From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail144.messagelabs.com (mail144.messagelabs.com [216.82.254.51]) by kanga.kvack.org (Postfix) with ESMTP id 87CF06B0092 for ; Tue, 18 Jan 2011 18:28:54 -0500 (EST) Date: Tue, 18 Jan 2011 15:28:44 -0800 From: Andrew Morton Subject: Re: [PATCH v4] mm: add replace_page_cache_page() function Message-Id: <20110118152844.88cfdc2c.akpm@linux-foundation.org> In-Reply-To: References: Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Miklos Szeredi Cc: minchan.kim@gmail.com, kamezawa.hiroyu@jp.fujitsu.com, nishimura@mxp.nes.nec.co.jp, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org List-ID: On Tue, 18 Jan 2011 12:18:11 +0100 Miklos Szeredi wrote: > +int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask) > +{ > + int error; > + struct mem_cgroup *memcg = NULL; I'm suspecting that the unneeded initialisation was added to suppress a warning? I removed it, and didn't get a warning. I expected to. Really, uninitialized_var() is better. It avoids adding extra code and, unlike "= 0" it is self-documenting. > + VM_BUG_ON(!PageLocked(old)); > + VM_BUG_ON(!PageLocked(new)); > + VM_BUG_ON(new->mapping); > + > + /* > + * This is not page migration, but prepare_migration and > + * end_migration does enough work for charge replacement. > + * > + * In the longer term we probably want a specialized function > + * for moving the charge from old to new in a more efficient > + * manner. > + */ > + error = mem_cgroup_prepare_migration(old, new, &memcg, gfp_mask); > + if (error) > + return error; > + > + error = radix_tree_preload(gfp_mask & ~__GFP_HIGHMEM); > + if (!error) { > + struct address_space *mapping = old->mapping; > + pgoff_t offset = old->index; > + > + page_cache_get(new); > + new->mapping = mapping; > + new->index = offset; > + > + spin_lock_irq(&mapping->tree_lock); > + __remove_from_page_cache(old); > + error = radix_tree_insert(&mapping->page_tree, offset, new); > + BUG_ON(error); > + mapping->nrpages++; > + __inc_zone_page_state(new, NR_FILE_PAGES); > + if (PageSwapBacked(new)) > + __inc_zone_page_state(new, NR_SHMEM); > + spin_unlock_irq(&mapping->tree_lock); > + radix_tree_preload_end(); > + page_cache_release(old); > + mem_cgroup_end_migration(memcg, old, new, true); This is all pretty ugly and inefficient. We call __remove_from_page_cache() which does a radix-tree lookup and then fiddles a bunch of accounting things. Then we immediately do the same radix-tree lookup and then undo the accounting changes which we just did. And we do it in an open-coded fashion, thus giving the kernel yet another code site where various operations need to be kept in sync. Would it not be better to do a single radix_tree_lookup_slot(), overwrite the pointer therein and just leave all the ancilliary accounting unaltered? > + } else { > + mem_cgroup_end_migration(memcg, old, new, false); > + } > + > + return error; > +} -- 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/ . Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: email@kvack.org