From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: Re: [PATCH v4] mm: add replace_page_cache_page() function Date: Thu, 8 Sep 2011 16:52:22 -0700 Message-ID: <20110908165222.18875260.akpm@linux-foundation.org> References: <20110118152844.88cfdc2c.akpm@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit To: Miklos Szeredi , minchan.kim@gmail.com, kamezawa.hiroyu@jp.fujitsu.com, nishimura@mxp.nes.nec.co.jp, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, Return-path: In-Reply-To: <20110118152844.88cfdc2c.akpm@linux-foundation.org> Sender: owner-linux-mm@kvack.org List-Id: linux-fsdevel.vger.kernel.org On Tue, 18 Jan 2011 15:28:44 -0800 Andrew Morton wrote: > 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? > Poke? -- 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 internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org