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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id 006d021491bc7-5c1d58ff7e1sm1873751eaf.39.2024.06.25.13.12.28 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 25 Jun 2024 13:12:30 -0700 (PDT) Date: Tue, 25 Jun 2024 13:12:17 -0700 (PDT) From: Hugh Dickins To: David Hildenbrand cc: Hugh Dickins , Andrew Morton , Barry Song <21cnbao@gmail.com>, baolin.wang@linux.alibaba.com, chrisl@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, mhocko@suse.com, ryan.roberts@arm.com, shy828301@gmail.com, surenb@google.com, v-songbaohua@oppo.com, willy@infradead.org, ying.huang@intel.com, yosryahmed@google.com, yuanshuai@oppo.com, yuzhao@google.com Subject: Re: [PATCH mm-unstable] mm: folio_add_new_anon_rmap() careful __folio_set_swapbacked() In-Reply-To: <9e8d71f4-69dc-4bf4-a40d-e1b89586f5c9@redhat.com> Message-ID: References: <0a41d5fc-d1a1-4b1b-873e-a701b20bbcb3@redhat.com> <9e8d71f4-69dc-4bf4-a40d-e1b89586f5c9@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 16B21100007 X-Stat-Signature: axuju33dcesj3k7hqb65ehdtxxdonkut X-Rspam-User: X-HE-Tag: 1719346351-496353 X-HE-Meta: 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 x5nJEBh/ v4nKGfO5YXa4Gi5vvApBwovRGUQM3eoZhGLizEiNr73re2tsrQQVtAL0PMBxq7MxNqxA7LByPYr1J1iNbWVNuQWOyhP0+BJpsFqDpl5uiSW8poWbypwi3UrYSE9Oa1T9vc61lSlFwt+EwbKOwBAouRWX23qiX7ubBvHYMWJD9jEQjRAs+bkQXPIquz2n7MLfZBsaqIXoPJWPejaHvoHM6n6zCTSz92LDB9mDGofBdn/HKwER8/d1c4GpymFVcqSungsJ2XcISqPoHtvXA0U09N7Gwx6rMVcCnniyGjKPO+9v44i6/ysZI8i0d0KPeTQw3+c0s8KxF8ldrqN3jH+gY83YpPS4nLI5mn/8jRDLjVGLSOKoltDNWTkFFG5CY/t/zvCH6 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, 25 Jun 2024, David Hildenbrand wrote: > On 25.06.24 21:37, Hugh Dickins wrote: > > On Tue, 25 Jun 2024, David Hildenbrand wrote: > >> > >> I'll point out that it's sufficient for a PFN walker to do a tryget + > >> trylock > >> to cause trouble. > > > > That surprises me. I thought a racer's tryget was irrelevant (touching > > a different field) and its trylock not a problem, since "we" hold the > > folio lock throughout. If my mental model is too naive there, please > > explain in more detail: we all need to understand this better. > > Sorry, I was imprecise. > > tryget+trylock should indeed not be a problem, tryget+lock would be, because > IIRC folio_wait_bit_common()->folio_set_waiters() would be messing with folio > flags. Interesting observation, thanks. I had imagined that a folio locker was safe, but think you're right that (before the fix) this could have erased its PG_waiters. Typically, I guess something else would come along sooner or later to lock the folio, and that succeed in waking up the earlier one: so probably not an issue that would be detected in testing, but not good. Hugh