From: Qu Wenruo Date: Wed, 17 Sep 2014 03:53:35 +0000 (+0800) Subject: btrfs: Fix and enhance merge_extent_mapping() to insert best fitted extent map X-Git-Tag: fixes-against-v3.18-rc2~120^2~24 X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=e6c4efd87ab04e5ead363f24e6ac35ed3506d401;p=pandora-kernel.git btrfs: Fix and enhance merge_extent_mapping() to insert best fitted extent map The following commit enhanced the merge_extent_mapping() to reduce fragment in extent map tree, but it can't handle case which existing lies before map_start: 51f39 btrfs: Use right extent length when inserting overlap extent map. [BUG] When existing extent map's start is before map_start, the em->len will be minus, which will corrupt the extent map and fail to insert the new extent map. This will happen when someone get a large extent map, but when it is going to insert it into extent map tree, some one has already commit some write and split the huge extent into small parts. [REPRODUCER] It is very easy to tiger using filebench with randomrw personality. It is about 100% to reproduce when using 8G preallocated file in 60s randonrw test. [FIX] This patch can now handle any existing extent position. Since it does not directly use existing->start, now it will find the previous and next extent around map_start. So the old existing->start < map_start bug will never happen again. [ENHANCE] This patch will insert the best fitted extent map into extent map tree, other than the oldest [map_start, map_start + sectorsize) or the relatively newer but not perfect [map_start, existing->start). The patch will first search existing extent that does not intersects with the desired map range [map_start, map_start + len). The existing extent will be either before or behind map_start, and based on the existing extent, we can find out the previous and next extent around map_start. So the best fitted extent would be [prev->end, next->start). For prev or next is not found, em->start would be prev->end and em->end wold be next->start. With this patch, the fragment in extent map tree should be reduced much more than the 51f39 commit and reduce an unneeded extent map tree search. Reported-by: Tsutomu Itoh Signed-off-by: Qu Wenruo Signed-off-by: Chris Mason --- Reading git-diff-tree failed