Re: [RFC 10/16] Variable Order Page Cache: Readahead fixups

[Fengguang Wu <fengguang.wu@gmail.com>]

On Wed, Apr 25, 2007 at 08:56:12AM -0700, Christoph Lameter wrote:
> On Wed, 25 Apr 2007, Mel Gorman wrote:
> 
> > > +		/*
> > > +		 * FIXME: Note the 2M constant here that may prove to
> > > +		 * be a problem if page sizes become bigger than one megabyte.
> > > +		 */
> > > +		unsigned long this_chunk = page_cache_index(mapping, 2 * 1024 * 1024);
> > >
> > 
> > Should readahead just be disabled when the compound page size is as
> > large or larger than what readahead normally reads?
> 
> I am not sure how to solve that one yet. With the above fix we stay at the 
> 2M sized readahead. As the compound order increases so the number of pages
> is reduced. We could keep the number of pages constant but then very high
> orders may cause a excessive use of memory for readahead.

Do we need to support very high orders(i.e. >2MB)?
If not, we can define a MAX_PAGE_CACHE_SIZE=2MB, and limit page orders
under that threshold. Now large readahead can be done in
MAX_PAGE_CACHE_SIZE chunks.

The attached patch is derived from yours, hope you like it :)
(not tested/compiled yet)

Changes include:

- Introduce MAX_PAGE_CACHE_SIZE, the upper limit of compound page size.
- scale readahead size with the page cache size in file_ra_state_init().
- simplify max_sane_readahead() calls by moving them into readahead routines.

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
---
 include/linux/mm.h      |    2 +-
 include/linux/pagemap.h |    1 +
 mm/fadvise.c            |    4 ++--
 mm/filemap.c            |    5 ++---
 mm/madvise.c            |    2 +-
 mm/readahead.c          |   25 +++++++++++++++----------
 6 files changed, 22 insertions(+), 17 deletions(-)

--- linux-2.6.22-rc1-mm1.orig/mm/fadvise.c
+++ linux-2.6.22-rc1-mm1/mm/fadvise.c
@@ -86,10 +86,10 @@ asmlinkage long sys_fadvise64_64(int fd,
 		nrpages = end_index - start_index + 1;
 		if (!nrpages)
 			nrpages = ~0UL;
-		
+
 		ret = force_page_cache_readahead(mapping, file,
 				start_index,
-				max_sane_readahead(nrpages));
+				nrpages);
 		if (ret > 0)
 			ret = 0;
 		break;
--- linux-2.6.22-rc1-mm1.orig/mm/filemap.c
+++ linux-2.6.22-rc1-mm1/mm/filemap.c
@@ -1287,8 +1287,7 @@ do_readahead(struct address_space *mappi
 	if (!mapping || !mapping->a_ops || !mapping->a_ops->readpage)
 		return -EINVAL;
 
-	force_page_cache_readahead(mapping, filp, index,
-					max_sane_readahead(nr));
+	force_page_cache_readahead(mapping, filp, index, nr);
 	return 0;
 }
 
@@ -1426,7 +1425,7 @@ retry_find:
 			count_vm_event(PGMAJFAULT);
 		}
 		did_readaround = 1;
-		ra_pages = max_sane_readahead(file->f_ra.ra_pages);
+		ra_pages = file->f_ra.ra_pages;
 		if (ra_pages) {
 			pgoff_t start = 0;
 
--- linux-2.6.22-rc1-mm1.orig/mm/madvise.c
+++ linux-2.6.22-rc1-mm1/mm/madvise.c
@@ -124,7 +124,7 @@ static long madvise_willneed(struct vm_a
 	end = ((end - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
 
 	force_page_cache_readahead(file->f_mapping,
-			file, start, max_sane_readahead(end - start));
+			file, start, end - start);
 	return 0;
 }
 
