exofs: Override read-ahead to align on stripe_size

* Set all inode->i_mapping->backing_dev_info to point to
  the per super-block sb->s_bdi.

* Calculating a read_ahead that is:
  - preferable 2 stripes long
    (Future patch will add a mount option to override this)
  - Minimum 128K aligned up to stripe-size
  - Caped to maximum-IO-sizes round down to stripe_size.
    (Max sizes are governed by max bio-size that fits in a page
     times number-of-devices)

CC: Marc Dionne <marc.c.dionne@gmail.com>
Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>

fs/exofs/exofs.h

@@ -256,6 +256,8 @@ static inline int exofs_oi_read(struct exofs_i_info *oi,
 }
 
 /* inode.c               */
+unsigned exofs_max_io_pages(struct exofs_layout *layout,
+			    unsigned expected_pages);
 int exofs_setattr(struct dentry *, struct iattr *);
 int exofs_write_begin(struct file *file, struct address_space *mapping,
 		loff_t pos, unsigned len, unsigned flags,

fs/exofs/inode.c

@@ -43,6 +43,17 @@ enum { BIO_MAX_PAGES_KMALLOC =
 		PAGE_SIZE / sizeof(struct page *),
 };
 
+unsigned exofs_max_io_pages(struct exofs_layout *layout,
+			    unsigned expected_pages)
+{
+	unsigned pages = min_t(unsigned, expected_pages, MAX_PAGES_KMALLOC);
+
+	/* TODO: easily support bio chaining */
+	pages =  min_t(unsigned, pages,
+		       layout->group_width * BIO_MAX_PAGES_KMALLOC);
+	return pages;
+}
+
 struct page_collect {
 	struct exofs_sb_info *sbi;
 	struct inode *inode;
@@ -97,8 +108,7 @@ static void _pcol_reset(struct page_collect *pcol)
 
 static int pcol_try_alloc(struct page_collect *pcol)
 {
-	unsigned pages = min_t(unsigned, pcol->expected_pages,
+	unsigned pages;
-			  MAX_PAGES_KMALLOC);
 
 	if (!pcol->ios) { /* First time allocate io_state */
 		int ret = exofs_get_io_state(&pcol->sbi->layout, &pcol->ios);
@@ -108,8 +118,7 @@ static int pcol_try_alloc(struct page_collect *pcol)
 	}
 
 	/* TODO: easily support bio chaining */
-	pages =  min_t(unsigned, pages,
+	pages =  exofs_max_io_pages(&pcol->sbi->layout, pcol->expected_pages);
-		       pcol->sbi->layout.group_width * BIO_MAX_PAGES_KMALLOC);
 
 	for (; pages; pages >>= 1) {
 		pcol->pages = kmalloc(pages * sizeof(struct page *),
@@ -1049,6 +1058,7 @@ struct inode *exofs_iget(struct super_block *sb, unsigned long ino)
 		memcpy(oi->i_data, fcb.i_data, sizeof(fcb.i_data));
 	}
 
+	inode->i_mapping->backing_dev_info = sb->s_bdi;
 	if (S_ISREG(inode->i_mode)) {
 		inode->i_op = &exofs_file_inode_operations;
 		inode->i_fop = &exofs_file_operations;
@@ -1149,6 +1159,7 @@ struct inode *exofs_new_inode(struct inode *dir, int mode)
 
 	sbi = sb->s_fs_info;
 
+	inode->i_mapping->backing_dev_info = sb->s_bdi;
 	sb->s_dirt = 1;
 	inode_init_owner(inode, dir, mode);
 	inode->i_ino = sbi->s_nextid++;

fs/exofs/super.c

@@ -390,6 +390,23 @@ static int _read_and_match_data_map(struct exofs_sb_info *sbi, unsigned numdevs,
 	return 0;
 }
 
+static unsigned __ra_pages(struct exofs_layout *layout)
+{
+	const unsigned _MIN_RA = 32; /* min 128K read-ahead */
+	unsigned ra_pages = layout->group_width * layout->stripe_unit /
+				PAGE_SIZE;
+	unsigned max_io_pages = exofs_max_io_pages(layout, ~0);
+
+	ra_pages *= 2; /* two stripes */
+	if (ra_pages < _MIN_RA)
+		ra_pages = roundup(_MIN_RA, ra_pages / 2);
+
+	if (ra_pages > max_io_pages)
+		ra_pages = max_io_pages;
+
+	return ra_pages;
+}
+
 /* @odi is valid only as long as @fscb_dev is valid */
 static int exofs_devs_2_odi(struct exofs_dt_device_info *dt_dev,
 			     struct osd_dev_info *odi)
@@ -623,6 +640,7 @@ static int exofs_fill_super(struct super_block *sb, void *data, int silent)
 	}
 
 	/* set up operation vectors */
+	sbi->bdi.ra_pages = __ra_pages(&sbi->layout);
 	sb->s_bdi = &sbi->bdi;
 	sb->s_fs_info = sbi;
 	sb->s_op = &exofs_sops;