* Andrew Morton akpm@linux-foundation.org [230327 15:38]:
On Mon, 27 Mar 2023 14:55:32 -0400 "Liam R. Howlett" Liam.Howlett@oracle.com wrote:
Use the maple tree in RCU mode for VMA tracking.
The maple tree tracks the stack and is able to update the pivot (lower/upper boundary) in-place to allow the page fault handler to write to the tree while holding just the mmap read lock. This is safe as the writes to the stack have a guard VMA which ensures there will always be a NULL in the direction of the growth and thus will only update a pivot.
It is possible, but not recommended, to have VMAs that grow up/down without guard VMAs. syzbot has constructed a testcase which sets up a VMA to grow and consume the empty space. Overwriting the entire NULL entry causes the tree to be altered in a way that is not safe for concurrent readers; the readers may see a node being rewritten or one that does not match the maple state they are using.
Enabling RCU mode allows the concurrent readers to see a stable node and
This differs from what had. Intended?
Yes, this is not necessary. The scope of this tree is limited to the function do_vmi_align_munmap() and so we don't need to free the nodes with RCU.
Thanks, Liam
--- a/mm/mmap.c~mm-enable-maple-tree-rcu-mode-by-default-v8 +++ a/mm/mmap.c @@ -2277,8 +2277,7 @@ do_vmi_align_munmap(struct vma_iterator int count = 0; int error = -ENOMEM; MA_STATE(mas_detach, &mt_detach, 0, 0);
- mt_init_flags(&mt_detach, vmi->mas.tree->ma_flags &
(MT_FLAGS_LOCK_MASK | MT_FLAGS_USE_RCU));
- mt_init_flags(&mt_detach, vmi->mas.tree->ma_flags & MT_FLAGS_LOCK_MASK); mt_set_external_lock(&mt_detach, &mm->mmap_lock);
/* _