Date:	Mon, 16 Apr 2012 11:35:44 +0800
From:	Robin Dong <hao.bigrat@...il.com>
To:	"Ted Ts'o" <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 1/3] ext4: s_freeclusters_counter should not tranform to
 unit of block before assigning to "free_clusters" in ext4_has_free_cluste

在 2012年3月25日 上午5:05，Ted Ts'o <tytso@....edu> 写道：
> On Tue, Mar 13, 2012 at 07:38:16PM +0800, Robin Dong wrote:
>> Creating 4-byte files until ENOSPC in a delay-allocation and bigalloc ext4 fs and then sync it, the dmseg will report like:
>>
>>       [  482.154538] EXT4-fs (sdb6): delayed block allocation failed for inode 1664 at logical offset 0 with max blocks 1 with error -28
>>       [  482.154540] EXT4-fs (sdb6): This should not happen!! Data will be lost
>>
>> The reason is ext4_has_free_clusters reporting wrong
>> result. Actually, the unit of sbi->s_dirtyclusters_counter is block,
>> so we should tranform it to cluster for argument "dirty_clusters",
>> just like "free_clusters".
>
> We have a bigger problem here, which is that this is not the only
> place where s_dirty_clusters_counter is being used in units of
> clusters.  (See ext4_claim_free_clusters, which when called by mballoc
> is using units of clusters.)
>
> We definitely have brokeness here, but this is not the whole story.
> We need to take a step back here and decide whether the correct units
> is clusters or blocks.  Ultimately I think it does need to be
> clusters, because we can't just convert blocks and clusters by using
> B2C; we could dirty 3 blocks, but if those 3 blocks span two 64-block
> clusters, what's important is that we have to reserve space for 2
> clusters.  We can't just calculate "3 >> 6" and assume that we can
> reserve 0 clusters and be done with it!

Hi, Ted

Actually, I have complete this logic in my "[PATCH 2/3] ext4: modify
the implementation of quota reservation in bigalloc-delay-allocation"
(http://marc.info/?l=linux-ext4&m=133163876628132&w=2)  patch,
when those 3 blocks span two clusters, the logic in
ext4_ext_map_blocks() will check whether a block is already in a
delay-allocated-cluster (or already be
allocated in disk) and ultimately reserved 2 clusters for the 3 blocks.

Imaging block0 and block1 in cluster0, block2 in cluster1. When
ext4_da_map_blocks() process block0, we will call
ext4_da_reserve_space() (which will
reserve a cluster) and bump s_dirtycluster_counter , but when process
block1, ext4_ext_map_blocks will return flag with
EXT4_MAP_FROM_CLUSTER
and we will not bump s_dirtycluster_counter this time since block1 is
belong to a already reserved cluster.

Maybe I misunderstanding your meaning, could you please point out my fault?

Thanks.

>
> This is one of the places where I think we need to solve things by
> having a better data structure for tracking which pages have been
> subject to delayed allocation, since if we touch another block in a
> cluster where we've done a delayed allocation, we don't need to bump
> s_dirtyclusters_counter.  However, if this is the first time we've
> touched a block in a particular cluster, then we *do* need to bump
> s_dirtyclusters_counter --- and if we need to search all of the pages
> in the page cache to make this determination, it's going to be
> painful....
>
>                                        - Ted



-- 
--
Best Regard
Robin Dong
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