Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757249AbZFIGtH (ORCPT ); Tue, 9 Jun 2009 02:49:07 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755263AbZFIGs5 (ORCPT ); Tue, 9 Jun 2009 02:48:57 -0400 Received: from mga14.intel.com ([143.182.124.37]:56932 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752719AbZFIGs4 (ORCPT ); Tue, 9 Jun 2009 02:48:56 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.41,329,1241420400"; d="scan'208";a="152136502" Date: Tue, 9 Jun 2009 14:48:55 +0800 From: Wu Fengguang To: Nai Xia Cc: Andi Kleen , Nick Piggin , "hugh@veritas.com" , "riel@redhat.com" , "akpm@linux-foundation.org" , "chris.mason@oracle.com" , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" Subject: Re: [PATCH] [13/16] HWPOISON: The high level memory error handler in the VM v3 Message-ID: <20090609064855.GB5490@localhost> References: <20090528082616.GG6920@wotan.suse.de> <20090528093141.GD1065@one.firstfloor.org> <20090528120854.GJ6920@wotan.suse.de> <20090528134520.GH1065@one.firstfloor.org> <20090528145021.GA5503@localhost> <20090607160225.GA24315@localhost> <20090608123133.GA7944@localhost> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 9959 Lines: 225 On Mon, Jun 08, 2009 at 10:46:53PM +0800, Nai Xia wrote: > On Mon, Jun 8, 2009 at 8:31 PM, Wu Fengguang wrote: > > On Mon, Jun 08, 2009 at 07:06:12PM +0800, Nai Xia wrote: > >> On Mon, Jun 8, 2009 at 12:02 AM, Wu Fengguang wrote: > >> > On Thu, Jun 04, 2009 at 02:25:24PM +0800, Nai Xia wrote: > >> >> On Thu, May 28, 2009 at 10:50 PM, Wu Fengguang wrote: > >> >> > On Thu, May 28, 2009 at 09:45:20PM +0800, Andi Kleen wrote: > >> >> >> On Thu, May 28, 2009 at 02:08:54PM +0200, Nick Piggin wrote: > >> >> > > >> >> > [snip] > >> >> > > >> >> >> > > >> >> >> > BTW. I don't know if you are checking for PG_writeback often enough? > >> >> >> > You can't remove a PG_writeback page from pagecache. The normal > >> >> >> > pattern is lock_page(page); wait_on_page_writeback(page); which I > >> >> >> > >> >> >> So pages can be in writeback without being locked? I still > >> >> >> wasn't able to find such a case (in fact unless I'm misreading > >> >> >> the code badly the writeback bit is only used by NFS and a few > >> >> >> obscure cases) > >> >> > > >> >> > Yes the writeback page is typically not locked. Only read IO requires > >> >> > to be exclusive. Read IO is in fact page *writer*, while writeback IO > >> >> > is page *reader* :-) > >> >> > >> >> Sorry for maybe somewhat a little bit off topic, > >> >> I am trying to get a good understanding of PG_writeback & PG_locked ;) > >> >> > >> >> So you are saying PG_writeback & PG_locked are acting like a read/write lock? > >> >> I notice wait_on_page_writeback(page) seems always called with page locked -- > >> > > >> > No. Note that pages are not locked in wait_on_page_writeback_range(). > >> > >> I see. This function seems mostly called  on the sync path, > >> it just waits for data being synchronized to disk. > >> No writers from the pages' POV, so no lock. > >> I missed this case, but my argument about the role of read/write lock. > >> seems still consistent. :) > > > > It's more constrained. Normal read/write locks allow concurrent readers, > > however fsync() must wait for previous IO to finish before starting > > its own IO. > > Oh, yes, this is what I called "mixed roles". One for lock, one for > status flag, twisted together in the same path, making the read lock > semantics totally broken. > > > > >> > > >> >> that is the semantics of a writer waiting to get the lock while it's > >> >> acquired by > >> >> some reader:The caller(e.g. truncate_inode_pages_range()  and > >> >> invalidate_inode_pages2_range()) are the writers waiting for > >> >> writeback readers (as you clarified ) to finish their job, right ? > >> > > >> > Sorry if my metaphor confused you. But they are not typical > >> > reader/writer problems, but more about data integrities. > >> > >> No, you didn't :) > >> Actually, you make me clear about the mixed roles for > >> those bits. > >> > >> > > >> > Pages have to be "not under writeback" when truncated. > >> > Otherwise data lost is possible: > >> > > >> > 1) create a file with one page (page A) > >> > 2) truncate page A that is under writeback > >> > 3) write to file, which creates page B > >> > 4) sync file, which sends page B to disk quickly > >> > > >> > Now if page B reaches disk before A, the new data will be overwritten > >> > by truncated old data, which corrupts the file. > >> > >> I fully understand this scenario which you had already clarified in a > >> previous message. :) > >> > >> 1. someone make index1-> page A > >> 2. Path P1 is acting as a *reader* to a cache page at index1 by > >>     setting PG_writeback on, while at the same time as a *writer* to > >>     the corresponding file blocks. > >> 3. Another path P2 comes in and  truncate page A, he is the writer > >>     to the same cache page. > >> 4. Yet another path P3 comes  as the writer to the cache page > >>      making it points to page B: index1--> page B. > >> 5. Path P4 comes writing back the cache page(and set PG_writeback). > >>    He is the reader of the cache page and the writer to the file blocks. > >> > >> The corrupts occur because P1 & P4 races when writing file blocks. > >> But the _root_ of this racing is because nothing is used to serialize > >> them on the side of writing the file blocks and