From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Tue, 3 Feb 2015 14:48:28 -0500 From: Benjamin LaHaise Subject: Re: [PATCH 2/2] aio: make aio .mremap handle size changes Message-ID: <20150203194828.GU2974@kvack.org> References: <798fafb96373cfab0707457a266dd137016cd1e9.1422986358.git.shli@fb.com> <20150203192323.GT2974@kvack.org> <20150203193115.GA296459@devbig257.prn2.facebook.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150203193115.GA296459@devbig257.prn2.facebook.com> Sender: owner-linux-mm@kvack.org List-ID: To: Shaohua Li Cc: linux-mm@kvack.org, Kernel-team@fb.com, Andrew Morton On Tue, Feb 03, 2015 at 11:31:15AM -0800, Shaohua Li wrote: > On Tue, Feb 03, 2015 at 02:23:23PM -0500, Benjamin LaHaise wrote: > > On Tue, Feb 03, 2015 at 11:18:53AM -0800, Shaohua Li wrote: > > > mremap aio ring buffer to another smaller vma is legal. For example, > > > mremap the ring buffer from the begining, though after the mremap, some > > > ring buffer pages can't be accessed in userspace because vma size is > > > shrinked. The problem is ctx->mmap_size isn't changed if the new ring > > > buffer vma size is changed. Latter io_destroy will zap all vmas within > > > mmap_size, which might zap unrelated vmas. > > > > Nak. Shrinking the aio ring buffer is not a supported operation and will > > cause the application to lose events. Make the size changing mremap fail, > > as this patch will not make the system do the right thing. > > Yes, making the syscall fail (vma ops has .remap) is another option. If > the app uses io_getevents(), looks the app will not lose events, no? On > the other hand, I just want to make sure kernel does the right thing > (not zap unrelated vmas). If app does crazy things, it will break. But reading events out of the ring buffer is a supported mode of operation. Given that constraint, you should make an mremap changing the size of the ring buffer fail. -ben > Thanks, > Shaohua -- "Thought is the essence of where you are now." -- 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