在 2012年3月25日 上午5:05，Ted Ts'o <tytso@xxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
