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[124.209.175.111]) by smtp.gmail.com with ESMTPSA id fv23-20020a17090b0e9700b0027df6ff00eesm509871pjb.19.2023.10.18.21.33.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 18 Oct 2023 21:33:12 -0700 (PDT) Date: Thu, 19 Oct 2023 13:33:05 +0900 From: Sergey Senozhatsky To: Mike Kravetz Cc: Nick Desaulniers , Nathan Chancellor , Usama Arif , linux-mm@kvack.org, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, muchun.song@linux.dev, songmuchun@bytedance.com, fam.zheng@bytedance.com, liangma@liangbit.com, punit.agrawal@bytedance.com, Konrad Dybcio , llvm@lists.linux.dev Subject: Re: [PATCH] mm: hugetlb: Only prep and add allocated folios for non-gigantic pages Message-ID: <20231019043305.GB14965@google.com> References: <20231009145605.2150897-1-usama.arif@bytedance.com> <20231010012345.GA108129@monkey> <20231012000327.GA1855399@dev-arch.thelio-3990X> <20231012145318.GA5127@monkey> <20231013001203.GA3812@monkey> <20231014000450.GA253713@monkey> <20231018222003.GA21776@monkey> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20231018222003.GA21776@monkey> X-Rspamd-Queue-Id: A7D6A1C0009 X-Rspam-User: X-Rspamd-Server: rspam04 X-Stat-Signature: 7o4uoza51x4ut9zp1x5o16dru7pmt1t8 X-HE-Tag: 1697689993-969809 X-HE-Meta: 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 cba+L4td yFrg3jHdGvmRCCge4mkwu7Ki7Y9OsKHDdZtP3harha88aataOZXcwykNqJA6ZtAq6Dfg5t/fDHKumXMl3llrFUH+z/TL3JOYNOm+eF32CHGWUBWBqziup1BQMXwCHb0p6UrGNOo2b578ljIzREyYTBhevbZ4wktShNvqOkJ8lS82EMgsVlF9mdHW0mQzQGQ6ToXdLhwL66VMzqf+z01WhIXV4ZZv2R8CVR5Rit7c74JAhYPHs23qDTMrCrNZvPSaNWCwToiN4Qnq+oAzC4/5/bKlbqT9dk4ehfW343wf25jqTL08JNkHBIqqd6+9L2giOF8f8MWVqpADHYuq5JPWOEdpEwA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000081, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On (23/10/18 15:20), Mike Kravetz wrote: > > I think you need to initialize h, otherwise what value is passed to > > prep_and_add_bootmem_folios if the loop is not run because the list is > > empty. The compiler sees `h` is only given a value in the loop, so > > the loop must be run. That's obviously hazardous, but the compiler > > assumes there's no UB. At least that's my limited understanding > > looking at the IR diff Nathan got me in > > https://github.com/ClangBuiltLinux/linux/issues/1946. > > Thanks for looking closer at this Nick and Nathan! > > I think you are saying the compiler is running the loop because it wants > to initialize h before passing the value to another function. It does > this even if the explicit loop entry condition is false. Is that correct? The loop is getting promoted to "infinite" loop, there is no &pos->member != (head) condition check in the generated code at all (at least on my machine). I wish we could at least get the "possibly uninitialized variable" warning from the compiler in this case, which we'd translate to "hold my beer, I'm going to try one thing".