On Thu, Aug 10, 2023 at 06:24:15AM +0000, Suren Baghdasaryan wrote:
> Ok, I think I found the issue. wp_page_shared() ->
> fault_dirty_shared_page() can drop mmap_lock (see the comment saying
> "Drop the mmap_lock before waiting on IO, if we can...", therefore we
> have to ensure we are not doing this under per-VMA lock.
... or we could change maybe_unlock_mmap_for_io() the same way
that we changed folio_lock_or_retry():
+++ b/mm/internal.h
@@ -706,7 +706,7 @@ static inline struct file *maybe_unlock_mmap_for_io(struct vm_fault *vmf,
if (fault_flag_allow_retry_first(flags) &&
!(flags & FAULT_FLAG_RETRY_NOWAIT)) {
fpin = get_file(vmf->vma->vm_file);
- mmap_read_unlock(vmf->vma->vm_mm);
+ release_fault_lock(vmf);
}
return fpin;
}
What do you think?
> I think what happens is that this path is racing with another page
> fault which took mmap_lock for read. fault_dirty_shared_page()
> releases this lock which was taken by another page faulting thread and
> that thread generates an assertion when it finds out the lock it just
> took got released from under it.
I'm confused that our debugging didn't catch this earlier. lockdep
should always catch this.
