Page host virtual assist: minor fault optimization.

PATCH In review Martin Schwidefsky 1 participant 1 message Apr 24, 2006

Subsystems (5)

Memory management MM core Page cache MM swap +1 more subsystems

lore

Share & tools

lore

Martin Schwidefsky

author

2006-04-24 12:37 UTC #0 lore raw

PATCH 6/8 Page host virtual assist: minor fault optimization.

[patch 6/8] Page host virtual assist: minor fault optimization.

On of the challenges of hva is the cost for the state transitions. If the cost gets too big the whole concept of page state information is in question. Therefore it is very important to avoid the state transitions for minor faults. Why change the page state to stable in find_get_page and back in page_add_anon_rmap/page_add_file_rmap if the discarded pages can be handled by the discard fault handler? If the page is in page/swap cache just map it even if it is already discarded. The first access to the page will cause a discard fault which needs to be able to deal with this kind of situation anyway because of races in the memory management.

To do this the special find_get_page_nohv variant introduced for volatile swap cache is used which does not change the page state. The call to find_get_page in filemap_nopage and filemap_getpage are replaced with find_get_page_nohv. By the use of this function a new race condition is created. If a minor fault races with the discard of a page the page may not get mapped to the page table because the discard handler removed the page from the cache which removes the page->mapping that is needed to find the page table entry. A check for the PG_discarded bit is added to do_swap_page and do_no_page. The page table lock for the pte takes care of the synchronization.

After that there is only one state transition left in the minor fault. page_add_anon_rmap/page_add_file_rmap try to get the page into volatile state. If these two calls are removed we end up with almost all pages in stable. The reason is that if a page is not uptodate yet, there is an additional reference acquired from filemap_nopage. After the page has been brought uptodate a page_hva_make_volatile needs to be done with an offset of 2 (page cache reference + additional reference from filemap_nopage).

That removes the state transitions on the minor fault path. A page that has been mapped will eventually be unmapped again. On the unmap path each page that has been removed from the page table is freed with a call to page_cache_release. In general that causes an unnecessary page state transition from volatile to volatile. Not what we want. To get rid of these state transitions as well special variants of put_page_testzero/ page_cache_release are introduced that do not attempt to make the page volatile. page_cache_release_nohv is then used in free_page_and_swap_cache and release_pages. This makes the unmap of ptes state transitions free.

Signed-off-by Martin Schwidefsky <[email protected]>

---

 include/linux/mm.h      |   11 +++++--
 include/linux/pagemap.h |    5 +++
 include/linux/swap.h    |    2 -
 mm/filemap.c            |   73 +++++++++++++++++++++++++++++++++++++++++++-----
 mm/fremap.c             |    1 
 mm/memory.c             |    6 ++-
 mm/rmap.c               |    4 --
 mm/swap.c               |   26 ++++++++++++++++-
 mm/swap_state.c         |    4 +-
 9 files changed, 113 insertions(+), 19 deletions(-)

include/linux/mm.h +8 -3

diff -urpN linux-2.6/include/linux/mm.h linux-2.6-patched/include/linux/mm.h
@@ -307,11 +307,15 @@ struct page {
  * put_page_testzero checks if the page can be made volatile if the page
  * still has users and the page host virtual assist is enabled.
  */
-static inline int put_page_testzero(struct page *page)
+static inline int put_page_testzero_nohv(struct page *page)
 {
-	int ret;
 	VM_BUG_ON(atomic_read(&page->_count) == 0);
-	ret = atomic_dec_and_test(&page->_count);
+	return atomic_dec_and_test(&page->_count);
+}
+
+static inline int put_page_testzero(struct page *page)
+{
+	int ret = put_page_testzero_nohv(page);
 	if (!ret)
 		page_hva_make_volatile(page, 1);
 	return ret;

@@ -354,6 +358,7 @@ static inline void init_page_count(struc
 }
 
 void put_page(struct page *page);
+void put_page_nohv(struct page *page);
 
 void split_page(struct page *page, unsigned int order);

include/linux/pagemap.h +5 -0

diff -urpN linux-2.6/include/linux/pagemap.h linux-2.6-patched/include/linux/pagemap.h
@@ -49,6 +49,11 @@ static inline void mapping_set_gfp_mask(
 
 #define page_cache_get(page)		get_page(page)
 #define page_cache_release(page)	put_page(page)
+#if defined(CONFIG_PAGE_HVA)
+#define page_cache_release_nohv(page)	put_page_nohv(page)
+#else
+#define page_cache_release_nohv(page)	put_page(page)
+#endif
 void release_pages(struct page **pages, int nr, int cold);
 
