From mboxrd@z Thu Jan 1 00:00:00 1970 Subject: [PATCH] fix page->count discrepancy for zero page From: Dave Hansen Content-Type: multipart/mixed; boundary="=-cV4YYfqp3aExvEPFBNz3" Message-Id: <1088460353.5471.406.camel@nighthawk> Mime-Version: 1.0 Date: Mon, 28 Jun 2004 15:05:53 -0700 Sender: owner-linux-mm@kvack.org Return-Path: To: Linux Kernel Mailing List Cc: linux-mm , "BRADLEY CHRISTIANSEN [imap]" , Andrew Morton List-ID: --=-cV4YYfqp3aExvEPFBNz3 Content-Type: text/plain Content-Transfer-Encoding: 7bit While writing some analysis tools for memory hot-remove, we came across a single page which had a ->count that always increased, without bound. It ended up always being the zero page, and it was caused by a leaked reference in some do_wp_page() code that ends up avoiding PG_reserved pages. Basically what happens is that page_cache_release()/put_page() ignore PG_reserved pages, while page_cache_get()/get_page() go ahead and take the reference. So, each time there's a COW fault on the zero-page, you get a leaked page->count increment. It's pretty rare to have a COW fault on anything that's PG_reserved, in fact, I can't think of anything else that this applies to other than the zero page. In any case, it the bug doesn't cause any real problems, but it is a bit of an annoyance and is obviously incorrect. We've been running with this patch for about 3 months now, and haven't run into any problems with it. Attached patch is against 2.6.7 and applies to -mm3 properly. -- Dave --=-cV4YYfqp3aExvEPFBNz3 Content-Disposition: attachment; filename=patch-2.6.7-zpage Content-Type: text/x-patch; name=patch-2.6.7-zpage; charset=ANSI_X3.4-1968 Content-Transfer-Encoding: 7bit diff -urp linux-2.6.7/mm/memory.c linux-2.6.7-zpage/mm/memory.c --- linux-2.6.7/mm/memory.c Tue Jun 15 22:19:22 2004 +++ linux-2.6.7-zpage/mm/memory.c Thu Jun 24 12:08:42 2004 @@ -1064,7 +1064,8 @@ static int do_wp_page(struct mm_struct * /* * Ok, we need to copy. Oh, well.. */ - page_cache_get(old_page); + if (!PageReserved(old_page)) + page_cache_get(old_page); spin_unlock(&mm->page_table_lock); if (unlikely(anon_vma_prepare(vma))) --=-cV4YYfqp3aExvEPFBNz3-- -- 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: aart@kvack.org