From d9fe526a83b84edc9c5ff217a00c896bfc20b2ce Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Mon, 4 Feb 2008 22:28:51 -0800 Subject: [PATCH] tmpfs: allow filepage alongside swappage tmpfs has long allowed for a fresh filepage to be created in pagecache, just before shmem_getpage gets the chance to match it up with the swappage which already belongs to that offset. But unionfs_writepage now does a find_or_create_page, divorced from shmem_getpage, which leaves conflicting filepage and swappage outstanding indefinitely, when unionfs is over tmpfs. Therefore shmem_writepage (where a page is swizzled from file to swap) must now be on the lookout for existing swap, ready to free it in favour of the more uptodate filepage, instead of BUGging on that clash. And when the add_to_page_cache fails in shmem_unuse_inode, it must defer to an uptodate filepage, otherwise swapoff would hang. Whereas when add_to_page_cache fails in shmem_getpage, it should retry in the same way it already does. Signed-off-by: Hugh Dickins Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/shmem.c | 69 ++++++++++++++++++++++++++++++++++-------------------- 1 file changed, 44 insertions(+), 25 deletions(-) diff --git a/mm/shmem.c b/mm/shmem.c index e577adf4ae85..4ae47f54c822 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -827,6 +827,7 @@ static int shmem_unuse_inode(struct shmem_inode_info *info, swp_entry_t entry, s struct page *subdir; swp_entry_t *ptr; int offset; + int error; idx = 0; ptr = info->i_direct; @@ -884,7 +885,20 @@ static int shmem_unuse_inode(struct shmem_inode_info *info, swp_entry_t entry, s found: idx += offset; inode = &info->vfs_inode; - if (add_to_page_cache(page, inode->i_mapping, idx, GFP_ATOMIC) == 0) { + error = add_to_page_cache(page, inode->i_mapping, idx, GFP_ATOMIC); + if (error == -EEXIST) { + struct page *filepage = find_get_page(inode->i_mapping, idx); + if (filepage) { + /* + * There might be a more uptodate page coming down + * from a stacked writepage: forget our swappage if so. + */ + if (PageUptodate(filepage)) + error = 0; + page_cache_release(filepage); + } + } + if (!error) { delete_from_swap_cache(page); set_page_dirty(page); info->flags |= SHMEM_PAGEIN; @@ -937,44 +951,45 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc) struct inode *inode; BUG_ON(!PageLocked(page)); - /* - * shmem_backing_dev_info's capabilities prevent regular writeback or - * sync from ever calling shmem_writepage; but a stacking filesystem - * may use the ->writepage of its underlying filesystem, in which case - * we want to do nothing when that underlying filesystem is tmpfs - * (writing out to swap is useful as a response to memory pressure, but - * of no use to stabilize the data) - just redirty the page, unlock it - * and claim success in this case. AOP_WRITEPAGE_ACTIVATE, and the - * page_mapped check below, must be avoided unless we're in reclaim. - */ - if (!wbc->for_reclaim) { - set_page_dirty(page); - unlock_page(page); - return 0; - } - BUG_ON(page_mapped(page)); - mapping = page->mapping; index = page->index; inode = mapping->host; info = SHMEM_I(inode); if (info->flags & VM_LOCKED) goto redirty; - swap = get_swap_page(); - if (!swap.val) + if (!total_swap_pages) goto redirty; + /* + * shmem_backing_dev_info's capabilities prevent regular writeback or + * sync from ever calling shmem_writepage; but a stacking filesystem + * may use the ->writepage of its underlying filesystem, in which case + * tmpfs should write out to swap only in response to memory pressure, + * and not for pdflush or sync. However, in those cases, we do still + * want to check if there's a redundant swappage to be discarded. + */ + if (wbc->for_reclaim) + swap = get_swap_page(); + else + swap.val = 0; + spin_lock(&info->lock); - shmem_recalc_inode(inode); if (index >= info->next_index) { BUG_ON(!(info->flags & SHMEM_TRUNCATE)); goto unlock; } entry = shmem_swp_entry(info, index, NULL); - BUG_ON(!entry); - BUG_ON(entry->val); + if (entry->val) { + /* + * The more uptodate page coming down from a stacked + * writepage should replace our old swappage. + */ + free_swap_and_cache(*entry); + shmem_swp_set(info, entry, 0); + } + shmem_recalc_inode(inode); - if (add_to_swap_cache(page, swap, GFP_ATOMIC) == 0) { + if (swap.val && add_to_swap_cache(page, swap, GFP_ATOMIC) == 0) { remove_from_page_cache(page); shmem_swp_set(info, entry, swap.val); shmem_swp_unmap(entry); @@ -986,6 +1001,7 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc) spin_unlock(&shmem_swaplist_lock); } swap_duplicate(swap); + BUG_ON(page_mapped(page)); page_cache_release(page); /* pagecache ref */ set_page_dirty(page); unlock_page(page); @@ -998,7 +1014,10 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc) swap_free(swap); redirty: set_page_dirty(page); - return AOP_WRITEPAGE_ACTIVATE; /* Return with the page locked */ + if (wbc->for_reclaim) + return AOP_WRITEPAGE_ACTIVATE; /* Return with page locked */ + unlock_page(page); + return 0; } #ifdef CONFIG_NUMA