From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx137.postini.com [74.125.245.137]) by kanga.kvack.org (Postfix) with SMTP id 927E46B004D for ; Sat, 17 Nov 2012 20:48:22 -0500 (EST) Received: by mail-pb0-f41.google.com with SMTP id xa7so2940568pbc.14 for ; Sat, 17 Nov 2012 17:48:21 -0800 (PST) Message-ID: <50A83E5E.9060300@gmail.com> Date: Sun, 18 Nov 2012 09:48:14 +0800 From: Jaegeuk Hanse MIME-Version: 1.0 Subject: Re: [PATCH] tmpfs: fix shmem_getpage_gfp VM_BUG_ON 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> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Hugh Dickins Cc: Dave Jones , Andrew Morton , Johannes Weiner , linux-mm@kvack.org, linux-kernel@vger.kernel.org On 11/17/2012 12:48 PM, Hugh Dickins wrote: > Further offtopic.. Hi Hugh, - I see you add this in vfs.txt: + fallocate: called by the VFS to preallocate blocks or punch a hole. I want to know if it's necessary to add it to man page since users still don't know fallocate can punch a hole from man fallocate. - in function shmem_fallocate: + else if (shmem_falloc.nr_unswapped > shmem_falloc.nr_falloced) + error = -ENOMEM; If this changelog "shmem_fallocate() compare counts and give up once the reactivated pages have started to coming back to writepage (approximately: some zones would in fact recycle faster than others)." describe why need this change? If the answer is yes, I have two questions. 1) how can guarantee it really don't need preallocation if just one or a few pages always reactivated, in this scene, nr_unswapped maybe grow bigger enough than shmem_falloc.nr_falloced 2) why return -ENOMEM, it's not really OOM, is it a trick or ...? Regards, Jaegeuk > > 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