linux-kernel - Re: Using userfaultfd with KVM's async page fault handling causes processes to hung waiting for mmap

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Message-ID: <20230719211631.890995-1-axelrasmussen@google.com>
Date:   Wed, 19 Jul 2023 14:16:31 -0700
From:   Axel Rasmussen <axelrasmussen@...gle.com>
To:     Dimitris Siakavaras <jimsiak@...ab.ece.ntua.gr>
Cc:     viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Peter Xu <peterx@...hat.com>,
        linux-mm@...ck.org, Axel Rasmussen <axelrasmussen@...gle.com>
Subject: Re: Using userfaultfd with KVM's async page fault handling causes
 processes to hung waiting for mmap_lock to be released

Thanks for the detailed report Dimitris! I've CCed the MM mailing list and some
folks who work on userfaultfd.

I took a look at this today, but I haven't quite come up with a solution.

I thought it might be as easy as changing userfaultfd_release() to set released
*after* taking the lock. But no such luck, the ordering is what it is to deal
with another subtle case:

	WRITE_ONCE(ctx->released, true);

	if (!mmget_not_zero(mm))
		goto wakeup;

	/*
	 * Flush page faults out of all CPUs. NOTE: all page faults
	 * must be retried without returning VM_FAULT_SIGBUS if
	 * userfaultfd_ctx_get() succeeds but vma->vma_userfault_ctx
	 * changes while handle_userfault released the mmap_lock. So
	 * it's critical that released is set to true (above), before
	 * taking the mmap_lock for writing.
	 */
	mmap_write_lock(mm);

I think perhaps the right thing to do is to have handle_userfault() release
mmap_lock when it returns VM_FAULT_NOPAGE, and to have GUP deal with that
appropriately? But, some investigation is required to be sure that's okay to do
in the other non-GUP ways we can end up in handle_userfault().