From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail137.messagelabs.com (mail137.messagelabs.com [216.82.249.19]) by kanga.kvack.org (Postfix) with SMTP id 95A536B004D for ; Fri, 16 Oct 2009 00:50:42 -0400 (EDT) Received: by yxe10 with SMTP id 10so1441530yxe.12 for ; Thu, 15 Oct 2009 21:50:41 -0700 (PDT) Message-ID: <4AD7FB57.2030403@vflare.org> Date: Fri, 16 Oct 2009 10:19:27 +0530 From: Nitin Gupta Reply-To: ngupta@vflare.org MIME-Version: 1.0 Subject: Re: [PATCH 7/9] swap_info: swap count continuations References: In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Hugh Dickins Cc: Andrew Morton , KAMEZAWA Hiroyuki , hongshin@gmail.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org List-ID: On 10/15/2009 06:26 AM, Hugh Dickins wrote: > Swap is duplicated (reference count incremented by one) whenever the same > swap page is inserted into another mm (when forking finds a swap entry in > place of a pte, or when reclaim unmaps a pte to insert the swap entry). > > swap_info_struct's vmalloc'ed swap_map is the array of these reference > counts: but what happens when the unsigned short (or unsigned char since > the preceding patch) is full? (and its high bit is kept for a cache flag) > > We then lose track of it, never freeing, leaving it in use until swapoff: > at which point we _hope_ that a single pass will have found all instances, > assume there are no more, and will lose user data if we're wrong. > > Swapping of KSM pages has not yet been enabled; but it is implemented, > and makes it very easy for a user to overflow the maximum swap count: > possible with ordinary process pages, but unlikely, even when pid_max > has been raised from PID_MAX_DEFAULT. > > This patch implements swap count continuations: when the count overflows, > a continuation page is allocated and linked to the original vmalloc'ed > map page, and this used to hold the continuation counts for that entry > and its neighbours. These continuation pages are seldom referenced: > the common paths all work on the original swap_map, only referring to > a continuation page when the low "digit" of a count is incremented or > decremented through SWAP_MAP_MAX. > I think the patch can be simplified a lot if we have just 2 levels (hard-coded) of swap_map, each level having 16-bit count -- combined 32-bit count should be sufficient for about anything. Saving 1-byte for level-1 swap_map and then having arbitrary levels of swap_map doesn't look like its worth the complexity. Nitin -- 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