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Mon, 02 Feb 2026 07:28:41 -0800 (PST) Received: from localhost ([2a03:2880:10ff:2::]) by smtp.gmail.com with ESMTPSA id 586e51a60fabf-409570f63b4sm11999451fac.3.2026.02.02.07.28.40 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 02 Feb 2026 07:28:40 -0800 (PST) From: Joshua Hahn To: Ingyu Jang Cc: linux-mm@kvack.org, akpm@linux-foundation.org, david@kernel.org Subject: Re: [Question] Dead code in copy_pud_range() for copy_huge_pud() error check? Date: Mon, 2 Feb 2026 07:28:38 -0800 Message-ID: <20260202152839.221116-1-joshua.hahnjy@gmail.com> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260131172854.3370590-1-ingyujang25@korea.ac.kr> References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: A1D0F4000F X-Stat-Signature: 1p3wmr8hea3qpzew73m58up1tnmn1nnk X-Rspam-User: X-HE-Tag: 1770046122-243704 X-HE-Meta: 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 /Wj8plJM 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000004, 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 Sun, 1 Feb 2026 02:28:54 +0900 Ingyu Jang wrote: Hello Ingyu, I hope you are doing well! I'm by no means an expert in this area, but thought I would reply with what I found in the commit log. > Hi, > > I noticed that in mm/memory.c, the function copy_pud_range() checks > if copy_huge_pud() returns -ENOMEM (at line 1421): > > err = copy_huge_pud(dst_mm, src_mm, > dst_pud, src_pud, addr, src_vma); > if (err == -ENOMEM) > return -ENOMEM; > > However, looking at copy_huge_pud() in mm/huge_memory.c (line 1966), > it only returns two values: > - 0 on success (line 1994) > - -EAGAIN when !pud_trans_huge(pud) (line 1978) > > There is no code path that returns -ENOMEM, which means the check > "if (err == -ENOMEM)" at line 1421 is unreachable dead code. > > Is this intentional defensive coding for potential future changes, > or could this unreachable error check be cleaned up? It seems like commit a00cc7d9dd93 ("mm, x86: add support for PUD-sized transparent hugepages") introduced both the if (err == -ENOMEM) check and copy_huge_pud, back in 2017. And at the time these were introduced, copy_huge_pud only had return values for -EAGAIN and 0, as you pointed out. So it doesn't seem like there's any bug or missed case, just an extra case. My guess is that copy_pud_range tried to imitate copy_pmd_range and added the -ENOMEM check. copy_huge_pmd can return -ENOMEM though, so it seems like this is what was missed when copying the function over. If you have other changes in this area, this might be a good candidate for a small fix-up to be included in the series? : -) The whole if block inside pud_trans_huge(...) can be simplified quite a bit: do { ... if (pud_trans_huge(*src_pud)) { VM_BUG_ON_VMA(next-addr != HPAGE_PUD_SIZE, src_vma); if (!copy_huge_pud(dst_mm, src_mm, dst_pud, src_pud, addr, src_vma)) continue; } ... } Anyways... Hope this answers your question! Have a great day, Joshua