--- linux-2.6.22-rc1-mm1.orig/mm/readahead.c
+++ linux-2.6.22-rc1-mm1/mm/readahead.c
@@ -44,7 +44,8 @@ EXPORT_SYMBOL_GPL(default_backing_dev_in
 void
 file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
 {
-	ra->ra_pages = mapping->backing_dev_info->ra_pages;
+	ra->ra_pages = DIV_ROUND_UP(mapping->backing_dev_info->ra_pages,
+				    page_cache_size(mapping));
 	ra->prev_index = -1;
 }
 EXPORT_SYMBOL_GPL(file_ra_state_init);
@@ -84,7 +85,7 @@ int read_cache_pages(struct address_spac
 			put_pages_list(pages);
 			break;
 		}
-		task_io_account_read(PAGE_CACHE_SIZE);
+		task_io_account_read(page_cache_size(mapping));
 	}
 	pagevec_lru_add(&lru_pvec);
 	return ret;
@@ -151,7 +152,7 @@ __do_page_cache_readahead(struct address
 	if (isize == 0)
 		goto out;
 
-	end_index = ((isize - 1) >> PAGE_CACHE_SHIFT);
+ 	end_index = page_cache_index(mapping, isize - 1);
 
 	/*
 	 * Preallocate as many pages as we will need.
@@ -193,8 +194,8 @@ out:
 }
 
 /*
- * Chunk the readahead into 2 megabyte units, so that we don't pin too much
- * memory at once.
+ * Chunk the readahead into MAX_PAGE_CACHE_SIZE(2M) units, so that we don't pin
+ * too much memory at once.
  */
 int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
 		pgoff_t offset, unsigned long nr_to_read)
@@ -204,10 +205,11 @@ int force_page_cache_readahead(struct ad
 	if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
 		return -EINVAL;
 
+	nr_to_read = max_sane_readahead(nr_to_read, mapping_order(mapping));
 	while (nr_to_read) {
 		int err;
 
-		unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_CACHE_SIZE;
+		unsigned long this_chunk = page_cache_index(mapping, MAX_PAGE_CACHE_SIZE);
 
 		if (this_chunk > nr_to_read)
 			this_chunk = nr_to_read;
@@ -237,17 +239,20 @@ int do_page_cache_readahead(struct addre
 	if (bdi_read_congested(mapping->backing_dev_info))
 		return -1;
 
+	nr_to_read = max_sane_readahead(nr_to_read, mapping_order(mapping));
 	return __do_page_cache_readahead(mapping, filp, offset, nr_to_read, 0);
 }
 
 /*
- * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
+ * Given a desired number of page order readahead pages, return a
  * sensible upper limit.
  */
-unsigned long max_sane_readahead(unsigned long nr)
+unsigned long max_sane_readahead(unsigned long nr, int order)
 {
-	return min(nr, (node_page_state(numa_node_id(), NR_INACTIVE)
-		+ node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2);
+	unsigned long base_pages = node_page_state(numa_node_id(), NR_INACTIVE)
+			+ node_page_state(numa_node_id(), NR_FREE_PAGES);
+
+	return min(nr, (base_pages / 2) >> order);
 }
 
 /*
--- linux-2.6.22-rc1-mm1.orig/include/linux/mm.h
+++ linux-2.6.22-rc1-mm1/include/linux/mm.h
@@ -1163,7 +1163,7 @@ unsigned long page_cache_readahead_ondem
 			  struct page *page,
 			  pgoff_t offset,
 			  unsigned long size);
-unsigned long max_sane_readahead(unsigned long nr);
+unsigned long max_sane_readahead(unsigned long nr, int order);
 
 /* Do stack extension */
 extern int expand_stack(struct vm_area_struct *vma, unsigned long address);
--- linux-2.6.22-rc1-mm1.orig/include/linux/pagemap.h
+++ linux-2.6.22-rc1-mm1/include/linux/pagemap.h
@@ -57,6 +57,7 @@ static inline void mapping_set_gfp_mask(
 #define PAGE_CACHE_SIZE		PAGE_SIZE
 #define PAGE_CACHE_MASK		PAGE_MASK
 #define PAGE_CACHE_ALIGN(addr)	(((addr)+PAGE_CACHE_SIZE-1)&PAGE_CACHE_MASK)
+#define MAX_PAGE_CACHE_SIZE	(2 * 1024 * 1024)
 
 #define page_cache_get(page)		get_page(page)
 #define page_cache_release(page)	put_page(page)

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