From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f48.google.com (mail-pa0-f48.google.com [209.85.220.48]) by kanga.kvack.org (Postfix) with ESMTP id C12E56B0037 for ; Thu, 10 Oct 2013 03:01:30 -0400 (EDT) Received: by mail-pa0-f48.google.com with SMTP id bj1so2274859pad.35 for ; Thu, 10 Oct 2013 00:01:30 -0700 (PDT) Received: from eucpsbgm1.samsung.com (unknown [203.254.199.244]) by mailout1.w1.samsung.com (Oracle Communications Messaging Server 7u4-24.01(7.0.4.24.0) 64bit (built Nov 17 2011)) with ESMTP id <0MUF00C4LY6ETT10@mailout1.w1.samsung.com> for linux-mm@kvack.org; Thu, 10 Oct 2013 08:01:26 +0100 (BST) Message-id: <1381388484.21461.16.camel@AMDC1943> Subject: Re: [PATCH v3 5/6] zswap: replace tree in zswap with radix tree in zbud From: Krzysztof Kozlowski Date: Thu, 10 Oct 2013 09:01:24 +0200 In-reply-to: <20131009171617.GA21057@variantweb.net> References: <1381238980-2491-1-git-send-email-k.kozlowski@samsung.com> <1381238980-2491-6-git-send-email-k.kozlowski@samsung.com> <20131009153022.GB5406@variantweb.net> <20131009171617.GA21057@variantweb.net> Content-type: text/plain; charset=UTF-8 Content-transfer-encoding: 7bit MIME-version: 1.0 Sender: owner-linux-mm@kvack.org List-ID: To: Seth Jennings Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Bob Liu , Mel Gorman , Bartlomiej Zolnierkiewicz , Marek Szyprowski , Tomasz Stanislawski , Kyungmin Park , Dave Hansen , Minchan Kim On Wed, 2013-10-09 at 12:16 -0500, Seth Jennings wrote: > On Wed, Oct 09, 2013 at 10:30:22AM -0500, Seth Jennings wrote: > > In my approach, I was also looking at allowing the zbud pools to use > > HIGHMEM pages, since the handle is no longer an address. This requires > > the pages that are being mapped to be kmapped (atomic) which will > > disable preemption. This isn't an additional overhead since the > > map/unmap corresponds with a compress/decompress operation at the zswap > > level which uses per-cpu variables that disable preemption already. > > On second though, lets not mess with the HIGHMEM page support for now. > Turns out it is tricker than I thought since the unbuddied lists are > linked through the zbud header stored in the page. But we can still > disable preemption to allow per-cpu tracking of the current mapping and > avoid a lookup (and races) in zbud_unmap(). This tracking of current mapping could solve another problem I encountered with new one-radix-tree approach with storage of duplicated entries. The problem is in zbud_unmap() API using offset to unmap (if duplicated entries are overwritten): - thread 1: zswap_fronstwap_load() of some offset - zbud_map() maps this offset -> zhdr1 - thread 2: zswap_frontswap_store() stores new data for this offset - zbud_alloc() allocated new zhdr2 and replaces zhdr1 in radix tree under this offset - new compressed data is stored by zswap - thread 1: tries to zbud_unmap() of this offset, but now the old zhdr1 is not present in radix tree so unmap will either fail or use zhdr2 which is wrong To solve this issue I experimented with unmapping by zbud_mapped_entry instead of offset (so zbud_unmap() won't search zbud_header in radix tree at all): ########################## int zbud_unmap(struct zbud_pool *pool, pgoff_t offset, struct zbud_mapped_entry *entry) { struct zbud_header *zhdr = handle_to_zbud_header((unsigned long)entry->addr); VM_BUG_ON((offset != zhdr->first_offset) && (offset != zhdr->last_offset)); spin_lock(&pool->lock); if (put_map_count(zhdr, offset)) { /* Racing zbud_free() could not free the offset because * it was still mapped so it is our job to free. */ zbud_header_free(pool, zhdr, offset); spin_unlock(&pool->lock); return -EFAULT; } put_zbud_page(zhdr); spin_unlock(&pool->lock); return 0; } ########################## However getting rid of first/last_map_count seems much more simpler! Best regards, Krzysztof -- 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