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SubjectRe: [patch 6/7] mm: lockless pagecache
On Thu, Aug 11, 2005 at 10:28:04PM +1000, Nick Piggin wrote:
> 6/7
>
> --
> SUSE Labs, Novell Inc.
>

> Use the speculative get_page and the lockless radix tree lookups
> to introduce lockless page cache lookups (ie. no mapping->tree_lock).
>
> The only atomicity changes this should introduce is the use of a
> non atomic pagevec lookup for truncate, however what atomicity
> guarantees there were are probably not too useful anyway.

I don't understand the placement of the rcu_read_lock() and
rcu_read_unlock() calls. Again, possibly because I don't understand
the overall algorithm yet. And again, search for blank lines.

> Index: linux-2.6/mm/filemap.c
> ===================================================================
> --- linux-2.6.orig/mm/filemap.c
> +++ linux-2.6/mm/filemap.c
> @@ -379,18 +379,25 @@ int add_to_page_cache(struct page *page,
> int error = radix_tree_preload(gfp_mask & ~__GFP_HIGHMEM);
>
> if (error == 0) {
> + page_cache_get(page);
> + __SetPageLocked(page);
> + page->mapping = mapping;
> + page->index = offset;
> +
> write_lock_irq(&mapping->tree_lock);
> error = radix_tree_insert(&mapping->page_tree, offset, page);
> if (!error) {
> - page_cache_get(page);
> - SetPageLocked(page);
> - page->mapping = mapping;
> - page->index = offset;
> mapping->nrpages++;
> pagecache_acct(1);
> }
> write_unlock_irq(&mapping->tree_lock);
> radix_tree_preload_end();
> +
> + if (error) {
> + page->mapping = NULL;
> + __put_page(page);
> + __ClearPageLocked(page);
> + }
> }
> return error;
> }
> @@ -500,13 +507,15 @@ EXPORT_SYMBOL(__lock_page);
> */
> struct page * find_get_page(struct address_space *mapping, unsigned long offset)
> {
> - struct page *page;
> + struct page **pagep;
> + struct page *page = NULL;
>
> - 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();
> + pagep = (struct page **)radix_tree_lookup_slot(&mapping->page_tree,
> + offset);
> + if (pagep)
> + page = page_cache_get_speculative(pagep);

The data structures accessed by page_cache_get_speculative() don't
seem to be freed via RCU. My guess is that this is because these
data structures (struct page) never really go away. However, they
do get re-used, and this re-use must be protected against. It looks
to me that this protection takes the form of atomic instructions
in get_page_testone() and the like. If this is the case, then it
should be possible to push the rcu_read_lock() and rcu_read_unlock()
down into the radix_tree_lookup_slot().

> + rcu_read_unlock();
> return page;
> }
>
> @@ -519,12 +528,24 @@ struct page *find_trylock_page(struct ad
> {
> struct page *page;
>
> - read_lock_irq(&mapping->tree_lock);
> - page = radix_tree_lookup(&mapping->page_tree, offset);
> - if (page && TestSetPageLocked(page))
> - page = NULL;
> - read_unlock_irq(&mapping->tree_lock);
> - return page;
> + page = find_get_page(mapping, offset);
> + if (page) {
> + if (TestSetPageLocked(page))
> + goto out_failed;
> + /* Has the page been truncated before being locked? */
> + if (page->mapping != mapping || page->index != offset) {
> + unlock_page(page);
> + goto out_failed;
> + }
> +
> + /* Silly interface requires us to drop the refcount */
> + __put_page(page);
> + return page;
> +
> +out_failed:
> + page_cache_release(page);
> + }
> + return NULL;
> }
>
> EXPORT_SYMBOL(find_trylock_page);
> @@ -545,25 +566,17 @@ 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 (page->mapping != mapping || page->index != offset) {
> - unlock_page(page);
> - page_cache_release(page);
> - goto repeat;
> - }
> + lock_page(page);
> + /* Has the page been truncated before being locked? */
> + if (page->mapping != mapping || page->index != offset) {
> + unlock_page(page);
> + page_cache_release(page);
> + goto repeat;
> }
> }
> - read_unlock_irq(&mapping->tree_lock);
> return page;
> }
>
> @@ -646,6 +659,32 @@ unsigned find_get_pages(struct address_s
> return ret;
> }
>
> +unsigned find_get_pages_nonatomic(struct address_space *mapping, pgoff_t start,
> + unsigned int nr_pages, struct page **pages)
> +{
> + unsigned int i;
> + unsigned int ret;
> + unsigned int ret2;
> +
> + /*
> + * We do some unsightly casting to use the array first for storing
> + * pointers to the page pointers, and then for the pointers to
> + * the pages themselves that the caller wants.
> + */
> + rcu_read_lock();
> + ret = radix_tree_gang_lookup_slot(&mapping->page_tree,
> + (void ***)pages, start, nr_pages);
> + ret2 = 0;
> + for (i = 0; i < ret; i++) {
> + struct page *page;
> + page = page_cache_get_speculative(((struct page ***)pages)[i]);
> + if (page)
> + pages[ret2++] = page;
> + }

Same thing here -- I don't see why the rcu_read_lock() and rcu_read_unlock()
should not be pushed down into radix_tree_gang_lookup_slot().

