From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <49344FA5.90004@cs.columbia.edu> Date: Mon, 01 Dec 2008 15:57:09 -0500 From: Oren Laadan MIME-Version: 1.0 Subject: Re: [RFC v10][PATCH 05/13] Dump memory address space References: <1227747884-14150-1-git-send-email-orenl@cs.columbia.edu> <1227747884-14150-6-git-send-email-orenl@cs.columbia.edu> <20081128105351.GQ28946@ZenIV.linux.org.uk> <1228154412.2971.44.camel@nimitz> In-Reply-To: <1228154412.2971.44.camel@nimitz> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Dave Hansen Cc: Al Viro , Andrew Morton , Linus Torvalds , containers@lists.linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-api@vger.kernel.org, Thomas Gleixner , Serge Hallyn , Ingo Molnar , "H. Peter Anvin" List-ID: Dave Hansen wrote: > On Fri, 2008-11-28 at 10:53 +0000, Al Viro wrote: >>> +static int cr_ctx_checkpoint(struct cr_ctx *ctx, pid_t pid) >>> +{ >>> + ctx->root_pid = pid; >>> + >>> + /* >>> + * assume checkpointer is in container's root vfs >>> + * FIXME: this works for now, but will change with real containers >>> + */ >>> + ctx->vfsroot = ¤t->fs->root; >>> + path_get(ctx->vfsroot); >> This is going to break as soon as you get another thread doing e.g. chroot(2) >> while you are in there. > > Yeah, we do need at least a read_lock(¤t->fs->lock) to keep people > from chroot()'ing underneath us. True. (while adapting older and safer code I omitted these tests with no reason). > >> And it's a really, _really_ bad idea to take a >> pointer to shared object, increment refcount on the current *contents* of >> said object and assume that dropping refcount on the later contents of the >> same will balance out. > > Absolutely. I assume you mean get_fs_struct(current) instead of > path_get(). True. Should change the type of ctx->vfsroot to not be a pointer, and do: >>> + ctx->vfsroot = *current->fs->root; >>> + path_get(&ctx->vfsroot); and adjust accordingly in where the refcount is dropped. What we need here is a reference point (this will change later when we handle multiple fs-namespaces), which is the path of the "container root". Assuming locking is correct so that current->fs does not change under us, it's enough to get that path and later release that path. BW, the current->fs is assumed to not change during the checkpoint; if it does, then it's a mis-use of the checkpoint interface, and the resulting behavior is undefined - restart is guaranteed to restore the exact old state even if checkpoint succeeds. Thanks, Oren. -- 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