On 4/29/15 5:10 AM, Jan Kara wrote:
> On Tue 28-04-15 10:46:12, Eric Sandeen wrote:
...
>> Maybe we can do something similar here? I've kind of lost track
>> of how resize is working now, TBH.
> Well, the fact is we have to limit some fs parameters so that code can be
> reasonably simple. IMO already supporting 20 MB filesystem with a journal
> is a stretch and choice of number of reserved blocks looks arbitrary but
> I guess we shouldn't regress if possible.
>
> So 20 MB filesystem will get 1 MB journal. That means
> j_max_transaction_size 256 and thus we allow at most 128 credits in a single
> handle. The filesystem has 79 reserved GDT blocks and flex group size 16 so
> when resizing to 200 MB when adding full flex group, the resize code in
> ext4_flex_group_add() wants a handle with 4*16 + 79 credits which is too
> much (143).
*nod*
> That being said the calculation in ext4_flex_group_add() looks too
> pessimistic (in the flex_gd->count * 4 part). Sure we need to modify resize
> inode and its dindirect block but that's common for all the groups,
> similarly for superblock so we can change that to (3 + flex_gd->count) and
> even that is too pessimistic since we have EXT4_DESC_PER_BLOCK(sb)
> descriptors in one block. So we could shrink it to (3 + 1 + (flex_gd->count +
> EXT4_DESC_PER_BLOCK(sb) - 1) / EXT4_DESC_PER_BLOCK(sb)). That will shrink
> the total credit estimate down to 84 for that filesystem.
>
> The attached patch fixes the issue for me. Thoughts?
>
> Honza
I think I'm glad you understand it better than me. :) I was surprised at
the large reservation but hadn't worked out why it was a problem.
> From 37ecb96f1ab1e57bc5c1663c1662d658df2f84fd Mon Sep 17 00:00:00 2001
> From: Jan Kara <jack@xxxxxxx>
> Date: Wed, 29 Apr 2015 11:46:31 +0200
> Subject: [PATCH] ext4: Fix growing of tiny filesystems
>
> The estimate of necessary transaction credits in ext4_flex_group_add()
> is too pessimistic. It reserves credit for sb, resize inode, and resize
> inode dindirect block for each group added in a flex group although they
> are always the same block and thus it is enough to account them only
> once. Also the number of modified GDT block is overestimated since we
> fit EXT4_DESC_PER_BLOCK(sb) descriptors in one block.
>
> Make the estimation more precise. That reduces number of requested
> credits enough that we can grow 20 MB filesystem (which has 1 MB
> journal, 79 reserved GDT blocks, and flex group size 16 by default).
>
> Signed-off-by: Jan Kara <jack@xxxxxxx>
> ---
> fs/ext4/resize.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
> index 8a8ec6293b19..15b4b3605859 100644
> --- a/fs/ext4/resize.c
> +++ b/fs/ext4/resize.c
> @@ -1432,12 +1432,15 @@ static int ext4_flex_group_add(struct super_block *sb,
> goto exit;
> /*
> * We will always be modifying at least the superblock and GDT
> - * block. If we are adding a group past the last current GDT block,
> + * blocks. If we are adding a group past the last current GDT block,
> * we will also modify the inode and the dindirect block. If we
> * are adding a group with superblock/GDT backups we will also
> * modify each of the reserved GDT dindirect blocks.
> */
> - credit = flex_gd->count * 4 + reserved_gdb;
> + credit = 3; /* sb, resize inode, resize inode dindirect */
yay for excellent comments :)
> + credit += 1 + (flex_gd->count + EXT4_DESC_PER_BLOCK(sb) - 1) /
> + EXT4_DESC_PER_BLOCK(sb); /* GDT blocks */
maybe credit += 1 + DIV_ROUND_UP(flex_gd->count, EXT4_DESC_PER_BLOCK(sb)) ?
> + credit += reserved_gdb; /* Reserved GDT dindirect blocks */
> handle = ext4_journal_start_sb(sb, EXT4_HT_RESIZE, credit);
> if (IS_ERR(handle)) {
> err = PTR_ERR(handle);
Thanks!
-Eric
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