> + rcu_read_unlock();
> + return ret2;
> +}
> +
> /*
> * Like find_get_pages, except we only return pages which are tagged with
> * `tag'. We update *index to index the next page for the traversal.
> Index: linux-2.6/mm/readahead.c
> ===================================================================
> --- linux-2.6.orig/mm/readahead.c
> +++ linux-2.6/mm/readahead.c
> @@ -272,27 +272,26 @@ __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++) {
> unsigned long page_offset = offset + page_idx;
>
> if (page_offset > end_index)
> break;
>
> + /* Don't need mapping->tree_lock - lookup can be racy */
> + rcu_read_lock();
> page = radix_tree_lookup(&mapping->page_tree, page_offset);
> + rcu_read_unlock();

Here, they could clearly be pushed down. ;-)

> if (page)
> continue;
>
> - read_unlock_irq(&mapping->tree_lock);
> page = page_cache_alloc_cold(mapping);
> - read_lock_irq(&mapping->tree_lock);
> if (!page)
> break;
> page->index = page_offset;
> list_add(&page->lru, &page_pool);
> 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/swap_state.c
> ===================================================================
> --- linux-2.6.orig/mm/swap_state.c
> +++ linux-2.6/mm/swap_state.c
> @@ -76,19 +76,26 @@ static int __add_to_swap_cache(struct pa
> BUG_ON(PagePrivate(page));
> error = radix_tree_preload(gfp_mask);
> if (!error) {
> + page_cache_get(page);
> + SetPageLocked(page);
> + SetPageSwapCache(page);
> + page->private = entry.val;
> +
> write_lock_irq(&swapper_space.tree_lock);
> error = radix_tree_insert(&swapper_space.page_tree,
> entry.val, page);
> if (!error) {
> - page_cache_get(page);
> - SetPageLocked(page);
> - SetPageSwapCache(page);
> - page->private = entry.val;
> total_swapcache_pages++;
> pagecache_acct(1);
> }
> write_unlock_irq(&swapper_space.tree_lock);
> radix_tree_preload_end();
> +
> + if (error) {
> + __put_page(page);
> + ClearPageLocked(page);
> + ClearPageSwapCache(page);
> + }
> }
> return error;
> }
> Index: linux-2.6/include/linux/page-flags.h
> ===================================================================
> --- linux-2.6.orig/include/linux/page-flags.h
> +++ linux-2.6/include/linux/page-flags.h
> @@ -167,16 +167,13 @@ extern void __mod_page_state(unsigned lo
> /*
> * Manipulation of page state flags
> */
> -#define PageLocked(page) \
> - test_bit(PG_locked, &(page)->flags)
> -#define SetPageLocked(page) \
> - set_bit(PG_locked, &(page)->flags)
> -#define TestSetPageLocked(page) \
> - test_and_set_bit(PG_locked, &(page)->flags)
> -#define ClearPageLocked(page) \
> - clear_bit(PG_locked, &(page)->flags)
> -#define TestClearPageLocked(page) \
> - test_and_clear_bit(PG_locked, &(page)->flags)
> +#define PageLocked(page) test_bit(PG_locked, &(page)->flags)
> +#define SetPageLocked(page) set_bit(PG_locked, &(page)->flags)
> +#define __SetPageLocked(page) __set_bit(PG_locked, &(page)->flags)
> +#define TestSetPageLocked(page) test_and_set_bit(PG_locked, &(page)->flags)
> +#define ClearPageLocked(page) clear_bit(PG_locked, &(page)->flags)
> +#define __ClearPageLocked(page) __clear_bit(PG_locked, &(page)->flags)
> +#define TestClearPageLocked(page) test_and_clear_bit(PG_locked, &(page)->flags)
>
> #define PageError(page) test_bit(PG_error, &(page)->flags)
> #define SetPageError(page) set_bit(PG_error, &(page)->flags)
> Index: linux-2.6/include/linux/pagemap.h
> ===================================================================
> --- linux-2.6.orig/include/linux/pagemap.h
> +++ linux-2.6/include/linux/pagemap.h
> @@ -130,6 +130,8 @@ extern struct page * find_or_create_page
