ext4: Use readahead when reading an inode from the inode table

With modern hard drives, reading 64k takes roughly the same time as
reading a 4k block.  So request readahead for adjacent inode table
blocks to reduce the time it takes when iterating over directories
(especially when doing this in htree sort order) in a cold cache case.
With this patch, the time it takes to run "git status" on a kernel
tree after flushing the caches via "echo 3 > /proc/sys/vm/drop_caches"
is reduced by 21%.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 64 insertions, 70 deletions

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 22fcbb67cd88..ef4ca3d4abc0 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3833,41 +3833,6 @@ out_stop:
         ext4_journal_stop(handle);
 }
 
-static ext4_fsblk_t ext4_get_inode_block(struct super_block *sb,
-                unsigned long ino, struct ext4_iloc *iloc)
-{
-        ext4_group_t block_group;
-        unsigned long offset;
-        ext4_fsblk_t block;
-        struct ext4_group_desc *gdp;
-
-        if (!ext4_valid_inum(sb, ino)) {
-                /*
-                 * This error is already checked for in namei.c unless we are
-                 * looking at an NFS filehandle, in which case no error
-                 * report is needed
-                 */
-                return 0;
-        }
-
-        block_group = (ino - 1) / EXT4_INODES_PER_GROUP(sb);
-        gdp = ext4_get_group_desc(sb, block_group, NULL);
-        if (!gdp)
-                return 0;
-
-        /*
-         * Figure out the offset within the block group inode table
-         */
-        offset = ((ino - 1) % EXT4_INODES_PER_GROUP(sb)) *
-                EXT4_INODE_SIZE(sb);
-        block = ext4_inode_table(sb, gdp) +
-                (offset >> EXT4_BLOCK_SIZE_BITS(sb));
-
-        iloc->block_group = block_group;
-        iloc->offset = offset & (EXT4_BLOCK_SIZE(sb) - 1);
-        return block;
-}
-
 /*
  * ext4_get_inode_loc returns with an extra refcount against the inode's
  * underlying buffer_head on success. If 'in_mem' is true, we have all
@@ -3877,19 +3842,35 @@ static ext4_fsblk_t ext4_get_inode_block(struct super_block *sb,
 static int __ext4_get_inode_loc(struct inode *inode,
                                 struct ext4_iloc *iloc, int in_mem)
 {
-        ext4_fsblk_t block;
-        struct buffer_head *bh;
+        struct ext4_group_desc  *gdp;
+        struct buffer_head      *bh;
+        struct super_block      *sb = inode->i_sb;
+        ext4_fsblk_t            block;
+        int                     inodes_per_block, inode_offset;
+
+        iloc->bh = 0;
+        if (!ext4_valid_inum(sb, inode->i_ino))
+                return -EIO;
 
-        block = ext4_get_inode_block(inode->i_sb, inode->i_ino, iloc);
-        if (!block)
+        iloc->block_group = (inode->i_ino - 1) / EXT4_INODES_PER_GROUP(sb);
+        gdp = ext4_get_group_desc(sb, iloc->block_group, NULL);
+        if (!gdp)
                 return -EIO;
 
-        bh = sb_getblk(inode->i_sb, block);
+        /*
+         * Figure out the offset within the block group inode table
+         */
+        inodes_per_block = (EXT4_BLOCK_SIZE(sb) / EXT4_INODE_SIZE(sb));
+        inode_offset = ((inode->i_ino - 1) %
+                        EXT4_INODES_PER_GROUP(sb));
+        block = ext4_inode_table(sb, gdp) + (inode_offset / inodes_per_block);
+        iloc->offset = (inode_offset % inodes_per_block) * EXT4_INODE_SIZE(sb);
+
+        bh = sb_getblk(sb, block);
         if (!bh) {
-                ext4_error (inode->i_sb, "ext4_get_inode_loc",
-                                "unable to read inode block - "
-                                "inode=%lu, block=%llu",
-                                 inode->i_ino, block);
+                ext4_error(sb, "ext4_get_inode_loc", "unable to read "
+                           "inode block - inode=%lu, block=%llu",
+                           inode->i_ino, block);
                 return -EIO;
         }
         if (!buffer_uptodate(bh)) {
@@ -3917,28 +3898,12 @@ static int __ext4_get_inode_loc(struct inode *inode,
                  */
                 if (in_mem) {
                         struct buffer_head *bitmap_bh;
-                        struct ext4_group_desc *desc;
-                        int inodes_per_buffer;
-                        int inode_offset, i;
-                        ext4_group_t block_group;
-                        int start;
-
-                        block_group = (inode->i_ino - 1) /
-                                        EXT4_INODES_PER_GROUP(inode->i_sb);
-                        inodes_per_buffer = bh->b_size /
-                                EXT4_INODE_SIZE(inode->i_sb);
-                        inode_offset = ((inode->i_ino - 1) %
-                                        EXT4_INODES_PER_GROUP(inode->i_sb));
-                        start = inode_offset & ~(inodes_per_buffer - 1);
+                        int i, start;
 
