patches/old/mm-enable-maple-tree-rcu-mode-by-default.patch

From: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Subject: mm: enable maple tree RCU mode by default
Date: Mon, 27 Feb 2023 09:36:07 -0800

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
will return the expected result.

[Liam.Howlett@Oracle.com: we don't need to free the nodes with RCU[
Link: https://lore.kernel.org/linux-mm/000000000000b0a65805f663ace6@google.com/
Link: https://lkml.kernel.org/r/20230227173632.3292573-9-surenb@google.com
Fixes: d4af56c5c7c6 ("mm: start tracking VMAs with maple tree")
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reported-by: syzbot+8d95422d3537159ca390@syzkaller.appspotmail.com
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/mm_types.h |    3 ++-
 kernel/fork.c            |    3 +++
 mm/mmap.c                |    3 ++-
 3 files changed, 7 insertions(+), 2 deletions(-)

--- a/include/linux/mm_types.h~mm-enable-maple-tree-rcu-mode-by-default
+++ a/include/linux/mm_types.h
@@ -774,7 +774,8 @@ struct mm_struct {
 	unsigned long cpu_bitmap[];
 };
 
-#define MM_MT_FLAGS	(MT_FLAGS_ALLOC_RANGE | MT_FLAGS_LOCK_EXTERN)
+#define MM_MT_FLAGS	(MT_FLAGS_ALLOC_RANGE | MT_FLAGS_LOCK_EXTERN | \
+			 MT_FLAGS_USE_RCU)
 extern struct mm_struct init_mm;
 
 /* Pointer magic because the dynamic array size confuses some compilers. */
--- a/kernel/fork.c~mm-enable-maple-tree-rcu-mode-by-default
+++ a/kernel/fork.c
@@ -617,6 +617,7 @@ static __latent_entropy int dup_mmap(str
 	if (retval)
 		goto out;
 
+	mt_clear_in_rcu(vmi.mas.tree);
 	for_each_vma(old_vmi, mpnt) {
 		struct file *file;
 
@@ -700,6 +701,8 @@ static __latent_entropy int dup_mmap(str
 	retval = arch_dup_mmap(oldmm, mm);
 loop_out:
 	vma_iter_free(&vmi);
+	if (!retval)
+		mt_set_in_rcu(vmi.mas.tree);
 out:
 	mmap_write_unlock(mm);
 	flush_tlb_mm(oldmm);
--- a/mm/mmap.c~mm-enable-maple-tree-rcu-mode-by-default
+++ a/mm/mmap.c
@@ -2277,7 +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, MT_FLAGS_LOCK_EXTERN);
+	mt_init_flags(&mt_detach, vmi->mas.tree->ma_flags & MT_FLAGS_LOCK_MASK);
 	mt_set_external_lock(&mt_detach, &mm->mmap_lock);
 
 	/*
@@ -3037,6 +3037,7 @@ void exit_mmap(struct mm_struct *mm)
 	 */
 	set_bit(MMF_OOM_SKIP, &mm->flags);
 	mmap_write_lock(mm);
+	mt_clear_in_rcu(&mm->mm_mt);
 	free_pgtables(&tlb, &mm->mm_mt, vma, FIRST_USER_ADDRESS,
 		      USER_PGTABLES_CEILING);
 	tlb_finish_mmu(&tlb);
_