From 71a3f4edc11b9dd7af28d003acbbd33496003da1 Mon Sep 17 00:00:00 2001 From: Rusty Russell Date: Tue, 12 Aug 2008 17:52:53 -0500 Subject: lguest: use get_user_pages_fast() instead of get_user_pages() Using a simple page table thrashing program I measure a slight improvement. The program creates five processes. Each touches 1000 pages then schedules the next process. We repeat this 1000 times. As lguest only caches 4 cr3 values, this rebuilds a lot of shadow page tables requiring virt->phys mappings. Before: 5.93 seconds After: 5.40 seconds (Counts of slow vs fastpath in this usage are 6092 and 2852462 respectively.) And more importantly for lguest, the code is simpler. Signed-off-by: Rusty Russell --- drivers/lguest/page_tables.c | 25 +++++++++---------------- 1 file changed, 9 insertions(+), 16 deletions(-) (limited to 'drivers/lguest') diff --git a/drivers/lguest/page_tables.c b/drivers/lguest/page_tables.c index d93500f24fbb..81d0c6053447 100644 --- a/drivers/lguest/page_tables.c +++ b/drivers/lguest/page_tables.c @@ -108,9 +108,8 @@ static unsigned long gpte_addr(pgd_t gpgd, unsigned long vaddr) } /*:*/ -/*M:014 get_pfn is slow; it takes the mmap sem and calls get_user_pages. We - * could probably try to grab batches of pages here as an optimization - * (ie. pre-faulting). :*/ +/*M:014 get_pfn is slow: we could probably try to grab batches of pages here as + * an optimization (ie. pre-faulting). :*/ /*H:350 This routine takes a page number given by the Guest and converts it to * an actual, physical page number. It can fail for several reasons: the @@ -123,19 +122,13 @@ static unsigned long gpte_addr(pgd_t gpgd, unsigned long vaddr) static unsigned long get_pfn(unsigned long virtpfn, int write) { struct page *page; - /* This value indicates failure. */ - unsigned long ret = -1UL; - /* get_user_pages() is a complex interface: it gets the "struct - * vm_area_struct" and "struct page" assocated with a range of pages. - * It also needs the task's mmap_sem held, and is not very quick. - * It returns the number of pages it got. */ - down_read(¤t->mm->mmap_sem); - if (get_user_pages(current, current->mm, virtpfn << PAGE_SHIFT, - 1, write, 1, &page, NULL) == 1) - ret = page_to_pfn(page); - up_read(¤t->mm->mmap_sem); - return ret; + /* gup me one page at this address please! */ + if (get_user_pages_fast(virtpfn << PAGE_SHIFT, 1, write, &page) == 1) + return page_to_pfn(page); + + /* This value indicates failure. */ + return -1UL; } /*H:340 Converting a Guest page table entry to a shadow (ie. real) page table @@ -174,7 +167,7 @@ static pte_t gpte_to_spte(struct lg_cpu *cpu, pte_t gpte, int write) /*H:460 And to complete the chain, release_pte() looks like this: */ static void release_pte(pte_t pte) { - /* Remember that get_user_pages() took a reference to the page, in + /* Remember that get_user_pages_fast() took a reference to the page, in * get_pfn()? We have to put it back now. */ if (pte_flags(pte) & _PAGE_PRESENT) put_page(pfn_to_page(pte_pfn(pte))); -- cgit v1.2.3