Re: [PATCH] mm/damon/vaddr: remove redundant RCU lock

[Igor Putko]

Hi SJ,

Thanks for the reply and the pointers!
Regarding the issue Sashiko AI flagged - I dug into damon/vaddr.c, and I think it's a real bug, but it's isolated to __damon_va_three_regions().
damon_va_walk_page_range() is fine here, since lock_vma_under_rcu() handles stability on its own. __damon_va_three_regions() is the problematic one: right after mmap_read_lock(mm) is taken in the caller, it sets up a VMA_ITERATOR and runs for_each_vma(). But mmap_read_lock doesn't stop the OOM reaper from running concurrently, setting MMF_UNSTABLE, and leaving empty markers in the maple tree. Walking the tree in that state without a check_stable_address_space(mm) call could lead to a NULL pointer dereference.
Since this looks like a separate issue, I'll prepare a dedicated fix and send it as its own patch thread, so it doesn't hold up the current cleanup. Let me know if that approach works for you.

Thanks,
Igor