-                        /* Is the inode bitmap in cache? */
-                        desc = ext4_get_group_desc(inode->i_sb,
-                                                block_group, NULL);
-                        if (!desc)
-                                goto make_io;
+                        start = inode_offset & ~(inodes_per_block - 1);
 
-                        bitmap_bh = sb_getblk(inode->i_sb,
-                                ext4_inode_bitmap(inode->i_sb, desc));
+                        /* Is the inode bitmap in cache? */
+                        bitmap_bh = sb_getblk(sb, ext4_inode_bitmap(sb, gdp));
                         if (!bitmap_bh)
                                 goto make_io;
 
@@ -3951,14 +3916,14 @@ static int __ext4_get_inode_loc(struct inode *inode,
                                 brelse(bitmap_bh);
                                 goto make_io;
                         }
-                        for (i = start; i < start + inodes_per_buffer; i++) {
+                        for (i = start; i < start + inodes_per_block; i++) {
                                 if (i == inode_offset)
                                         continue;
                                 if (ext4_test_bit(i, bitmap_bh->b_data))
                                         break;
                         }
                         brelse(bitmap_bh);
-                        if (i == start + inodes_per_buffer) {
+                        if (i == start + inodes_per_block) {
                                 /* all other inodes are free, so skip I/O */
                                 memset(bh->b_data, 0, bh->b_size);
                                 set_buffer_uptodate(bh);
@@ -3969,6 +3934,36 @@ static int __ext4_get_inode_loc(struct inode *inode,
 
 make_io:
                 /*
+                 * If we need to do any I/O, try to pre-readahead extra
+                 * blocks from the inode table.
+                 */
+                if (EXT4_SB(sb)->s_inode_readahead_blks) {
+                        ext4_fsblk_t b, end, table;
+                        unsigned num;
+
+                        table = ext4_inode_table(sb, gdp);
+                        /* Make sure s_inode_readahead_blks is a power of 2 */
+                        while (EXT4_SB(sb)->s_inode_readahead_blks &
+                               (EXT4_SB(sb)->s_inode_readahead_blks-1))
+                                EXT4_SB(sb)->s_inode_readahead_blks = 
+                                   (EXT4_SB(sb)->s_inode_readahead_blks &
+                                    (EXT4_SB(sb)->s_inode_readahead_blks-1));
+                        b = block & ~(EXT4_SB(sb)->s_inode_readahead_blks-1);
+                        if (table > b)
+                                b = table;
+                        end = b + EXT4_SB(sb)->s_inode_readahead_blks;
+                        num = EXT4_INODES_PER_GROUP(sb);
+                        if (EXT4_HAS_RO_COMPAT_FEATURE(sb,
+                                       EXT4_FEATURE_RO_COMPAT_GDT_CSUM))
+                                num -= le16_to_cpu(gdp->bg_itable_unused);
+                        table += num / inodes_per_block;
+                        if (end > table)
+                                end = table;
+                        while (b <= end)
+                                sb_breadahead(sb, b++);
+                }
+
+                /*
                  * There are other valid inodes in the buffer, this inode
                  * has in-inode xattrs, or we don't have this inode in memory.
                  * Read the block from disk.
@@ -3978,10 +3973,9 @@ make_io:
                 submit_bh(READ_META, bh);
                 wait_on_buffer(bh);
                 if (!buffer_uptodate(bh)) {
-                        ext4_error(inode->i_sb, "ext4_get_inode_loc",
-                                        "unable to read inode block - "
-                                        "inode=%lu, block=%llu",
-                                        inode->i_ino, block);
+                        ext4_error(sb, __func__,
+                                   "unable to read inode block - inode=%lu, "
+                                   "block=%llu", inode->i_ino, block);
                         brelse(bh);
                         return -EIO;
                 }