linux-kernel - amusing SLUB compaction bug when CC_OPTIMIZE_FOR

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Message-ID: <ea96c78c-e1dc-1364-e91-51909f82388b@google.com>
Date:   Tue, 27 Sep 2022 22:16:35 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>
cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: amusing SLUB compaction bug when CC_OPTIMIZE_FOR_SIZE

It's a bug in linux-next, but taking me too long to identify which
commit is "to blame", so let me throw it over to you without more
delay: I think __PageMovable() now needs to check !PageSlab().

I had made a small experimental change somewhere, rebuilt and rebooted,
was not surprised to crash once swapping and compaction came in,
but was surprised to find the crash in isolate_movable_page(),
called by compaction's isolate_migratepages_block().

page->mapping was ffffffff811303aa, which qualifies as __PageMovable(),
which expects struct movable_operations at page->mapping minus low bits.
But ffffffff811303aa was the address of SLUB's rcu_free_slab(): I have
CONFIG_CC_OPTIMIZE_FOR_SIZE=y, so function addresses may have low bits set.

Over to you! Thanks,
Hugh