From: Hugh Dickins Good example of "swapper_space considered harmful": swap_unplug_io_fn was originally designed for calling via swapper_space.backing_dev_info; but that way it loses track of which device is to be unplugged, so had to unplug all swap devices. But now sync_page tests SwapCache anyway, can call swap_unplug_io_fn with page, which leads direct to the device. Reverted -mc4's CONFIG_SWAP=n fix, just add another NOTHING for it. Reverted -mc3's editorial adjustments to swap_backing_dev_info and swapper_space initializations: they document the few fields which are actually used now, as comment above them says (sound of slapped wrist). --- 25-akpm/include/linux/swap.h | 5 ++--- 25-akpm/mm/filemap.c | 2 +- 25-akpm/mm/nommu.c | 5 ----- 25-akpm/mm/swap_state.c | 4 ++-- 25-akpm/mm/swapfile.c | 23 +++++++++++++++-------- 5 files changed, 20 insertions(+), 19 deletions(-) diff -puN include/linux/swap.h~rmap-5-swap_unplug-page include/linux/swap.h --- 25/include/linux/swap.h~rmap-5-swap_unplug-page 2004-04-11 12:39:03.598313048 -0700 +++ 25-akpm/include/linux/swap.h 2004-04-11 12:39:03.608311528 -0700 @@ -181,8 +181,6 @@ extern int vm_swappiness; extern int shmem_unuse(swp_entry_t entry, struct page *page); #endif /* CONFIG_MMU */ -extern void swap_unplug_io_fn(struct backing_dev_info *); - #ifdef CONFIG_SWAP /* linux/mm/page_io.c */ extern int swap_readpage(struct file *, struct page *); @@ -218,7 +216,7 @@ extern sector_t map_swap_page(struct swa extern struct swap_info_struct *get_swap_info_struct(unsigned); extern int can_share_swap_page(struct page *); extern int remove_exclusive_swap_page(struct page *); -struct backing_dev_info; +extern void swap_unplug_io_fn(struct page *); extern struct swap_list_t swap_list; extern spinlock_t swaplock; @@ -252,6 +250,7 @@ extern spinlock_t swaplock; #define move_from_swap_cache(p, i, m) 1 #define __delete_from_swap_cache(p) /*NOTHING*/ #define delete_from_swap_cache(p) /*NOTHING*/ +#define swap_unplug_io_fn(p) /*NOTHING*/ static inline int remove_exclusive_swap_page(struct page *p) { diff -puN mm/filemap.c~rmap-5-swap_unplug-page mm/filemap.c --- 25/mm/filemap.c~rmap-5-swap_unplug-page 2004-04-11 12:39:03.600312744 -0700 +++ 25-akpm/mm/filemap.c 2004-04-11 12:39:03.609311376 -0700 @@ -127,7 +127,7 @@ static inline int sync_page(struct page if (mapping->a_ops && mapping->a_ops->sync_page) return mapping->a_ops->sync_page(page); } else if (PageSwapCache(page)) { - swap_unplug_io_fn(NULL); + swap_unplug_io_fn(page); } return 0; } diff -puN mm/nommu.c~rmap-5-swap_unplug-page mm/nommu.c --- 25/mm/nommu.c~rmap-5-swap_unplug-page 2004-04-11 12:39:03.601312592 -0700 +++ 25-akpm/mm/nommu.c 2004-04-11 12:39:03.610311224 -0700 @@ -18,7 +18,6 @@ #include #include #include -#include #include #include @@ -572,7 +571,3 @@ unsigned long get_unmapped_area(struct f void pte_chain_init(void) { } - -void swap_unplug_io_fn(struct backing_dev_info *) -{ -} diff -puN mm/swapfile.c~rmap-5-swap_unplug-page mm/swapfile.c --- 25/mm/swapfile.c~rmap-5-swap_unplug-page 2004-04-11 12:39:03.603312288 -0700 +++ 25-akpm/mm/swapfile.c 2004-04-11 12:39:03.612310920 -0700 @@ -86,19 +86,26 @@ static void remove_swap_bdev(struct bloc BUG(); } -void swap_unplug_io_fn(struct backing_dev_info *unused_bdi) +/* + * Unlike a standard unplug_io_fn, swap_unplug_io_fn is never called + * through swap's backing_dev_info (which is only used by shrink_list), + * but directly from sync_page when PageSwapCache: and takes the page + * as argument, so that it can find the right device from swp_entry_t. + */ +void swap_unplug_io_fn(struct page *page) { - int i; + swp_entry_t entry; down(&swap_bdevs_sem); - for (i = 0; i < MAX_SWAPFILES; i++) { - struct block_device *bdev = swap_bdevs[i]; + entry.val = page->private; + if (PageSwapCache(page)) { + struct block_device *bdev = swap_bdevs[swp_type(entry)]; struct backing_dev_info *bdi; - if (bdev == NULL) - break; - bdi = bdev->bd_inode->i_mapping->backing_dev_info; - (*bdi->unplug_io_fn)(bdi); + if (bdev) { + bdi = bdev->bd_inode->i_mapping->backing_dev_info; + (*bdi->unplug_io_fn)(bdi); + } } up(&swap_bdevs_sem); } diff -puN mm/swap_state.c~rmap-5-swap_unplug-page mm/swap_state.c --- 25/mm/swap_state.c~rmap-5-swap_unplug-page 2004-04-11 12:39:03.605311984 -0700 +++ 25-akpm/mm/swap_state.c 2004-04-11 12:39:03.611311072 -0700 @@ -25,13 +25,13 @@ static struct address_space_operations s }; static struct backing_dev_info swap_backing_dev_info = { - .memory_backed = 1, /* Does not contribute to dirty memory */ - .unplug_io_fn = swap_unplug_io_fn, + .state = 0, /* uncongested */ }; struct address_space swapper_space = { .page_tree = RADIX_TREE_INIT(GFP_ATOMIC), .tree_lock = SPIN_LOCK_UNLOCKED, + .nrpages = 0, /* total_swapcache_pages */ .a_ops = &swap_aops, .backing_dev_info = &swap_backing_dev_info, }; _