 #ifdef CONFIG_NUMA

include/linux/swap.h +1 -1

diff -urpN linux-2.6/include/linux/swap.h linux-2.6-patched/include/linux/swap.h
@@ -294,7 +294,7 @@ static inline void disable_swap_token(vo
 /* only sparc can not include linux/pagemap.h in this file
  * so leave page_cache_release and release_pages undeclared... */
 #define free_page_and_swap_cache(page) \
-	page_cache_release(page)
+	page_cache_release_nohv(page)
 #define free_pages_and_swap_cache(pages, nr) \
 	release_pages((pages), (nr), 0);

mm/filemap.c +66 -7

143 lines not on this page · raw message

mm/fremap.c +1 -0

diff -urpN linux-2.6/mm/fremap.c linux-2.6-patched/mm/fremap.c
@@ -83,6 +83,7 @@ int install_page(struct mm_struct *mm, s
 	page_hva_check_write(page, pte_val);
 	set_pte_at(mm, addr, pte, pte_val);
 	page_add_file_rmap(page);
+	page_hva_make_volatile(page, 1);
 	pte_val = *pte;
 	update_mmu_cache(vma, addr, pte_val);
 	err = 0;

mm/memory.c +4 -2

diff -urpN linux-2.6/mm/memory.c linux-2.6-patched/mm/memory.c
@@ -1942,7 +1942,8 @@ static int do_swap_page(struct mm_struct
 	 * Back out if somebody else already faulted in this pte.
 	 */
 	page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
-	if (unlikely(!pte_same(*page_table, orig_pte)))
+	if (unlikely(!pte_same(*page_table, orig_pte) ||
+		     (page_hva_enabled() && PageDiscarded(page))))
 		goto out_nomap;
 
 	if (unlikely(!PageUptodate(page))) {

@@ -2156,7 +2157,8 @@ retry:
 	 * handle that later.
 	 */
 	/* Only go through if we didn't race with anybody else... */
-	if (pte_none(*page_table)) {
+	if (pte_none(*page_table) &&
+	    !unlikely(page_hva_enabled() && PageDiscarded(new_page))) {
 		flush_icache_page(vma, new_page);
 		entry = mk_pte(new_page, vma->vm_page_prot);
 		if (write_access)

mm/rmap.c +1 -3

diff -urpN linux-2.6/mm/rmap.c linux-2.6-patched/mm/rmap.c
@@ -472,7 +472,6 @@ void page_add_anon_rmap(struct page *pag
 	if (atomic_inc_and_test(&page->_mapcount))
 		__page_set_anon_rmap(page, vma, address);
 	/* else checking page index and mapping is racy */
-	page_hva_make_volatile(page, 1);
 }
 
 /*

@@ -501,7 +500,6 @@ void page_add_file_rmap(struct page *pag
 {
 	if (atomic_inc_and_test(&page->_mapcount))
 		__inc_page_state(nr_mapped);
-	page_hva_make_volatile(page, 1);
 }
 
 /**

@@ -610,7 +608,7 @@ static int try_to_unmap_one(struct page 
 		dec_mm_counter(mm, file_rss);
 
 	page_remove_rmap(page);
-	page_cache_release(page);
+	page_cache_release_nohv(page);
 
 out_unmap:
 	pte_unmap_unlock(pte, ptl);

mm/swap.c +25 -1

diff -urpN linux-2.6/mm/swap.c linux-2.6-patched/mm/swap.c
@@ -55,6 +55,30 @@ void put_page(struct page *page)
 }
 EXPORT_SYMBOL(put_page);
 
+#if defined(CONFIG_PAGE_HVA)
+
+static void put_compound_page_nohv(struct page *page)
+{
+	page = (struct page *)page_private(page);
+	if (put_page_testzero_nohv(page)) {
+		void (*dtor)(struct page *page);
+
+		dtor = (void (*)(struct page *))page[1].lru.next;
+		(*dtor)(page);
+	}
+}
+
+void put_page_nohv(struct page *page)
+{
+	if (unlikely(PageCompound(page)))
+		put_compound_page_nohv(page);
+	else if (put_page_testzero_nohv(page))
+		__page_cache_release(page);
+}
+EXPORT_SYMBOL(put_page_nohv);
+
+#endif
+
 /*
  * Writeback is about to end against a page which has been marked for immediate
  * reclaim.  If it still appears to be reclaimable, move it to the tail of the

@@ -254,7 +278,7 @@ void release_pages(struct page **pages, 
 			continue;
 		}
 
-		if (!put_page_testzero(page))
+		if (!put_page_testzero_nohv(page))
 			continue;
 
 		if (PageLRU(page)) {

mm/swap_state.c +2 -2

diff -urpN linux-2.6/mm/swap_state.c linux-2.6-patched/mm/swap_state.c
@@ -271,7 +271,7 @@ static inline void free_swap_cache(struc
 void free_page_and_swap_cache(struct page *page)
 {
 	free_swap_cache(page);
-	page_cache_release(page);
+	page_cache_release_nohv(page);
 }
 
 /*

@@ -305,7 +305,7 @@ struct page * lookup_swap_cache(swp_entr
 {
 	struct page *page;
 
-	page = find_get_page(&swapper_space, entry.val);
+	page = find_get_page_nohv(&swapper_space, entry.val);
 
 	if (page)
 		INC_CACHE_INFO(find_success);

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to [email protected]. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"[email protected]"> [email protected] </a>