Re: [PATCH] btrfs: Fix and enhance merge_extent_mapping() to insert best fitted extent map

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

-------- Original Message --------
Subject: Re: [PATCH] btrfs: Fix and enhance merge_extent_mapping() to 
insert best fitted extent map
From: Filipe David Manana <fdmanana@gmail.com>
To: Qu Wenruo <quwenruo@cn.fujitsu.com>
Date: 2014年09月18日 21:16
> On Wed, Sep 17, 2014 at 4:53 AM, Qu Wenruo <quwenruo@cn.fujitsu.com> wrote:
>> 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.
> This sounds like very deterministic to me.
> Any reason to not add tests to the sanity tests that exercise
> this/these case/cases?
Yes, thanks for the informing.
Will add the test case for it soon.

Thanks,
Qu
>
> Thanks
>
>> [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>
>> ---
>>   fs/btrfs/inode.c | 79 ++++++++++++++++++++++++++++++++++++++++----------------
>>   1 file changed, 57 insertions(+), 22 deletions(-)
>>
>> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
>> index 016c403..8039021 100644
>> --- a/fs/btrfs/inode.c
>> +++ b/fs/btrfs/inode.c
>> @@ -6191,21 +6191,60 @@ out_fail_inode:
>>          goto out_fail;
>>   }
>>
>> +/* Find next extent map of a given extent map, caller needs to ensure locks */
>> +static struct extent_map *next_extent_map(struct extent_map *em)
>> +{
>> +       struct rb_node *next;
>> +
>> +       next = rb_next(&em->rb_node);
>> +       if (!next)
>> +               return NULL;
>> +       return container_of(next, struct extent_map, rb_node);
>> +}
>> +
>> +static struct extent_map *prev_extent_map(struct extent_map *em)
>> +{
>> +       struct rb_node *prev;
>> +
>> +       prev = rb_prev(&em->rb_node);
>> +       if (!prev)
>> +               return NULL;
>> +       return container_of(prev, struct extent_map, rb_node);
>> +}
>> +
>>   /* helper for btfs_get_extent.  Given an existing extent in the tree,
>> + * the existing extent is the nearest extent to map_start,
>>    * and an extent that you want to insert, deal with overlap and insert
>> - * the new extent into the tree.
>> + * the best fitted new extent into the tree.
>>    */
>>   static int merge_extent_mapping(struct extent_map_tree *em_tree,
>>                                  struct extent_map *existing,
>>                                  struct extent_map *em,
>>                                  u64 map_start)
>>   {
>> +       struct extent_map *prev;
>> +       struct extent_map *next;
>> +       u64 start;
>> +       u64 end;
>>          u64 start_diff;
>>
>>          BUG_ON(map_start < em->start || map_start >= extent_map_end(em));
>> -       start_diff = map_start - em->start;
>> -       em->start = map_start;
>> -       em->len = existing->start - em->start;
>> +
>> +       if (existing->start > map_start) {
>> +               next = existing;
>> +               prev = prev_extent_map(next);
>> +       } else {
>> +               prev = existing;
>> +               next = next_extent_map(prev);
>> +       }
>> +
>> +       start = prev ? extent_map_end(prev) : em->start;
>> +       start = max_t(u64, start, em->start);
>> +       end = next ? next->start : extent_map_end(em);
>> +       end = min_t(u64, end, extent_map_end(em));
>> +       start_diff = start - em->start;
>> +       em->start = start;
>> +       em->len = end - start;
>>          if (em->block_start < EXTENT_MAP_LAST_BYTE &&
>>              !test_bit(EXTENT_FLAG_COMPRESSED, &em->flags)) {
>>                  em->block_start += start_diff;
>> @@ -6482,25 +6521,21 @@ insert:
>>
>>                  ret = 0;
>>
>> -               existing = lookup_extent_mapping(em_tree, start, len);
>> -               if (existing && (existing->start > start ||
>> -                   existing->start + existing->len <= start)) {
>> +               existing = search_extent_mapping(em_tree, start, len);
>> +               /*
>> +                * existing will always be non-NULL, since there must be
>> +                * extent causing the -EEXIST.
>> +                */
>> +               if (start >= extent_map_end(existing) ||
>> +                   start + len <= existing->start) {
>> +                       /*
>> +                        * The existing extent map is the one nearest to
>> +                        * the [start, start + len) range which overlaps
>> +                        */
>> +                       err = merge_extent_mapping(em_tree, existing,
>> +                                                  em, start);
>>                          free_extent_map(existing);
>> -                       existing = NULL;
>> -               }
>> -               if (!existing) {
>> -                       existing = lookup_extent_mapping(em_tree, em->start,
>> -                                                        em->len);
>> -                       if (existing) {
>> -                               err = merge_extent_mapping(em_tree, existing,
>> -                                                          em, start);
>> -                               free_extent_map(existing);
>> -                               if (err) {
>> -                                       free_extent_map(em);
>> -                                       em = NULL;
>> -                               }
>> -                       } else {
>> -                               err = -EIO;
>> +                       if (err) {
>>                                  free_extent_map(em);
>>                                  em = NULL;
>>                          }
>> --
>> 2.1.0
>>
>> --
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