From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f41.google.com (mail-wm0-f41.google.com [74.125.82.41]) by kanga.kvack.org (Postfix) with ESMTP id EBC576B0038 for ; Fri, 13 Nov 2015 00:31:45 -0500 (EST) Received: by wmec201 with SMTP id c201so64932362wme.0 for ; Thu, 12 Nov 2015 21:31:45 -0800 (PST) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id jp7si12335988wjc.168.2015.11.12.21.31.44 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Thu, 12 Nov 2015 21:31:44 -0800 (PST) Date: Thu, 12 Nov 2015 21:31:37 -0800 From: Davidlohr Bueso Subject: Re: [PATCH, RESEND] ipc/shm: handle removed segments gracefully in shm_mmap() Message-ID: <20151113053137.GB3502@linux-uzut.site> References: <1447232220-36879-1-git-send-email-kirill.shutemov@linux.intel.com> <20151111170347.GA3502@linux-uzut.site> <20151111195023.GA17310@node.shutemov.name> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <20151111195023.GA17310@node.shutemov.name> Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: "Kirill A. Shutemov" , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Dmitry Vyukov On Wed, 11 Nov 2015, Kirill A. Shutemov wrote: >And I had concern about your approach: > > If I read it correctly, with the patch we would ignore locking > failure inside shm_open() and mmap will succeed in this case. So > the idea is to have shm_close() no-op and therefore symmetrical. Both open and close are no-ops in the case the segment has been removed, that's the symmetrical, and I'm not sure I follow -- we don't ignore locking failure in shm_open _at all_. Just like your approach, all I do is return if there's an error... > That's look fragile to me. We would silently miss some other > broken open/close pattern. Such cases, if any, should be fixed and handled appropriately, not hide it under the rung, methinks. >> >> o My shm_check_vma_validity() also deals with IPC_RMID as we do the >> ipc_valid_object() check. > >Mine too: > > shm_mmap() > __shm_open() > shm_lock() > ipc_lock() > ipc_valid_object() > >Or I miss something? Sorry, I meant ipc_obtain_object_idr, so EINVAL is also accounted for, we the segment is already deleted and not only marked as such. > >> o We have a new WARN where necessary, instead of having one now is shm_open. > >I'm not sure why you think that shm_close() which was never paired with >successful shm_open() doesn't deserve WARN(). > >> o My no-ops explicitly pair. > >As I said before, I don't think we should ignore locking error in >shm_open(). If we propagate the error back to caller shm_close() should >never happen, therefore no-op is unneeded in shm_close(): my patch trigger >WARN() there. Yes, you WARN() in shm_close, but you still make it a no-op... > >> > ret = sfd->file->f_op->mmap(sfd->file, vma); >> >- if (ret != 0) >> >+ if (ret) { >> >+ shm_close(vma); >> > return ret; >> >+ } >> >> Hmm what's this shm_close() about? > >Undo shp->shm_nattch++ in successful __shm_open(). Yeah that's just nasty. > >I've got impression that I miss something important about how locking in >IPC/SHM works, but I cannot grasp what.. Hm?. Could you be more specific? The only lock involved here is the ipc object lock, if you haven't, you might want to refer to ipc/util.c which has a brief ipc locking description. -- 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