On Fri 19-06-20 08:39:53, Eric Sandeen wrote: > On 6/19/20 1:41 AM, Lukas Czerner wrote: > > On Wed, Jun 17, 2020 at 02:19:04PM -0500, Eric Sandeen wrote: > >> If for any reason a directory passed to do_split() does not have enough > >> active entries to exceed half the size of the block, we can end up > >> iterating over all "count" entries without finding a split point. > >> > >> In this case, count == move, and split will be zero, and we will > >> attempt a negative index into map[]. > >> > >> Guard against this by detecting this case, and falling back to > >> split-to-half-of-count instead; in this case we will still have > >> plenty of space (> half blocksize) in each split block. > > ... > > >> + /* > >> + * map index at which we will split > >> + * > >> + * If the sum of active entries didn't exceed half the block size, just > >> + * split it in half by count; each resulting block will have at least > >> + * half the space free. > >> + */ > >> + if (i > 0) > >> + split = count - move; > >> + else > >> + split = count/2; > > > > Won't we have exactly the same problem as we did before your commit > > ef2b02d3e617cb0400eedf2668f86215e1b0e6af ? Since we do not know how much > > space we actually moved we might have not made enough space for the new > > entry ? > > I don't think so - while we don't have the original reproducer, I assume that > it was the case where the block was very full, and splitting by count left us > with one of the split blocks still over half full (because ensuring that we > split in half by size seemed to fix it) > > In this case, the sum of the active entries was <= half the block size. > So if we split by count, we're guaranteed to have >= half the block size free > in each side of the split. > > > Also since we have the move == count when the problem appears then it's > > clear that we never hit the condition > > > > 1865 → → /* is more than half of this entry in 2nd half of the block? */ > > 1866 → → if (size + map[i].size/2 > blocksize/2) > > 1867 → → → break; > > > > in the loop. This is surprising but it means the the entries must have > > gaps between them that are small enough that we can't fit the entry > > right in ? Should not we try to compact it before splitting, or is it > > the case that this should have been done somewhere else ? > > Yes, that's exactly what happened - see my 0/1 cover letter. Maybe that should > be in the patch description itself. ALso, yes compaction would help but I was > unclear as to whether that should be done here, is the side effect of some other > bug, etc. In general, we do seem to do compaction elsewhere and I don't know > how we got to this point. > > > If we really want ot be fair and we want to split it right in the middle > > of the entries size-wise then we need to keep track of of sum of the > > entries and decide based on that, not blocksize/2. But maybe the problem > > could be solved by compacting the entries together because the condition > > seems to rely on that. > > I thought about that as well, but it took a bit more code to do; we could make > make_map() return both count and total size, for example. But based on my > theory above that both sides of the split will have >= half block free, it > didn't seem necessary, particularly since this seems like an edge case? This didn't seem to conclude in any way? The patch looks good to me FWIW so feel free to add: Reviewed-by: Jan Kara <jack@xxxxxxx> Ted, can you please pick this patch up? Thanks! Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
