Re: Problem with delayed allocation

Aneesh Kumar K.V
2008-08-04T15:27:50Z

On Mon, Aug 04, 2008 at 08:22:49PM +0530, Aneesh Kumar K.V wrote:
> On Sat, Aug 02, 2008 at 04:07:19PM -0400, Theodore Ts'o wrote:
> >
> > Apparently __fsync_super(), which is called right before remounting a
> > filesystem read-only, isn't working correctly. To reproduce, create a
> > script which does this:
> >
> > #!/bin/sh
> > DEVICE=/dev/closure/test
> > mke2fs -t ext4dev /dev/closure/test
> > mount $DEVICE /mnt
> > cd /mnt
> > tar xfj /var/tmp/linux-2.6.26.tar.gz <----- or some really big file
> > du -s
> > cd ..
> > mount -o remount,ro /mnt
> > sync
> > dmesg > /tmp/dmesg.out <----- note all of the ext4_da_writepages error messages
> > umount /mnt
> > du -s /mnt
> > sync
> > mount $DEVICE /mnt
> > du -s /mnt <--- note that size of the unpacked hierarcy is much smaller
> >
> > This doesn't happen if the ext4 filesystem is mounted with nodelalloc,
> > so I assume the problem is in ext4_da_writepages().
> >
>
> Can you try this patch and see if it makes any difference ?
>
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 25adfc3..5a8a2d3 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -518,6 +518,7 @@ void generic_sync_sb_inodes(struct super_block *sb,
> spin_lock(&inode_lock);
> if (wbc->nr_to_write <= 0) {
> wbc->more_io = 1;
> + printk(KERN_CRIT "Breaking from the %s loop\n", __func__);
> break;
> }
> if (!list_empty(&sb->s_more_io))
> @@ -611,6 +612,8 @@ void sync_inodes_sb(struct super_block *sb, int wait)
> (inodes_stat.nr_inodes - inodes_stat.nr_unused) +
> nr_dirty + nr_unstable;
> wbc.nr_to_write += wbc.nr_to_write / 2; /* Bit more for luck */
> + wbc.nr_to_write = LONG_MAX;
> +
> sync_sb_inodes(sb, &wbc);
> }
>

I guess this could be the reason. I am not hitting the error during
remount, ro with this change. But I have other changes also accumulated
as a part of rewrite.

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 4a50445..ecabe77 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2225,8 +2288,10 @@ static int ext4_da_writepages(struct address_space *mapping,
if (!mapping->nrpages || !mapping_tagged(mapping, PAGECACHE_TAG_DIRTY))
return 0;

+#if 0
if (wbc->nr_to_write > mapping->nrpages)
wbc->nr_to_write = mapping->nrpages;
+#endif

if (!wbc->range_cyclic) {