mm: assert the mmap_lock is held in __anon_vma_prepare()
Patch series "Improve anon_vma scalability for anon VMAs".
We have a 3x throughput improvement reported by Intel's kernel test robot:
https://lore.kernel.org/all/202404261055.c5e24608-oliver.sang@intel.com/
This is from delaying taking the mmap_lock for page faults until we
actually need the mmap_lock in order to assign an anon_vma to the vma. It
cleans up the page fault path a little by making the anon fault handler
more similar to the file fault handler.
This patch (of 4):
Convert the comment into an assertion.
Link: https://lkml.kernel.org/r/20240426144506.1290619-1-willy@infradead.org
Link: https://lkml.kernel.org/r/20240426144506.1290619-2-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Jann Horn <jannh@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
diff --git a/mm/rmap.c b/mm/rmap.c
index 2608c40..619d4d6 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -182,8 +182,6 @@ static void anon_vma_chain_link(struct vm_area_struct *vma,
* for the new allocation. At the same time, we do not want
* to do any locking for the common case of already having
* an anon_vma.
- *
- * This must be called with the mmap_lock held for reading.
*/
int __anon_vma_prepare(struct vm_area_struct *vma)
{
@@ -191,6 +189,7 @@ int __anon_vma_prepare(struct vm_area_struct *vma)
struct anon_vma *anon_vma, *allocated;
struct anon_vma_chain *avc;
+ mmap_assert_locked(mm);
might_sleep();
avc = anon_vma_chain_alloc(GFP_KERNEL);
