From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx166.postini.com [74.125.245.166]) by kanga.kvack.org (Postfix) with SMTP id 34ACB6B0007 for ; Thu, 31 Jan 2013 00:11:43 -0500 (EST) Date: Thu, 31 Jan 2013 14:11:40 +0900 From: Minchan Kim Subject: Questin about swap_slot free and invalidate page Message-ID: <20130131051140.GB23548@blaptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Sender: owner-linux-mm@kvack.org List-ID: To: Nitin Gupta , Dan Magenheimer , Seth Jennings , Hugh Dickins , Konrad Rzeszutek Wilk Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton When I reviewed zswap, I was curious about frontswap_store. It said following as. * If frontswap already contains a page with matching swaptype and * offset, the frontswap implementation may either overwrite the data and * return success or invalidate the page from frontswap and return failure. It didn't say why it happens. we already have __frontswap_invalidate_page and call it whenever swap_slot frees. If we don't free swap slot, scan_swap_map can't find the slot for swap out so I thought overwriting of data shouldn't happen in frontswap. As I looked the code, the curplit is reuse_swap_page. It couldn't free swap slot if the page founded is PG_writeback but miss calling frontswap_invalidate_page so data overwriting on frontswap can happen. I'm not sure frontswap guys already discussed it long time ago. If we can fix it, we can remove duplication entry handling logic in all of backend of frontswap. All of backend should handle it although it's pretty rare. Of course, zram could be fixed. It might be trivial now but more there are many backend of frontswap, more it would be a headache. If we are trying to fix it in swap layer, we might fix it following as int reuse_swap_page(struct page *page) { .. .. if (count == 1) { if (!PageWriteback(page)) { delete_from_swap_cache(page); SetPageDirty(page); } else { frontswap_invalidate_page(); swap_slot_free_notify(); } } } But not sure, it is worth at the moment and there might be other places to be fixed.(I hope Hugh can point out if we are missing something if he has a time) If we are reluctant to it, at least, we should write out comment above frontswap_store about that to notice curious guys who spend many time to know WHY and smart guys who are going to fix it with nice way. Mr. Frontswap, What do you think about it? -- Kind regards, Minchan Kim -- 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