On Thu, Sep 03, 2015 at 03:35:51PM +0300, Kirill A. Shutemov wrote: > Kirill A. Shutemov (7): > mm: drop page->slab_page > slub: use page->rcu_head instead of page->lru plus cast > zsmalloc: use page->private instead of page->first_page > mm: pack compound_dtor and compound_order into one word in struct page > mm: make compound_head() robust > mm: use 'unsigned int' for page order > mm: use 'unsigned int' for compound_dtor/compound_order on 64BIT Reviewed-by: Andrea Arcangeli <aarcange@xxxxxxxxxx> The only other alternative solution that doesn't require finding a bit zero at the LSB in a field unused in tail pages, is to drop both PG_head and PG_tail, and reserve 4 bits from page->flags. This means a net loss of 2 bits from page->flags (and loss of 3 bits if !CONFIG_PAGEFLAGS_EXTENDED), but then everything becomes simple and there's no need of finding a LSB field that is guaranteed zero at all times. With those 4 bits, you clear them for not compound pages. When you create a compound page you encode the compound_order in those 4 bits of page->flags, equal for for all head and tail pages. compound_order() then becomes atomically available for tail pages too and compound_order goes away from struct page along with first_page (and there's no need to add a compound_head). In PageCompound you read the 4 bits, if they're not all zero it's compound, otherwise it's not. In PageHead/Tail, if the 4 bits are all zero it's not head/tail, otherwise you do the math on the page_to_pfn(page). If the pfn is naturally aligned against the order encoded in the 4 bits "!(pfn & (1<<order)-1)" it's a head, otherwise it's a tail. If it's a tail, for the compound_head then it's just a matter of doing "return page - (pfn & ((1<<order)-1)" (no need of pfn_to_page). This leverages the physical natural alignment of compound pages for all orders. It'd cover up to CONFIG_FORCE_MAX_ZONEORDER == 16 (128MBytes of order 15 with PAGE_SIZE 4kb). page_to_pfn can actually be replaced with (&NODE_DATA(page_to_nid(page))->node_mem_map-page) which is faster as page_to_nid only need to accesses page->flags which is already in L1. So then it costs only one cacheline access in the pgdat and a sub. Because of the two (or three) additional bits taken out of page->flags I doubt it's viable on 32bit, but I thought I'd mention it just in case. Thanks, Andrea -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>