above stream reading was > >> inconsistent because of the writers(P2 & P3) to cache page at index1. > >> > >> Note that the "sync file" is somewhat irrelevant, even without "sync file", > >> the racing still may exists. I know you must want to show me that this could > >> make the corruption more easy to occur. > >> > >> So I think the simple logic is: > >> 1) if you want to truncate/change the mapping from a index to a struct *page, > >> test writeback bit because the writebacker to the file blocks is the reader > >> of this mapping. > >> 2) if a writebacker want to start a read of this mapping with > >> test_set_page_writeback() > >> or set_page_writeback(), he'd be sure this page is locked to keep out the > >> writers to this mapping of index-->struct *page. > >> > >> This is really behavior of a read/write lock, right ? > > > > Please, that's a dangerous idea. A page can be written to at any time > > when writeback to disk is under way. Does PG_writeback (your reader > > lock) prevents page data writers?  NO. > > I meant PG_writeback stops writers to index---->struct page mapping. It's protected by the radix tree RCU locks. Period. If you are referring to the reverse mapping: page->mapping is procted by PG_lock. No one should make assumption that it won't change under page writeback. Thanks, Fengguang > I think I should make my statements more concise and the "reader/writer" > less vague. > > Here we care about the write/read operation for index---->struct page mapping. > Not for read/write operation for the page content. > > Anyone who wants to change this mapping is a writer, he should take > page lock. > Anyone who wants to reference this mapping is a reader, writers should > wait for him. And when this reader wants to get ref, he should wait for > anyone one who is changing this mapping(e.g. page truncater). > > When a path sets PG_writeback on a page, it need this index-->struct page > mapping be 100% valid right? (otherwise may leads to corruption.) > So writeback routines are readers of this index-->struct page mapping. > (oh, well if we can put the other role of PG_writeback aside) > > Ok,Ok, since PG_locked does mean much more than just protecting > the per-page mapping which makes the lock abstraction even less clear. > so indeed, forget about it. > > > > > Thanks, > > Fengguang > > > >> wait_on_page_writeback_range() looks different only because "sync" > >> operates on "struct page", it's not sensitive to index-->struct *page mapping. > >> It does care about if pages returned by pagevec_lookup_tag() are > >> still maintains the mapping when wait_on_page_writeback(page). > >> Here, PG_writeback is only a status flag for "struct page" not a lock bit for > >> index->struct *page mapping. > >> > >> > > >> >> So do you think the idea is sane to group the two bits together > >> >> to form a real read/write lock, which does not care about the _number_ > >> >> of readers ? > >> > > >> > We don't care number of readers here. So please forget about it. > >> Yeah, I meant number of readers is not important. > >> > >> I still hold that these two bits in some way act like a _sparse_ > >> read/write lock. > >> But I am going to drop the idea of making them a pure lock, since PG_writeback > >> does has other meaning -- the page is being writing back: for sync > >> path, it's only > >> a status flag. > >> Making a pure read/write lock definitely will lose that or at least distort it. > >> > >> > >> Hoping I've made my words understandable, correct me if wrong, and > >> many thanks for your time and patience. :-) > >> > >> > >> Nai Xia > >> > >> > > >> > Thanks, > >> > Fengguang > >> > > >> >> > The writeback bit is _widely_ used.  test_set_page_writeback() is > >> >> > directly used by NFS/AFS etc. But its main user is in fact > >> >> > set_page_writeback(), which is called in 26 places. > >> >> > > >> >> >> > think would be safest > >> >> >> > >> >> >> Okay. I'll just add it after the page lock. > >> >> >> > >> >> >> > (then you never have to bother with the writeback bit again) > >> >> >> > >> >> >> Until Fengguang does something fancy with it. > >> >> > > >> >> > Yes I'm going to do it without wait_on_page_writeback(). > >> >> > > >> >> > The reason truncate_inode_pages_range() has to wait on writeback page > >> >> > is to ensure data integrity. Otherwise if there comes two events: > >> >> >        truncate page A at offset X > >> >> >        populate page B at offset X > >> >> > If A and B are all writeback pages, then B can hit disk first and then > >> >> > be overwritten by A. Which corrupts the data at offset X from user's POV. > >> >> > > >> >> > But for hwpoison, there are no such worries. If A is poisoned, we do > >> >> > our best to isolate it as well as intercepting its IO. If the interception > >> >> > fails, it will trigger another machine check before hitting the disk. > >> >> > > >> >> > After all, poisoned A means the data at offset X is already corrupted. > >> >> > It doesn't matter if there comes another B page. > >> >> > > >> >> > Thanks, > >> >> > Fengguang > >> >> > -- > >> >> > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > >> >> > the body of a message to majordomo@vger.kernel.org > >> >> > More majordomo info at  http://vger.kernel.org/majordomo-info.html > >> >> > Please read the FAQ at  http://www.tux.org/lkml/ > >> >> > > >> > > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/