(sg_virt(sg), sg-length);
+   }
 }
 
 static void pa11_dma_sync_single_for_cpu(struct device *dev, dma_addr_t 
dma_handle, unsigned long offset, size_t size, enum dma_data_direction 
direction)
@@ -530,8 +533,10 @@ static void pa11_dma_sync_sg_for_cpu(struct device *dev, 
struct scatterlist *sgl
 
/* once we do combining we'll need to use 
phys_to_virt(sg_dma_address(sglist)) */
 
-   for_each_sg(sglist, sg, nents, i)
-   flush_kernel_vmap_range(sg_virt(sg), sg-length);
+   for_each_sg(sglist, sg, nents, i) {
+   if (sg_has_page(sg))
+   flush_kernel_vmap_range(sg_virt(sg), sg-length);
+   }
 }
 
 static void pa11_dma_sync_sg_for_device(struct device *dev, struct scatterlist 
*sglist, int nents, enum dma_data_direction direction)
@@ -541,8 +546,10 @@ static void pa11_dma_sync_sg_for_device(struct device 
*dev, struct scatterlist *
 
/* once we do combining we'll need to use 
phys_to_virt(sg_dma_address(sglist)) */
 
-   for_each_sg(sglist, sg, nents, i)
-   flush_kernel_vmap_range(sg_virt(sg), sg-length);
+   for_each_sg(sglist, sg, nents, i) {
+   if (sg_has_page(sg))
+   flush_kernel_vmap_range(sg_virt(sg), sg-length);
+   }
 }
 
 struct hppa_dma_ops pcxl_dma_ops = {
-- 
1.9.1

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