https://bugzilla.kernel.org/show_bug.cgi?id=151491 --- Comment #14 from Eric Whitney (enwlinux@xxxxxxxxx) --- I've been able to identify the unknown contributor to the delalloc accounting error as discussed previously (in comment 10). When ext4 is writing back a previously delalloc'ed extent that contains just a portion of a cluster, and then delallocs an extent that contains another disjoint portion of that same cluster, the count of delalloc'ed clusters (i_reserved_data_blocks) is incorrectly incremented. The cluster has been physically allocated during writeback, but the subsequent delalloc write does not discover that allocation. This is because the code in ext4_da_map_blocks() checks for a previously physically allocated block at the point of allocation rather than a previously physically allocated cluster spanning the point of allocation. The effect is to bump the delalloc'ed cluster count for clusters that will never be allocated (since they've already been allocated), and the overcount will therefore never be reduced. It's more likely this problem would occur when writing files sequentially if the test system was under memory pressure, resulting in writeback activity in parallel with delalloc writes. The magnitude of the overcount is also likely to be larger in this situation. This correlates well with the observation that the reproducer for the accounting errors is more likely to reproduce the problem on a test system with little free memory. I've been testing a prototype patch that appears to fix this problem. However, I've also identified at least two other unrelated delalloc accounting problems for bigalloc file systems whose effects are masked by the other contributor to overcounting in ext4_ext_map_blocks(). Fixing it results in failures caused by these other problems when running xfstests-bld on bigalloc. So, there's a lot of work yet to be done before it's time to post patches. -- You are receiving this mail because: You are watching the assignee of the bug.
