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Tue, 15 Oct 2024 10:32:44 -0400 (EDT) Date: Tue, 15 Oct 2024 17:32:39 +0300 From: "Kirill A. Shutemov" To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, syzbot+7d917f67c05066cec295@syzkaller.appspotmail.com, Andrew Morton , Jann Horn Subject: Re: [PATCH v1] mm/pagewalk: fix usage of pmd_leaf()/pud_leaf() without present check Message-ID: References: <20241015111236.1290921-1-david@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20241015111236.1290921-1-david@redhat.com> X-Stat-Signature: tqnbh1czduhfffpgxtawf3scrhyjsnbc X-Rspamd-Queue-Id: 1DCB0A002D X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1729002763-266608 X-HE-Meta: U2FsdGVkX1/j/6ljAFPEGv8YUfmA70s/NBjKQljJ4Fm6Uh3eQNTmcjx5lPQq5obLul3AYzsw6NYjxmap7/Vq0cel2AxML+HqocZQqWuPxRprZva0ZZcwPGTNrtViwiX6iziqrp32n7ooYWYegl0/98JCm6ciEBlkef3275dHk3oqGLrdXvZ3HWhwuoMsxPi7B3WlTIrbLki917m2pBgjhQEtMcyZZ0T1/vGmWLSf0xluByymt3HQhYjc+l1Ni7si4+Rdn3rheCpGF2/MtjPNnCafcQCUcw1Wn9bHuytAiQ/O/jJ1nkMdZ0N7TxKNjBzAyxrz1LOd3E+Qjvoo2FbyQzlaEuVNSfYoXMT6+ohbJ9RqFv2gtAVE3BLZD7M/vEYWP1XLUCIPeUAYo38BvFBCsz+LUds8+FpTR+f/lmK7cVmdO+zK6IrQ2HhsnfJeq5LYfTJglnkjhkcZGK8JDEV+/1CJv0amp+uNKquZnrKzGJZ3LJxu+UaE+fF8QGtKGUz9jJoACbtdyCy7wrW0u/hESR4YRpi4Xwi1ZuOkbC9YuMYYK6tahNfmg6HWX4QbiJGtm8VsXeHact4+s/mFFHmE7s8CY77IGW2orlS+x8Y31jd5uW7hGtmbMR2G77ASol06xk1TFHXZ3LsdX5Invbv6Gua2Uzd6IKn7JP5n1zGIt1oMF+zLiK3QGANiZk/xr5dBv3F3Ih6BI4sKIeu7Boz6vySLtM8lTsmp3xmHA7XTAcSCRY5l7hhLzvRXLjt0H1krVhaiB2DUzDtFRDdnRqA5Uwd6qyjw7hJEfHAOGV57KJ1eR61ElE8r9tK3FNRQVGN6u/WakQow8ZnLFLflJuQSEv0Tu0LnlZBKdHK2ly/Kn7hh3jOP1qQpGiOclkcJ/2DXp8oS9XZFzIMxbTaBPHpOI/AM34Aa/5fwBQaIeb0m3PT0W1MZBOpazF0eYETHffStFbdMnh9yoRLqsR72Hdy 06zwQcv9 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Oct 15, 2024 at 01:12:36PM +0200, David Hildenbrand wrote: > pmd_leaf()/pud_leaf() only implies a pmd_present()/pud_present() check on > some architectures. Should we clarify what behaviour we actually want from arch code? > We really should check for > pmd_present()/pud_present() first. > > This should explain the report we got on ppc64 (which has > CONFIG_PGTABLE_HAS_HUGE_LEAVES set in the config) that triggered: > VM_WARN_ON_ONCE(pmd_leaf(pmdp_get_lockless(pmdp))); > > Likely we had a PMD migration entry for which pmd_leaf() did not > trigger. We raced with restoring the PMD migration entry, and suddenly > saw a pmd_leaf(). In this case, pte_offset_map_lock() saved us from more > trouble, because it rechecks the PMD value, but we would not have processed > the migration entry -- which is not too bad because the only user of > FW_MIGRATION is KSM for unsharing, and KSM only applies to small folios. > > Further, we shouldn't re-read the PMD/PUD value for our warning, the > primary purpose of the VM_WARN_ON_ONCE() is to find spurious use of > pmd_leaf()/pud_leaf() without CONFIG_PGTABLE_HAS_HUGE_LEAVES. > > As a side note, we are currently not implementing FW_MIGRATION support > for PUD migration entries, which likely should exist due to hugetlb. Add > a TODO so this won't fall through the cracks if more FW_MIGRATION users > get added. > > Fixes: aa39ca6940f1 ("mm/pagewalk: introduce folio_walk_start() + folio_walk_end()") > Reported-by: syzbot+7d917f67c05066cec295@syzkaller.appspotmail.com > Closes: https://lkml.kernel.org/r/670d3248.050a0220.3e960.0064.GAE@google.com > Cc: Andrew Morton > Cc: Jann Horn > Signed-off-by: David Hildenbrand Acked-by: Kirill A. Shutemov -- Kiryl Shutsemau / Kirill A. Shutemov