> unsigned long index, unsigned int gfp_mask);
> unsigned find_get_pages(struct address_space *mapping, pgoff_t start,
> unsigned int nr_pages, struct page **pages);
> +unsigned find_get_pages_nonatomic(struct address_space *mapping, pgoff_t start,
> + unsigned int nr_pages, struct page **pages);
> unsigned find_get_pages_tag(struct address_space *mapping, pgoff_t *index,
> int tag, unsigned int nr_pages, struct page **pages);
>
> Index: linux-2.6/include/linux/pagevec.h
> ===================================================================
> --- linux-2.6.orig/include/linux/pagevec.h
> +++ linux-2.6/include/linux/pagevec.h
> @@ -25,6 +25,8 @@ void __pagevec_lru_add_active(struct pag
> void pagevec_strip(struct pagevec *pvec);
> unsigned pagevec_lookup(struct pagevec *pvec, struct address_space *mapping,
> pgoff_t start, unsigned nr_pages);
> +unsigned pagevec_lookup_nonatomic(struct pagevec *pvec,
> + struct address_space *mapping, pgoff_t start, unsigned nr_pages);
> unsigned pagevec_lookup_tag(struct pagevec *pvec,
> struct address_space *mapping, pgoff_t *index, int tag,
> unsigned nr_pages);
> Index: linux-2.6/mm/swap.c
> ===================================================================
> --- linux-2.6.orig/mm/swap.c
> +++ linux-2.6/mm/swap.c
> @@ -380,6 +380,19 @@ unsigned pagevec_lookup(struct pagevec *
> return pagevec_count(pvec);
> }
>
> +/**
> + * pagevec_lookup_nonatomic - non atomic pagevec_lookup
> + *
> + * This routine is non-atomic in that it may return blah.
> + */
> +unsigned pagevec_lookup_nonatomic(struct pagevec *pvec,
> + struct address_space *mapping, pgoff_t start, unsigned nr_pages)
> +{
> + pvec->nr = find_get_pages_nonatomic(mapping, start,
> + nr_pages, pvec->pages);
> + return pagevec_count(pvec);
> +}
> +
> unsigned pagevec_lookup_tag(struct pagevec *pvec, struct address_space *mapping,
> pgoff_t *index, int tag, unsigned nr_pages)
> {
> Index: linux-2.6/mm/truncate.c
> ===================================================================
> --- linux-2.6.orig/mm/truncate.c
> +++ linux-2.6/mm/truncate.c
> @@ -126,7 +126,7 @@ void truncate_inode_pages(struct address
>
> pagevec_init(&pvec, 0);
> next = start;
> - while (pagevec_lookup(&pvec, mapping, next, PAGEVEC_SIZE)) {
> + while (pagevec_lookup_nonatomic(&pvec, mapping, next, PAGEVEC_SIZE)) {
> for (i = 0; i < pagevec_count(&pvec); i++) {
> struct page *page = pvec.pages[i];
> pgoff_t page_index = page->index;
> @@ -160,7 +160,7 @@ void truncate_inode_pages(struct address
> next = start;
> for ( ; ; ) {
> cond_resched();
> - if (!pagevec_lookup(&pvec, mapping, next, PAGEVEC_SIZE)) {
> + if (!pagevec_lookup_nonatomic(&pvec, mapping, next, PAGEVEC_SIZE)) {
> if (next == start)
> break;
> next = start;
> @@ -206,7 +206,7 @@ unsigned long invalidate_mapping_pages(s
>
> pagevec_init(&pvec, 0);
> while (next <= end &&
> - pagevec_lookup(&pvec, mapping, next, PAGEVEC_SIZE)) {
> + pagevec_lookup_nonatomic(&pvec, mapping, next, PAGEVEC_SIZE)) {
> for (i = 0; i < pagevec_count(&pvec); i++) {
> struct page *page = pvec.pages[i];
>
> Index: linux-2.6/mm/page-writeback.c
> ===================================================================
> --- linux-2.6.orig/mm/page-writeback.c
> +++ linux-2.6/mm/page-writeback.c
> @@ -811,6 +811,7 @@ int mapping_tagged(struct address_space
> unsigned long flags;
> int ret;
>
> + /* XXX: radix_tree_tagged is safe to run without the lock? */
> read_lock_irqsave(&mapping->tree_lock, flags);
> ret = radix_tree_tagged(&mapping->page_tree, tag);
> read_unlock_irqrestore(&mapping->tree_lock, flags);

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