在 2012年2月15日 上午5:24，Aditya Kali [off-list ref] 写道：

Hi Robin,


On Tue, Feb 14, 2012 at 12:01 AM, Robin Dong [off-list ref] wrote:

quoted

在 2012年2月10日 上午10:55，Ted Ts'o [off-list ref] 写道：

quoted

On Thu, Feb 02, 2012 at 03:48:49PM +0800, Robin Dong wrote:

quoted

The reason is ext4_has_free_clusters reporting wrong
result. Actually, the unit of argument "dirty_clusters" is block, so
we should tranform the sbi->s_dirtyclusters_counter to block , just
like "free_clusters".

I've been looking at the the delayed allocation reservation code, and
I think it's a lot more confused that this.  Some places we are
playing with fields as if they are clusters, and some times as if they
are blocks, and I've definitely found places where we're not handling
quota correctly with bigalloc (ext4_map_blocks is calling
ext4_da_update_reserve_space() with a value which is clearly in units
of blocks, but we are treating that vale as if it is clusters, etc.)

So I think the problem is a lot larger than what you pointed out, and
we shouldn't fix it just by throwing in an EXT4_C2B call.  If
s_dirtyclusters_counter should be denominated in blocks, then we need
to rename it.  And there there's this comment which made me wince:

          * Note that in case of bigalloc, i_reserved_meta_blocks,
          * i_reserved_data_blocks, etc. refer to number of clusters.

Argh, no.  If something is denominated in clusters, then it should be
appropriately named as such.

Actually, I think the problem is a lot bigger than this, and it's a
conceptual one.  When we try writing to a block which does not have a
mapping, and bigalloc is enabled, there are three cases:

1) This is the first time the cluster has ever been written to; hence,
even though we are dirtying a block, we need to reserve room for the
entire cluster.

2) While the block has not been mapped (and so the data on disk is
uninitialized) the cluster has indeed been allocated, so we don't need
to reserve any additional room for the block.

3) One or more blocks in the cluster have already been written to via
delayed allocation, but the cluster itself has not been selected
(allocated) yet.  But since the space for the entire cluster was
reserved when the first block in the cluster was dirtied (case #1), we
don't need to reserve any more space.

We aren't handling these cases correctly today, since we don't have
this logic here.  These cases don't matter if you're only using
fallocate() and direct I/O, but if you are using buffered writes and
delayed allocation, they obviously matter a huge amount.

quoted

And I don't think we're handling it correctly.   Argh...

I thought we do handle these cases correctly after the 'ext4: attempt
to fix race in bigalloc code path' change. May be I missed something
or the code changed somewhere. If there is a testcase that fails, I
can take a look.

quoted

quoted

                                                - Ted

Hi, Ted and Aditya

I am trying to fix this "bigalloc quota reservation in
delay-allocation" problem recently, and I feel confusion about these
code below in ext4_ext_map_blocks():

       if (flags & EXT4_GET_BLOCKS_DELALLOC_RESERVE) {
               unsigned int reserved_clusters;
               /*
                * Check how many clusters we had reserved this allocated range
                */
               reserved_clusters = get_reserved_cluster_alloc(inode,
                                               map->m_lblk, allocated);
               if (map->m_flags & EXT4_MAP_FROM_CLUSTER) {
                       if (reserved_clusters) {
                               ......
                               ext4_da_update_reserve_space(inode,
                                               reserved_clusters, 0);
                       }
               } else {
                       BUG_ON(allocated_clusters < reserved_clusters);
                       /* We will claim quota for all newly allocated blocks.*/
                       ext4_da_update_reserve_space(inode, allocated_clusters,
                                                       1);
                       if (reserved_clusters < allocated_clusters) {
                               struct ext4_inode_info *ei = EXT4_I(inode);
                               int reservation = allocated_clusters -
                                                 reserved_clusters;
                               dquot_reserve_block(inode,
                                               EXT4_C2B(sbi, reservation));
                               spin_lock(&ei->i_block_reservation_lock);
                               ei->i_reserved_data_blocks += reservation;
                               spin_unlock(&ei->i_block_reservation_lock);
                       }
               }
       }

if we have allocated 3 clusters, why should we add "reservation" back
to i_reservd_data_blocks ?

Because we will end up here again next time when we write blocks
[10-11] and try to claim space for them. At this time, there will be
no more reserved blocks left (unless you give them back) and you will
hit the warning in ext4_da_update_reserve_space():

       if (unlikely(used > ei->i_reserved_data_blocks)) {
               ext4_msg(inode->i_sb, KERN_NOTICE, "%s: ino %lu, used %d "
                        "with only %d reserved data blocks\n",
                        __func__, inode->i_ino, used,
                        ei->i_reserved_data_blocks);
               WARN_ON(1);
               used = ei->i_reserved_data_blocks;
       }

quoted

(Aditya Kali 's long comment dose not make
sense to me) Maybe we don't need to do ext4_da_update_reserve_space
when the new block is allocated from "implied_cluster"? Why not just
change them to:

       if (flags & EXT4_GET_BLOCKS_DELALLOC_RESERVE) {
               if ( !(map->m_flags & EXT4_MAP_FROM_CLUSTER) )
                       /* We will claim quota for all newly allocated blocks.*/
                       ext4_da_update_reserve_space(inode, allocated_clusters,
                                                       1);
       }

This won't work in all cases, like the one given in the long comment.
With above change, ext4_da_update_reserve_space() will get called
again the next time for some delayed allocated blocks in the same
cluster, but there isn't any reserved blocks left to claim.
The flag 'EXT4_MAP_FROM_CLUSTER' is not a sufficient check to
determine 'how many' clusters we need to claim. This is because
get_implied_cluster_alloc() only checks within already allocated
extents (and not in the page cache). So, I had to add the function
get_reserved_cluster_alloc() to check if there are any delayed
allocated blocks still in the page-cache.

But I found the code in ext4_ext_map_blocks():

        if ((sbi->s_cluster_ratio > 1) &&
            ext4_find_delalloc_cluster(inode, map->m_lblk, 0))
                map->m_flags |= EXT4_MAP_FROM_CLUSTER;

thus, even the delayed allocated block in the page-cache (have not be
allocated) already have been checked. Therefore in my opinion, the
EXT4_MAP_FROM_CLUSTER is a sufficient check.

Follow the case of your long comment :) ,  process will go like this
in MY CODE ABOVE :

[0-3], [4-7], [8-11]

1. delay-allocation write blocks 10&11
    we reserve 1 cluster, the i_reserved_data_blocks is 1 now
2. delay-allocation write blocks 3 to 8
    we reserve other 2 clusters, so 3 clusters totally, the
i_reserved_data_blocks is 3 now
3. writeout the blocks 3 to 8
    claim all 3 clusters, i_reserved_data_blocks is 0 now
    At this moment, we really have allocated 3 clusters, and the
remain 10&11 block would never occupy another cluster (it would
definitely go into the [8-11] cluster), so, why need we reserve one
more cluster quota ?
4.  writeout the 10&11 blocks
    the 10&11 blocks will be set EXT4_MAP_FROM_CLUSTER flag ( by
get_implied_cluster_alloc as the 8~9 block have been allocated), and
it will not call ext4_da_update_reserve_space any more

As this, no warning, no reserve-and-release ping pong....at least in my imaging

Could you please point out what I missed ?

Thanks

-- 
--
Best Regard
Robin Dong
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