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 <t-itoh@jp.fujitsu.com> Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com> Signed-off-by: Chris Mason <clm@fb.com>
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@ -6249,21 +6249,60 @@ out_fail_inode:
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goto out_fail;
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}
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/* Find next extent map of a given extent map, caller needs to ensure locks */
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static struct extent_map *next_extent_map(struct extent_map *em)
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{
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struct rb_node *next;
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next = rb_next(&em->rb_node);
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if (!next)
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return NULL;
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return container_of(next, struct extent_map, rb_node);
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}
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static struct extent_map *prev_extent_map(struct extent_map *em)
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{
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struct rb_node *prev;
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prev = rb_prev(&em->rb_node);
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if (!prev)
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return NULL;
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return container_of(prev, struct extent_map, rb_node);
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}
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/* helper for btfs_get_extent. Given an existing extent in the tree,
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* the existing extent is the nearest extent to map_start,
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* and an extent that you want to insert, deal with overlap and insert
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* the new extent into the tree.
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* the best fitted new extent into the tree.
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*/
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static int merge_extent_mapping(struct extent_map_tree *em_tree,
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struct extent_map *existing,
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struct extent_map *em,
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u64 map_start)
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{
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struct extent_map *prev;
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struct extent_map *next;
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u64 start;
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u64 end;
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u64 start_diff;
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BUG_ON(map_start < em->start || map_start >= extent_map_end(em));
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start_diff = map_start - em->start;
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em->start = map_start;
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em->len = existing->start - em->start;
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if (existing->start > map_start) {
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next = existing;
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prev = prev_extent_map(next);
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} else {
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prev = existing;
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next = next_extent_map(prev);
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}
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start = prev ? extent_map_end(prev) : em->start;
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start = max_t(u64, start, em->start);
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end = next ? next->start : extent_map_end(em);
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end = min_t(u64, end, extent_map_end(em));
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start_diff = start - em->start;
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em->start = start;
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em->len = end - start;
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if (em->block_start < EXTENT_MAP_LAST_BYTE &&
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!test_bit(EXTENT_FLAG_COMPRESSED, &em->flags)) {
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em->block_start += start_diff;
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@ -6540,16 +6579,17 @@ insert:
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ret = 0;
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existing = lookup_extent_mapping(em_tree, start, len);
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if (existing && (existing->start > start ||
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existing->start + existing->len <= start)) {
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free_extent_map(existing);
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existing = NULL;
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}
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if (!existing) {
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existing = lookup_extent_mapping(em_tree, em->start,
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em->len);
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if (existing) {
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existing = search_extent_mapping(em_tree, start, len);
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/*
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* existing will always be non-NULL, since there must be
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* extent causing the -EEXIST.
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*/
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if (start >= extent_map_end(existing) ||
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start + len <= existing->start) {
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/*
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* The existing extent map is the one nearest to
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* the [start, start + len) range which overlaps
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*/
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err = merge_extent_mapping(em_tree, existing,
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em, start);
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free_extent_map(existing);
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@ -6557,11 +6597,6 @@ insert:
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free_extent_map(em);
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em = NULL;
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}
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} else {
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err = -EIO;
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free_extent_map(em);
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em = NULL;
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}
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} else {
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free_extent_map(em);
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em = existing;
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