From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx104.postini.com [74.125.245.104]) by kanga.kvack.org (Postfix) with SMTP id 518FA6B005A for ; Fri, 16 Nov 2012 23:48:48 -0500 (EST) Received: by mail-gh0-f169.google.com with SMTP id r1so709637ghr.14 for ; Fri, 16 Nov 2012 20:48:47 -0800 (PST) Date: Fri, 16 Nov 2012 20:48:46 -0800 (PST) From: Hugh Dickins Subject: Re: [PATCH] tmpfs: fix shmem_getpage_gfp VM_BUG_ON In-Reply-To: <50A6089B.7010708@gmail.com> Message-ID: References: <20121025023738.GA27001@redhat.com> <20121101191052.GA5884@redhat.com> <20121101232030.GA25519@redhat.com> <20121102014336.GA1727@redhat.com> <20121106135402.GA3543@redhat.com> <50A30ADD.9000209@gmail.com> <50A49C46.9040406@gmail.com> <50A6089B.7010708@gmail.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Jaegeuk Hanse Cc: Dave Jones , Andrew Morton , Johannes Weiner , linux-mm@kvack.org, linux-kernel@vger.kernel.org Further offtopic.. On Fri, 16 Nov 2012, Jaegeuk Hanse wrote: > Some questions about your shmem/tmpfs: misc and fallocate patchset. > > - Since shmem_setattr can truncate tmpfs files, why need add another similar > codes in function shmem_fallocate? What's the trick? I don't know if I understand you. In general, hole-punching is different from truncation. Supporting the hole-punch mode of the fallocate system call is different from supporting truncation. They're closely related, and share code, but meet different specifications. > - in tmpfs: support fallocate preallocation patch changelog: > "Christoph Hellwig: What for exactly? Please explain why preallocating on > tmpfs would make any sense. > Kay Sievers: To be able to safely use mmap(), regarding SIGBUS, on files on > the /dev/shm filesystem. The glibc fallback loop for -ENOSYS [or > -EOPNOTSUPP] on fallocate is just ugly." > Could shmem/tmpfs fallocate prevent one process truncate the file which the > second process mmap() and get SIGBUS when the second process access mmap but > out of current size of file? Again, I don't know if I understand you. fallocate does not prevent truncation or races or SIGBUS. I believe that Kay meant that without using fallocate to allocate the memory in advance, systemd found it hard to protect itself from the possibility of getting a SIGBUS, if access to a shmem mapping happened to run out of memory/space in the middle. I never grasped why writing the file in advance was not good enough: fallocate happened to be what they hoped to use, and it was hard to deny it, given that tmpfs already supported hole-punching, and was about to convert to the fallocate interface for that. Hugh -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org