* Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> [230327 15:38]: > On Mon, 27 Mar 2023 14:55:32 -0400 "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx> 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); > > /* > _ >
