[PATCH v4 01/13] drm/shmem-helper: Simplify page offset calculation in fault handler

[Loïc Molinari]

For a fault at address addr, the page offset is
  page_offset = (vmf->address - vma->vm_start) >> PAGE_SHIFT
              = ((addr & PAGE_MASK) - vma->vm_start) >> PAGE_SHIFT
              = (addr - vma->vm_start) >> PAGE_SHIFT

Since the faulty logical page offset based on VMA is
  vmf->pgoff = vma->vm_pgoff + ((addr - vma->vm_start) >> PAGE_SHIFT)

We can slightly simplify the calculation using
  page_offset = vmf->pgoff - vma->vm_pgoff

Signed-off-by: Loïc Molinari <loic.molin...@collabora.com>
---
 drivers/gpu/drm/drm_gem_shmem_helper.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c 
b/drivers/gpu/drm/drm_gem_shmem_helper.c
index dc94a27710e5..be89be1c804c 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -577,8 +577,8 @@ static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
        struct page *page;
        pgoff_t page_offset;
 
-       /* We don't use vmf->pgoff since that has the fake offset */
-       page_offset = (vmf->address - vma->vm_start) >> PAGE_SHIFT;
+       /* Offset to faulty address in the VMA (without the fake offset). */
+       page_offset = vmf->pgoff - vma->vm_pgoff;
 
        dma_resv_lock(shmem->base.resv, NULL);
 
-- 
2.47.3