From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wg0-f51.google.com (mail-wg0-f51.google.com [74.125.82.51]) by kanga.kvack.org (Postfix) with ESMTP id 1E5656B0031 for ; Thu, 21 Nov 2013 16:44:33 -0500 (EST) Received: by mail-wg0-f51.google.com with SMTP id l18so358845wgh.30 for ; Thu, 21 Nov 2013 13:44:32 -0800 (PST) Received: from mail-wg0-x22a.google.com (mail-wg0-x22a.google.com [2a00:1450:400c:c00::22a]) by mx.google.com with ESMTPS id hi12si1519602wib.1.2013.11.21.13.44.31 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Thu, 21 Nov 2013 13:44:31 -0800 (PST) Received: by mail-wg0-f42.google.com with SMTP id k14so622357wgh.5 for ; Thu, 21 Nov 2013 13:44:31 -0800 (PST) MIME-Version: 1.0 In-Reply-To: References: <1384976909-32671-1-git-send-email-ddstreet@ieee.org> From: Dan Streetman Date: Thu, 21 Nov 2013 16:44:11 -0500 Message-ID: Subject: Re: [PATCH] mm/zswap: don't allow entry eviction if in use by load Content-Type: text/plain; charset=UTF-8 Sender: owner-linux-mm@kvack.org List-ID: To: Weijie Yang Cc: Seth Jennings , linux-mm@kvack.org, linux-kernel , Bob Liu , Minchan Kim , Weijie Yang On Wed, Nov 20, 2013 at 8:59 PM, Weijie Yang wrote: > Hello Dan > > On Thu, Nov 21, 2013 at 3:48 AM, Dan Streetman wrote: >> The changes in commit 0ab0abcf511545d1fddbe72a36b3ca73388ac937 >> introduce a bug in writeback, if an entry is in use by load >> it will be evicted anyway, which isn't correct (technically, >> the code currently in zbud doesn't actually care much what the >> zswap evict function returns, but that could change). > > Thanks for your work. Howerver it is not a bug. > > I have thought about this situation, and it will never happen. > If entry is being loaded, its corresponding page must be in swapcache > so zswap_get_swap_cache_page() will return ZSWAP_SWAPCACHE_EXIST ah, ok. While you do imply that with the fail: comment, I personally think it should also be stated in the refcount check comment; a comment indicating failure can happen due to concurrent load does not make clear that it will *always* fail in cases of concurrent load and so that case doesn't need to be checked for in the success path. Additionally, the lack of a check here is assuming that zswap won't be updated to ever inc the refcount anywhere besides the load function, which might cause unexpected breakage later; i.e., this is coding to the current implementation, not to the entry->refcount api. Can I also ask why you do a rb_search instead of just checking the entry->refcount? Doing the search is going to take longer than just checking the refcount; is there some case where the entry will not be in the rb but will have a nonzero refcount? -- 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