From 6922c0c7abd387374255801f7739624867e8acad Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Wed, 3 Aug 2011 16:21:25 -0700 Subject: [PATCH] tmpfs: convert shmem_writepage and enable swap Convert shmem_writepage() to use shmem_delete_from_page_cache() to use shmem_radix_tree_replace() to substitute swap entry for page pointer atomically in the radix tree. As with shmem_add_to_page_cache(), it's not entirely satisfactory to be copying such code from delete_from_swap_cache, but again judged easier to sell than making its other callers go through the extras. Remove the toy implementation's shmem_put_swap() and shmem_get_swap(), now unreferenced, and the hack to disable swap: it's now good to go. The way things have worked out, info->lock no longer helps to guard the shmem_swaplist: we increment swapped under shmem_swaplist_mutex only. That global mutex exclusion between shmem_writepage() and shmem_unuse() is not pretty, and we ought to find another way; but it's been forced on us by recent race discoveries, not a consequence of this patchset. And what has become of the WARN_ON_ONCE(1) free_swap_and_cache() if a swap entry was found already present? That's no longer possible, the (unknown) one inserting this page into filecache would hit the swap entry occupying that slot. Signed-off-by: Hugh Dickins Acked-by: Rik van Riel Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/shmem.c | 88 +++++++++++++++++++++++------------------------------- 1 file changed, 37 insertions(+), 51 deletions(-) diff --git a/mm/shmem.c b/mm/shmem.c index 13ef2d7e912d..0f094a258526 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -6,7 +6,8 @@ * 2000-2001 Christoph Rohland * 2000-2001 SAP AG * 2002 Red Hat Inc. - * Copyright (C) 2002-2005 Hugh Dickins. + * Copyright (C) 2002-2011 Hugh Dickins. + * Copyright (C) 2011 Google Inc. * Copyright (C) 2002-2005 VERITAS Software Corporation. * Copyright (C) 2004 Andi Kleen, SuSE Labs * @@ -219,19 +220,6 @@ static void shmem_recalc_inode(struct inode *inode) } } -static void shmem_put_swap(struct shmem_inode_info *info, pgoff_t index, - swp_entry_t swap) -{ - if (index < SHMEM_NR_DIRECT) - info->i_direct[index] = swap; -} - -static swp_entry_t shmem_get_swap(struct shmem_inode_info *info, pgoff_t index) -{ - return (index < SHMEM_NR_DIRECT) ? - info->i_direct[index] : (swp_entry_t){0}; -} - /* * Replace item expected in radix tree by a new item, while holding tree lock. */ @@ -299,6 +287,25 @@ static int shmem_add_to_page_cache(struct page *page, return error; } +/* + * Like delete_from_page_cache, but substitutes swap for page. + */ +static void shmem_delete_from_page_cache(struct page *page, void *radswap) +{ + struct address_space *mapping = page->mapping; + int error; + + spin_lock_irq(&mapping->tree_lock); + error = shmem_radix_tree_replace(mapping, page->index, page, radswap); + page->mapping = NULL; + mapping->nrpages--; + __dec_zone_page_state(page, NR_FILE_PAGES); + __dec_zone_page_state(page, NR_SHMEM); + spin_unlock_irq(&mapping->tree_lock); + page_cache_release(page); + BUG_ON(error); +} + /* * Like find_get_pages, but collecting swap entries as well as pages. */ @@ -664,14 +671,10 @@ int shmem_unuse(swp_entry_t swap, struct page *page) mutex_lock(&shmem_swaplist_mutex); list_for_each_safe(this, next, &shmem_swaplist) { info = list_entry(this, struct shmem_inode_info, swaplist); - if (!info->swapped) { - spin_lock(&info->lock); - if (!info->swapped) - list_del_init(&info->swaplist); - spin_unlock(&info->lock); - } if (info->swapped) found = shmem_unuse_inode(info, swap, page); + else + list_del_init(&info->swaplist); cond_resched(); if (found) break; @@ -694,10 +697,10 @@ out: static int shmem_writepage(struct page *page, struct writeback_control *wbc) { struct shmem_inode_info *info; - swp_entry_t swap, oswap; struct address_space *mapping; - pgoff_t index; struct inode *inode; + swp_entry_t swap; + pgoff_t index; BUG_ON(!PageLocked(page)); mapping = page->mapping; @@ -720,55 +723,38 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc) WARN_ON_ONCE(1); /* Still happens? Tell us about it! */ goto redirty; } - - /* - * Disable even the toy swapping implementation, while we convert - * functions one by one to having swap entries in the radix tree. - */ - if (index < ULONG_MAX) - goto redirty; - swap = get_swap_page(); if (!swap.val) goto redirty; /* * Add inode to shmem_unuse()'s list of swapped-out inodes, - * if it's not already there. Do it now because we cannot take - * mutex while holding spinlock, and must do so before the page - * is moved to swap cache, when its pagelock no longer protects + * if it's not already there. Do it now before the page is + * moved to swap cache, when its pagelock no longer protects * the inode from eviction. But don't unlock the mutex until - * we've taken the spinlock, because shmem_unuse_inode() will - * prune a !swapped inode from the swaplist under both locks. + * we've incremented swapped, because shmem_unuse_inode() will + * prune a !swapped inode from the swaplist under this mutex. */ mutex_lock(&shmem_swaplist_mutex); if (list_empty(&info->swaplist)) list_add_tail(&info->swaplist, &shmem_swaplist); - spin_lock(&info->lock); - mutex_unlock(&shmem_swaplist_mutex); - - oswap = shmem_get_swap(info, index); - if (oswap.val) { - WARN_ON_ONCE(1); /* Still happens? Tell us about it! */ - free_swap_and_cache(oswap); - shmem_put_swap(info, index, (swp_entry_t){0}); - info->swapped--; - } - shmem_recalc_inode(inode); - if (add_to_swap_cache(page, swap, GFP_ATOMIC) == 0) { - delete_from_page_cache(page); - shmem_put_swap(info, index, swap); - info->swapped++; swap_shmem_alloc(swap); + shmem_delete_from_page_cache(page, swp_to_radix_entry(swap)); + + spin_lock(&info->lock); + info->swapped++; + shmem_recalc_inode(inode); spin_unlock(&info->lock); + + mutex_unlock(&shmem_swaplist_mutex); BUG_ON(page_mapped(page)); swap_writepage(page, wbc); return 0; } - spin_unlock(&info->lock); + mutex_unlock(&shmem_swaplist_mutex); swapcache_free(swap, NULL); redirty: set_page_dirty(page);