Re: [PATCH] mm: fix page_lock_anon_vma leaving mutex locked

On Sat, 2011-05-28 at 13:20 -0700, Hugh Dickins wrote:

On one machine I've been getting hangs, a page fault's anon_vma_prepare()
waiting in anon_vma_lock(), other processes waiting for that page's lock.

This is a replay of last year's f18194275c39
"mm: fix hang on anon_vma->root->lock".

The new page_lock_anon_vma() places too much faith in its refcount: when
it has acquired the mutex_trylock(), it's possible that a racing task in
anon_vma_alloc() has just reallocated the struct anon_vma, set refcount
to 1, and is about to reset its anon_vma->root.

Fix this by saving anon_vma->root, and relying on the usual page_mapped()
check instead of a refcount check: if page is still mapped, the anon_vma
is still ours; if page is not still mapped, we're no longer interested.

Interesting race.. but can we guarantee that the page didn't get
remapped meanwhile?

The updated comment by page_get_anon_vma() describes the lack of
serialization against page_remove_rmap() but fails to mention the
page_add_anon_rmap cases (bad me, I know I checked at the time, but
can't for the life of me remember what it was now).

_IFF_ we are serialized, your patch should suffice, since then
page_mapped() implies a >0 refcount, if not however, I think we need
both tests since in that case the page might be mapped again against a
different anon_vma and our current anon_vma (the one we locked against)
might have refcount == 0 and already be past the mutex_is_locked() test
in anon_vma_free(), at which point we're up shit creek since then the
anon_vma we're returning can disappear the moment we do
rcu_read_unlock().

Or am I delusional due to lack of sleep?

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