[PATCH 22/64] mm: avoid mmap_sem trylock in vm_insert_page()

[Davidlohr Bueso]

From: Davidlohr Bueso <dave@xxxxxxxxxxxx>

The rules to this function state that mmap_sem must
be acquired by the caller:

- for write if used in f_op->mmap() (by far the most common case)
- for read if used from vma_op->fault()(with VM_MIXEDMAP)

The only exception is:
  mmap_vmcore()
   remap_vmalloc_range_partial()
      mmap_vmcore()

But there is no concurrency here, thus mmap_sem is not held.
After auditing the kernel, the following drivers use the fault
path and correctly set VM_MIXEDMAP):

.fault = etnaviv_gem_fault
.fault = udl_gem_fault
tegra_bo_fault()

As such, drop the reader trylock BUG_ON() for the common case.
This avoids having file_operations know about mmranges, as
mmap_sem is held during, mmap() for example.

Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
---
 mm/memory.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index 5adcdc7dee80..7c69674cd9da 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1773,7 +1773,6 @@ int vm_insert_page(struct vm_area_struct *vma, unsigned long addr,
 	if (!page_count(page))
 		return -EINVAL;
 	if (!(vma->vm_flags & VM_MIXEDMAP)) {
-		BUG_ON(down_read_trylock(&vma->vm_mm->mmap_sem));
 		BUG_ON(vma->vm_flags & VM_PFNMAP);
 		vma->vm_flags |= VM_MIXEDMAP;
 	}
-- 
2.13.6