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 <rusty@rustcorp.com.au>

1 files changed, 9 insertions, 16 deletions

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(&current->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(&current->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)));