From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail144.messagelabs.com (mail144.messagelabs.com [216.82.254.51]) by kanga.kvack.org (Postfix) with ESMTP id DF8F56B007B for ; Wed, 15 Sep 2010 04:40:19 -0400 (EDT) Date: Wed, 15 Sep 2010 10:40:12 +0200 (CEST) From: Richard Guenther Subject: Re: [PATCH] After swapout/swapin private dirty mappings become clean In-Reply-To: <201009151030.36012.knikanth@suse.de> Message-ID: References: <20100915092239.C9D9.A69D9226@jp.fujitsu.com> <201009151007.43232.knikanth@suse.de> <20100915134347.C9EB.A69D9226@jp.fujitsu.com> <201009151030.36012.knikanth@suse.de> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org To: Nikanth Karthikesan Cc: KOSAKI Motohiro , linux-mm@kvack.org, Andrew Morton , balbir@linux.vnet.ibm.com, Michael Matz , Matt Mackall , linux-kernel@vger.kernel.org List-ID: On Wed, 15 Sep 2010, Nikanth Karthikesan wrote: > On Wednesday 15 September 2010 10:16:36 KOSAKI Motohiro wrote: > > > On Wednesday 15 September 2010 05:54:31 KOSAKI Motohiro wrote: > > > > > /proc/$pid/smaps broken: After swapout/swapin private dirty mappings > > > > > become clean. > > > > > > > > > > When a page with private file mapping becomes dirty, the vma will be > > > > > in both i_mmap tree and anon_vma list. The /proc/$pid/smaps will > > > > > account these pages as dirty and backed by the file. > > > > > > > > > > But when those dirty pages gets swapped out, and when they are read > > > > > back from swap, they would be marked as clean, as it should be, as > > > > > they are part of swap cache now. > > > > > > > > > > But the /proc/$pid/smaps would report the vma as a mapping of a file > > > > > and it is clean. The pages are actually in same state i.e., dirty > > > > > with respect to file still, but which was once reported as dirty is > > > > > now being reported as clean to user-space. > > > > > > > > > > This confuses tools like gdb which uses this information. Those tools > > > > > think that those pages were never modified and it creates problem > > > > > when they create dumps. > > > > > > > > > > The file mapping of the vma also cannot be broken as pages never read > > > > > earlier, will still have to come from the file. Just that those dirty > > > > > pages have become clean anonymous pages. > > > > > > > > > > During swaping in, restoring the exact state as dirty file-backed > > > > > pages before swapout would be useless, as there in no real bug. > > > > > Breaking the vma with only anonymous pages as seperate vmas > > > > > unnecessary may not be a good thing as well. So let us just export > > > > > the information that a file-backed vma has anonymous dirty pages. > > > > > > > > Why can't gdb check Swap: field in smaps? I think Swap!=0 mean we need > > > > dump out. > > > > > > Yes. When the page is swapped out it is accounted in "Swap:". > > > > > > > Am I missing anything? > > > > > > But when it gets swapped in back to memory, it is removed from "Swap:" > > > and added to "Private_Clean:" instead of "Private_Dirty:". > > > > Here is the code. > > I think the page will become dirty, again. > > > > -------------------------------------------------------------- > > int try_to_free_swap(struct page *page) > > { > > VM_BUG_ON(!PageLocked(page)); > > > > if (!PageSwapCache(page)) > > return 0; > > if (PageWriteback(page)) > > return 0; > > if (page_swapcount(page)) > > return 0; > > > > delete_from_swap_cache(page); > > SetPageDirty(page); > > return 1; > > } > > > > I think this gets called only when the swap space gets freed. But when the > page is just swapped out and swapped in, and the page is still part of > SwapCache, it will be marked as clean, when the I/O read from swap completes. And it will be still marked clean if I do a swapoff -a after it has been swapped in again. Thus /proc/smaps shows it as file-backed, private, clean and not swapped. Which is wrong. Richard. -- 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