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[209.85.220.73]) by mx.google.com with SMTPS id l6sor12673956qte.24.2019.02.11.08.32.09 for (Google Transport Security); Mon, 11 Feb 2019 08:32:09 -0800 (PST) Received-SPF: pass (google.com: domain of 3ianhxaukcealcppjiqqing.eqonkpwz-oomxcem.qti@flex--jannh.bounces.google.com designates 209.85.220.73 as permitted sender) client-ip=209.85.220.73; Authentication-Results: mx.google.com; dkim=pass header.i=@google.com header.s=20161025 header.b=resTFMXs; spf=pass (google.com: domain of 3ianhxaukcealcppjiqqing.eqonkpwz-oomxcem.qti@flex--jannh.bounces.google.com designates 209.85.220.73 as permitted sender) smtp.mailfrom=3iaNhXAUKCEAlcppjiqqing.eqonkpwz-oomxcem.qti@flex--jannh.bounces.google.com; dmarc=pass (p=REJECT sp=REJECT dis=NONE) header.from=google.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:message-id:mime-version:subject:from:to:cc; bh=g8j9Hq6BgrsLF5iTVvtuSKrF1yqV6cwx6ZevVFumJ1k=; b=resTFMXsv362eCHTHAhm5Pol1eeM3yr3nRd3wz29koaQbZo6yLCKX/5Iz9fnDy0/iM W5Abf2BqNvyPOiEYtn1TKiH/Y+79hUfjuYembRmNRQLa9uMj5rK9z+Nm6zo0AgmEKzyb ygJf284PlgcabSBZA4heIXW+UluHeJLvN0N0naYgRVZFp06laMAqzY93A+h3kKSt9n/Z 4HO5rQkeR73XBwOGSL3J0LrRLVtRNWFFRKsVIjIPTbiNCiI5qNSfT8bVL78/p3TLHI7H 7Jk+cR/7eB2951vuDVYip7w/yx+iV9Y3oKC5rpadIoRVThjxF94wI8Oh/eEBUxjBenu/ HLhQ== X-Google-Smtp-Source: AHgI3IZMONOPzWkMyGMVEbG8/au4SeEZWoLdpHNKemt6FuIiFFZh84ZC0chVudOiLVo7JAWl0PGhIF8vIA== X-Received: by 2002:ac8:1bf2:: with SMTP id m47mr10477070qtk.27.1549902729411; Mon, 11 Feb 2019 08:32:09 -0800 (PST) Date: Mon, 11 Feb 2019 17:32:03 +0100 Message-Id: <20190211163203.33477-1-jannh@google.com> Mime-Version: 1.0 X-Mailer: git-send-email 2.20.1.791.gb4d0f1c61a-goog Subject: [PATCH] mmap.2: fix description of treatment of the hint From: Jann Horn To: mtk.manpages@gmail.com, jannh@google.com Cc: linux-man@vger.kernel.org, linux-mm@kvack.org X-Bogosity: Ham, tests=bogofilter, spamicity=0.033510, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: X-Virus-Scanned: ClamAV using ClamSMTP The current manpage reads to me as if the kernel will always pick a free space close to the requested address, but that's not the case: mmap(0x600000000000, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x600000000000 mmap(0x600000000000, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f5042859000 You can also see this in the various implementations of ->get_unmapped_area() - if the specified address isn't available, the kernel basically ignores the hint (apart from the 5level paging hack). Clarify how this works a bit. Signed-off-by: Jann Horn --- man2/mmap.2 | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/man2/mmap.2 b/man2/mmap.2 index fccfb9b3e..8556bbfeb 100644 --- a/man2/mmap.2 +++ b/man2/mmap.2 @@ -71,7 +71,12 @@ If .I addr is not NULL, then the kernel takes it as a hint about where to place the mapping; -on Linux, the mapping will be created at a nearby page boundary. +on Linux, the kernel will pick a nearby page boundary (but always above +or equal to the value specified by +.IR /proc/sys/vm/mmap_min_addr ) +and attempt to create the mapping there. +If another mapping already exists there, the kernel picks a new +address, independent of the hint. .\" Before Linux 2.6.24, the address was rounded up to the next page .\" boundary; since 2.6.24, it is rounded down! The address of the new mapping is returned as the result of the call.