On Tue, Jun 26, 2007 at 11:49:15PM -0400, Andreas Dilger wrote:
> On Jun 27, 2007  09:14 +1000, David Chinner wrote:
> > Someone on the XFs list had an interesting request - preallocated
> > swap files. You can't use unwritten extents for this because
> > of sys_swapon()s use of bmap() (XFS returns holes for reading
> > unwritten extents), so we need a method of preallocating that does
> > not zero or mark the extent unread. i.e. FA_MKSWAP.
> 
> Is there a reason why unwritten extents return 0 to bmap()?

It's a fallout of xfs_get_blocks not mapping unwritten extents
on read because we want do_mpage_readpage() to treat them
as a hole. i.e. zero fill them instead of doing I/O. This is
the way XFS was shoehorned into the generic read path :/

> This
> would seem to be the only impediment from using fallocated files
> for swap files.  Maybe if FIEMAP was used by mkswap to get an
> "UNWRITTEN" flag back instead of "HOLE" it wouldn't be a problem.

Probably. If we taught do_mpage_readpage() about unwritten mappings,
then would could map them on read if and then sys_swapon can remain
blissfully unaware of unwritten extents.

I think this is pretty much all I need to do to acheive that is
(untested):

---

Teach do_mpage_readpage() about unwritten extents so we can
always map them in get_blocks rather than they are are holes on
read. Allows setup_swap_extents() to use preallocated files on XFS
filesystems for swap files without ever needing to convert them.

Signed-Off-By: Dave Chinner <[EMAIL PROTECTED]>

---
 fs/mpage.c                  |    5 +++--
 fs/xfs/linux-2.6/xfs_aops.c |   13 +++----------
 2 files changed, 6 insertions(+), 12 deletions(-)

Index: 2.6.x-xfs-new/fs/mpage.c
===================================================================
--- 2.6.x-xfs-new.orig/fs/mpage.c       2007-05-29 16:17:59.000000000 +1000
+++ 2.6.x-xfs-new/fs/mpage.c    2007-06-27 22:39:35.568852270 +1000
@@ -207,7 +207,8 @@ do_mpage_readpage(struct bio *bio, struc
         * Map blocks using the result from the previous get_blocks call first.
         */
        nblocks = map_bh->b_size >> blkbits;
-       if (buffer_mapped(map_bh) && block_in_file > *first_logical_block &&
+       if (buffer_mapped(map_bh) && !buffer_unwritten(map_bh) &&
+                       block_in_file > *first_logical_block &&
                        block_in_file < (*first_logical_block + nblocks)) {
                unsigned map_offset = block_in_file - *first_logical_block;
                unsigned last = nblocks - map_offset;
@@ -242,7 +243,7 @@ do_mpage_readpage(struct bio *bio, struc
                        *first_logical_block = block_in_file;
                }
 
-               if (!buffer_mapped(map_bh)) {
+               if (!buffer_mapped(map_bh) || buffer_unwritten(map_bh)) {
                        fully_mapped = 0;
                        if (first_hole == blocks_per_page)
                                first_hole = page_block;
Index: 2.6.x-xfs-new/fs/xfs/linux-2.6/xfs_aops.c
===================================================================
--- 2.6.x-xfs-new.orig/fs/xfs/linux-2.6/xfs_aops.c      2007-06-05 
22:14:39.000000000 +1000
+++ 2.6.x-xfs-new/fs/xfs/linux-2.6/xfs_aops.c   2007-06-27 22:39:29.545636749 
+1000
@@ -1340,16 +1340,9 @@ __xfs_get_blocks(
                return 0;
 
        if (iomap.iomap_bn != IOMAP_DADDR_NULL) {
-               /*
-                * For unwritten extents do not report a disk address on
-                * the read case (treat as if we're reading into a hole).
-                */
-               if (create || !(iomap.iomap_flags & IOMAP_UNWRITTEN)) {
-                       xfs_map_buffer(bh_result, &iomap, offset,
-                                      inode->i_blkbits);
-               }
-               if (create && (iomap.iomap_flags & IOMAP_UNWRITTEN)) {
-                       if (direct)
+               xfs_map_buffer(bh_result, &iomap, offset, inode->i_blkbits);
+               if (iomap.iomap_flags & IOMAP_UNWRITTEN) {
+                       if (create && direct)
                                bh_result->b_private = inode;
                        set_buffer_unwritten(bh_result);
                }


Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
-
To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
the body of a message to [EMAIL PROTECTED]
More majordomo info at  http://vger.kernel.org/majordomo-info.html

Reply via email to