From 41d78ba55037468e6c86c53e3076d1a74841de39 Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Tue, 14 Feb 2006 13:52:58 -0800 Subject: [PATCH] compound page: use page[1].lru If a compound page has its own put_page_testzero destructor (the only current example is free_huge_page), that is noted in page[1].mapping of the compound page. But that's rather a poor place to keep it: functions which call set_page_dirty_lock after get_user_pages (e.g. Infiniband's __ib_umem_release) ought to be checking first, otherwise set_page_dirty is liable to crash on what's not the address of a struct address_space. And now I'm about to make that worse: it turns out that every compound page needs a destructor, so we can no longer rely on hugetlb pages going their own special way, to avoid further problems of page->mapping reuse. For example, not many people know that: on 50% of i386 -Os builds, the first tail page of a compound page purports to be PageAnon (when its destructor has an odd address), which surprises page_add_file_rmap. Keep the compound page destructor in page[1].lru.next instead. And to free up the common pairing of mapping and index, also move compound page order from index to lru.prev. Slab reuses page->lru too: but if we ever need slab to use compound pages, it can easily stack its use above this. (akpm: decoded version of the above: the tail pages of a compound page now have ->mapping==NULL, so there's no need for the set_page_dirty[_lock]() caller to check that they're not compund pages before doing the dirty). Signed-off-by: Hugh Dickins Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/page_alloc.c | 15 ++++++--------- 1 file changed, 6 insertions(+), 9 deletions(-) (limited to 'mm/page_alloc.c') diff --git a/mm/page_alloc.c b/mm/page_alloc.c index dde04ff4be3..eec89ab39bb 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -169,20 +169,17 @@ static void bad_page(struct page *page) * All pages have PG_compound set. All pages have their ->private pointing at * the head page (even the head page has this). * - * The first tail page's ->mapping, if non-zero, holds the address of the - * compound page's put_page() function. - * - * The order of the allocation is stored in the first tail page's ->index - * This is only for debug at present. This usage means that zero-order pages - * may not be compound. + * The first tail page's ->lru.next holds the address of the compound page's + * put_page() function. Its ->lru.prev holds the order of allocation. + * This usage means that zero-order pages may not be compound. */ static void prep_compound_page(struct page *page, unsigned long order) { int i; int nr_pages = 1 << order; - page[1].mapping = NULL; - page[1].index = order; + page[1].lru.next = NULL; /* set dtor */ + page[1].lru.prev = (void *)order; for (i = 0; i < nr_pages; i++) { struct page *p = page + i; @@ -196,7 +193,7 @@ static void destroy_compound_page(struct page *page, unsigned long order) int i; int nr_pages = 1 << order; - if (unlikely(page[1].index != order)) + if (unlikely((unsigned long)page[1].lru.prev != order)) bad_page(page); for (i = 0; i < nr_pages; i++) { -- cgit v1.2.3