Commit 41d78ba5 authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds

[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: 's avatarHugh Dickins <hugh@veritas.com>
Signed-off-by: 's avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: 's avatarLinus Torvalds <torvalds@osdl.org>
parent 72772323
......@@ -85,7 +85,7 @@ void free_huge_page(struct page *page)
BUG_ON(page_count(page));
INIT_LIST_HEAD(&page->lru);
page[1].mapping = NULL;
page[1].lru.next = NULL; /* reset dtor */
spin_lock(&hugetlb_lock);
enqueue_huge_page(page);
......@@ -105,7 +105,7 @@ struct page *alloc_huge_page(struct vm_area_struct *vma, unsigned long addr)
}
spin_unlock(&hugetlb_lock);
set_page_count(page, 1);
page[1].mapping = (void *)free_huge_page;
page[1].lru.next = (void *)free_huge_page; /* set dtor */
for (i = 0; i < (HPAGE_SIZE/PAGE_SIZE); ++i)
clear_user_highpage(&page[i], addr);
return page;
......
......@@ -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++) {
......
......@@ -40,7 +40,7 @@ static void put_compound_page(struct page *page)
if (put_page_testzero(page)) {
void (*dtor)(struct page *page);
dtor = (void (*)(struct page *))page[1].mapping;
dtor = (void (*)(struct page *))page[1].lru.next;
(*dtor)(page);
}
}
......
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