[PATCH] uprobes: __replace_page() avoid BUG in munlock_vma_page()

[Hugh Dickins]

syzbot crashed on the VM_BUG_ON_PAGE(PageTail) in munlock_vma_page(),
when called from uprobes __replace_page().  Which of many ways to fix it?
Settled on not calling when PageCompound (since Head and Tail are equals
in this context, PageCompound the usual check in uprobes.c, and the prior
use of FOLL_SPLIT_PMD will have cleared PageMlocked already).

Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx>
Fixes: 5a52c9df62b4 ("uprobe: use FOLL_SPLIT_PMD instead of FOLL_SPLIT")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # v5.4+
---
This one is not a 5.9-rc regression, but still good to fix.

 kernel/events/uprobes.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- v5.9-rc/kernel/events/uprobes.c	2020-08-12 19:46:50.851196584 -0700
+++ linux/kernel/events/uprobes.c	2020-08-16 13:18:35.292821674 -0700
@@ -205,7 +205,7 @@ static int __replace_page(struct vm_area
 		try_to_free_swap(old_page);
 	page_vma_mapped_walk_done(&pvmw);
 
-	if (vma->vm_flags & VM_LOCKED)
+	if ((vma->vm_flags & VM_LOCKED) && !PageCompound(old_page))
 		munlock_vma_page(old_page);
 	put_page(old_page);