[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 <hugh@veritas.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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3 changed files with 9 additions and 12 deletions
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@ -85,7 +85,7 @@ void free_huge_page(struct page *page)
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BUG_ON(page_count(page));
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INIT_LIST_HEAD(&page->lru);
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page[1].mapping = NULL;
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page[1].lru.next = NULL; /* reset dtor */
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spin_lock(&hugetlb_lock);
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enqueue_huge_page(page);
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@ -105,7 +105,7 @@ struct page *alloc_huge_page(struct vm_area_struct *vma, unsigned long addr)
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}
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spin_unlock(&hugetlb_lock);
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set_page_count(page, 1);
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page[1].mapping = (void *)free_huge_page;
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page[1].lru.next = (void *)free_huge_page; /* set dtor */
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for (i = 0; i < (HPAGE_SIZE/PAGE_SIZE); ++i)
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clear_user_highpage(&page[i], addr);
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return page;
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@ -169,20 +169,17 @@ static void bad_page(struct page *page)
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* All pages have PG_compound set. All pages have their ->private pointing at
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* the head page (even the head page has this).
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*
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* The first tail page's ->mapping, if non-zero, holds the address of the
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* compound page's put_page() function.
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*
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* The order of the allocation is stored in the first tail page's ->index
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* This is only for debug at present. This usage means that zero-order pages
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* may not be compound.
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* The first tail page's ->lru.next holds the address of the compound page's
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* put_page() function. Its ->lru.prev holds the order of allocation.
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* This usage means that zero-order pages may not be compound.
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*/
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static void prep_compound_page(struct page *page, unsigned long order)
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{
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int i;
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int nr_pages = 1 << order;
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page[1].mapping = NULL;
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page[1].index = order;
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page[1].lru.next = NULL; /* set dtor */
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page[1].lru.prev = (void *)order;
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for (i = 0; i < nr_pages; i++) {
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struct page *p = page + i;
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@ -196,7 +193,7 @@ static void destroy_compound_page(struct page *page, unsigned long order)
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int i;
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int nr_pages = 1 << order;
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if (unlikely(page[1].index != order))
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if (unlikely((unsigned long)page[1].lru.prev != order))
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bad_page(page);
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for (i = 0; i < nr_pages; i++) {
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@ -40,7 +40,7 @@ static void put_compound_page(struct page *page)
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if (put_page_testzero(page)) {
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void (*dtor)(struct page *page);
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dtor = (void (*)(struct page *))page[1].mapping;
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dtor = (void (*)(struct page *))page[1].lru.next;
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(*dtor)(page);
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}
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}
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