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Subject[PATCH] O_DIRECT with hardware blocksize alignment
Folks,
Some major users of O_DIRECT (Oracle, for instance) align and
size I/O based on the 512byte hardware blocksize common to most hard
disk drives. The current O_DIRECT code enforces that the alignment and
size of the I/O match the software blocksize (inot->i_sb->s_blocksize).
This patch relaxes that restriction to a minimum of the hardware
blocksize. In the interest of efficiency,
min(I/O alignment, s_blocksize) is used as the effective
blocksize. eg:

I/O alignment s_blocksize final blocksize
8192 4096 4096
4096 4096 4096
512 4096 512

Joel


diff -uNr linux-2.4.17/fs/buffer.c linux-2.4.17-od/fs/buffer.c
--- linux-2.4.17/fs/buffer.c Fri Dec 21 09:41:55 2001
+++ linux-2.4.17-od/fs/buffer.c Wed Jan 9 10:55:52 2002
@@ -2003,6 +2003,17 @@
{
int i, nr_blocks, retval;
unsigned long * blocks = iobuf->blocks;
+ int i_bscale, i_blocknr, i_blockoff;
+
+ /* Calculate I/O blocksize to sw blocksize scaling factor */
+ i_bscale = 1;
+ if (blocksize != inode->i_sb->s_blocksize)
+ {
+ if ((inode->i_sb->s_blocksize < blocksize) ||
+ ((inode->i_sb->s_blocksize % blocksize) != 0))
+ BUG();
+ i_bscale = inode->i_sb->s_blocksize / blocksize;
+ }

nr_blocks = iobuf->length / blocksize;
/* build the blocklist */
@@ -2013,7 +2024,13 @@
bh.b_dev = inode->i_dev;
bh.b_size = blocksize;

- retval = get_block(inode, blocknr, &bh, rw == READ ? 0 : 1);
+ /* Convert blocknr to the software blocksize */
+ if (!i_bscale)
+ BUG();
+ i_blocknr = blocknr / i_bscale;
+ i_blockoff = blocknr % i_bscale;
+
+ retval = get_block(inode, i_blocknr, &bh, rw == READ ? 0 : 1);
if (retval)
goto out;

@@ -2031,7 +2048,12 @@
if (!buffer_mapped(&bh))
BUG();
}
- blocks[i] = bh.b_blocknr;
+
+ /*
+ * Convert the returned blocknr back to the
+ * I/O blocksize.
+ */
+ blocks[i] = (bh.b_blocknr * i_bscale) + i_blockoff;
}

retval = brw_kiovec(rw, 1, &iobuf, inode->i_dev, iobuf->blocks, blocksize);
diff -uNr linux-2.4.17/mm/filemap.c linux-2.4.17-od/mm/filemap.c
--- linux-2.4.17/mm/filemap.c Fri Dec 21 09:42:04 2001
+++ linux-2.4.17-od/mm/filemap.c Wed Jan 9 10:58:13 2002
@@ -1491,6 +1491,7 @@
{
ssize_t retval;
int new_iobuf, chunk_size, blocksize_mask, blocksize, blocksize_bits, iosize, progress;
+ int b_bsize, b_bmask;
struct kiobuf * iobuf;
struct address_space * mapping = filp->f_dentry->d_inode->i_mapping;
struct inode * inode = mapping->host;
@@ -1508,14 +1509,36 @@
new_iobuf = 1;
}

+ chunk_size = KIO_MAX_ATOMIC_IO << 10;
+
+ retval = -EINVAL;
+
+ /*
+ * Starting at the software blocksize, check size
+ * and alignment of the I/O. Shift the blocksize
+ * down until we get an alignment that works, or we hit
+ * the hardware blocksize and fail.
+ */
+ b_bsize = get_hardsect_size(inode->i_dev);
+ b_bmask = (b_bsize - 1);
+
blocksize = 1 << inode->i_blkbits;
blocksize_bits = inode->i_blkbits;
blocksize_mask = blocksize - 1;
- chunk_size = KIO_MAX_ATOMIC_IO << 10;
+ while (blocksize >= b_bsize)
+ {
+ if (! ((offset & blocksize_mask) ||
+ (count & blocksize_mask) ||
+ ((unsigned long)buf & blocksize_mask)))
+ break;

- retval = -EINVAL;
- if ((offset & blocksize_mask) || (count & blocksize_mask))
+ blocksize >>= 1;
+ blocksize_mask = (blocksize - 1);
+ blocksize_bits--;
+ }
+ if (blocksize < b_bsize)
goto out_free;
+
if (!mapping->a_ops->direct_IO)
goto out_free;

--
"Every new beginning comes from some other beginning's end."

http://www.jlbec.org/
jlbec@evilplan.org
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