From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nick Piggin Message-Id: <20060922172120.22370.4933.sendpatchset@linux.site> In-Reply-To: <20060922172042.22370.62513.sendpatchset@linux.site> References: <20060922172042.22370.62513.sendpatchset@linux.site> Subject: [patch 4/9] mm: lockless pagecache lookups Date: Fri, 22 Sep 2006 21:22:48 +0200 (CEST) Sender: owner-linux-mm@kvack.org Return-Path: To: Andrew Morton Cc: Nick Piggin , Linux Memory Management List-ID: Combine page_cache_get_speculative with lockless radix tree lookups to introduce lockless page cache lookups (ie. no mapping->tree_lock on the read-side). The only atomicity changes this introduces is that the gang pagecache lookup functions now behave as if they are implemented with multiple find_get_page calls, rather than operating on a snapshot of the pages. In practice, this atomicity guarantee is not used anyway, and it is difficult to see how it could be. Gang pagecache lookups are designed to replace individual lookups, so these semantics are natural. Swapcache can no longer use find_get_page, because it has a different method of encoding swapcache position into the page. Introduce a new find_get_swap_page for it. Signed-off-by: Nick Piggin Index: linux-2.6/mm/filemap.c =================================================================== --- linux-2.6.orig/mm/filemap.c +++ linux-2.6/mm/filemap.c @@ -590,9 +590,9 @@ int probe_page(struct address_space *map { int exists; - read_lock_irq(&mapping->tree_lock); + rcu_read_lock(); exists = __probe_page(mapping, offset); - read_unlock_irq(&mapping->tree_lock); + rcu_read_unlock(); return exists; } @@ -666,15 +666,31 @@ void fastcall __lock_page_nosync(struct * Is there a pagecache struct page at the given (mapping, offset) tuple? * If yes, increment its refcount and return it; if no, return NULL. */ -struct page * find_get_page(struct address_space *mapping, unsigned long offset) +struct page *find_get_page(struct address_space *mapping, unsigned long offset) { + void **pagep; struct page *page; - read_lock_irq(&mapping->tree_lock); - page = radix_tree_lookup(&mapping->page_tree, offset); - if (page) - page_cache_get(page); - read_unlock_irq(&mapping->tree_lock); + rcu_read_lock(); +repeat: + page = NULL; + pagep = radix_tree_lookup_slot(&mapping->page_tree, offset); + if (pagep) { + page = radix_tree_deref_slot(pagep); + if (unlikely(!page || page == RADIX_TREE_RETRY)) + goto repeat; + + if (!page_cache_get_speculative(page)) + goto repeat; + + /* Has the page moved? */ + if (unlikely(page != *pagep)) { + page_cache_release(page); + goto repeat; + } + } + rcu_read_unlock(); + return page; } EXPORT_SYMBOL(find_get_page); @@ -714,26 +730,19 @@ struct page *find_lock_page(struct addre { struct page *page; - read_lock_irq(&mapping->tree_lock); repeat: - page = radix_tree_lookup(&mapping->page_tree, offset); + page = find_get_page(mapping, offset); if (page) { - page_cache_get(page); - if (TestSetPageLocked(page)) { - read_unlock_irq(&mapping->tree_lock); - __lock_page(page); - read_lock_irq(&mapping->tree_lock); - - /* Has the page been truncated while we slept? */ - if (unlikely(page->mapping != mapping || - page->index != offset)) { - unlock_page(page); - page_cache_release(page); - goto repeat; - } + lock_page(page); + /* Has the page been truncated? */ + if (unlikely(page->mapping != mapping + || page->index != offset)) { + unlock_page(page); + page_cache_release(page); + goto repeat; } } - read_unlock_irq(&mapping->tree_lock); + return page; } EXPORT_SYMBOL(find_lock_page); @@ -803,13 +812,39 @@ unsigned find_get_pages(struct address_s { unsigned int i; unsigned int ret; + unsigned int nr_found; - read_lock_irq(&mapping->tree_lock); - ret = radix_tree_gang_lookup(&mapping->page_tree, - (void **)pages, start, nr_pages); - for (i = 0; i < ret; i++) - page_cache_get(pages[i]); - read_unlock_irq(&mapping->tree_lock); + rcu_read_lock(); +restart: + nr_found = radix_tree_gang_lookup_slot(&mapping->page_tree, + (void ***)pages, start, nr_pages); + ret = 0; + for (i = 0; i < nr_found; i++) { + struct page *page; +repeat: + page = radix_tree_deref_slot((void **)pages[i]); + if (unlikely(!page)) + continue; + /* + * this can only trigger if nr_found == 1, making livelock + * a non issue. + */ + if (unlikely(page == RADIX_TREE_RETRY)) + goto restart; + + if (!page_cache_get_speculative(page)) + goto repeat; + + /* Has the page moved? */ + if (unlikely(page != *((void **)pages[i]))) { + page_cache_release(page); + goto repeat; + } + + pages[ret] = page; + ret++; + } + rcu_read_unlock(); return ret; } EXPORT_SYMBOL(find_get_pages); @@ -831,19 +866,44 @@ unsigned find_get_pages_contig(struct ad { unsigned int i; unsigned int ret; + unsigned int nr_found; - read_lock_irq(&mapping->tree_lock); - ret = radix_tree_gang_lookup(&mapping->page_tree, - (void **)pages, index, nr_pages); - for (i = 0; i < ret; i++) { - if (pages[i]->mapping == NULL || pages[i]->index != index) + rcu_read_lock(); +restart: + nr_found = radix_tree_gang_lookup_slot(&mapping->page_tree, + (void ***)pages, index, nr_pages); + ret = 0; + for (i = 0; i < nr_found; i++) { + struct page *page; +repeat: + page = radix_tree_deref_slot((void **)pages[i]); + if (unlikely(!page)) + continue; + /* + * this can only trigger if nr_found == 1, making livelock + * a non issue. + */ + if (unlikely(page == RADIX_TREE_RETRY)) + goto restart; + + if (page->mapping == NULL || page->index != index) break; - page_cache_get(pages[i]); + if (!page_cache_get_speculative(page)) + goto repeat; + + /* Has the page moved? */ + if (unlikely(page != *((void **)pages[i]))) { + page_cache_release(page); + goto repeat; + } + + pages[ret] = page; + ret++; index++; } - read_unlock_irq(&mapping->tree_lock); - return i; + rcu_read_unlock(); + return ret; } EXPORT_SYMBOL(find_get_pages_tag); @@ -865,6 +925,7 @@ unsigned find_get_pages_tag(struct addre unsigned int ret; read_lock_irq(&mapping->tree_lock); + /* TODO: implement lookup_tag_slot and make this lockless */ ret = radix_tree_gang_lookup_tag(&mapping->page_tree, (void **)pages, *index, nr_pages, tag); for (i = 0; i < ret; i++) Index: linux-2.6/mm/readahead.c =================================================================== --- linux-2.6.orig/mm/readahead.c +++ linux-2.6/mm/readahead.c @@ -429,21 +429,20 @@ __do_page_cache_readahead(struct address /* * Preallocate as many pages as we will need. */ - read_lock_irq(&mapping->tree_lock); for (page_idx = 0; page_idx < nr_to_read; page_idx++) { pgoff_t page_offset = offset + page_idx; if (page_offset > end_index) break; + rcu_read_lock(); page = radix_tree_lookup(&mapping->page_tree, page_offset); + rcu_read_unlock(); if (page) continue; - read_unlock_irq(&mapping->tree_lock); page = page_cache_alloc_cold(mapping); cond_resched(); - read_lock_irq(&mapping->tree_lock); if (!page) break; page->index = page_offset; @@ -452,7 +451,6 @@ __do_page_cache_readahead(struct address SetPageReadahead(page); ret++; } - read_unlock_irq(&mapping->tree_lock); /* * Now start the IO. We ignore I/O errors - if the page is not Index: linux-2.6/mm/page-writeback.c =================================================================== --- linux-2.6.orig/mm/page-writeback.c +++ linux-2.6/mm/page-writeback.c @@ -967,17 +967,15 @@ void writeback_congestion_end(void) EXPORT_SYMBOL(writeback_congestion_end); /* - * Return true if any of the pages in the mapping are marged with the + * Return true if any of the pages in the mapping are marked with the * passed tag. */ int mapping_tagged(struct address_space *mapping, int tag) { - unsigned long flags; int ret; - - read_lock_irqsave(&mapping->tree_lock, flags); + rcu_read_lock(); ret = radix_tree_tagged(&mapping->page_tree, tag); - read_unlock_irqrestore(&mapping->tree_lock, flags); + rcu_read_unlock(); return ret; } EXPORT_SYMBOL(mapping_tagged); -- 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