[PATCH v3 0/2] btrfs: defrag: bring back the old file extent search behavior and address merged extent map generation problem

Qu Wenruo
2022-02-11T06:46:36Z

Filipe reported that the old defrag code using btrfs_search_forward() to
do the following optimization:

- Don't cache extent maps
To save memory in the long run

- Skip entire file ranges which doesn't meet generation requirement

- Don't use merged extent maps which will have unreliable geneartion

The first patch will bring back the old behavior, along with the old
optimizations.

However the 3rd problem is not that easy to solve, as data
read/readahead can also load extent maps into the cache, and causing
extent maps being merged.

Such already cached and merged extent maps will still confuse autodefrag,
as if we found cached extent maps, we will not try to read them from
disk again.

So to completely prevent merged extent maps tricking autodefrag, here
comes the 2nd patch, to mark merged extent maps for defrag.

If we hit an merged extent, and its generation meets our requirement, we
will not trust it but read from disk to get a reliable generation.

This should reduce defrag IO caused by the hidden extent map merging
behavior.

Changelog:
v2:
- Make defrag_get_em() to be more flexiable to handle file extent
iteartion
Now it will not reject item key which is smaller than our target but
doesn't have the wanted type/objectid.
It will continue go next next instead, to prevent skipping an extent.

- Properly reduce path.slots[0]
There is a bug where I want to put "if (path.slots[0] == 0)" but I put
"if (btrfs_header_nritems(path.slots[0]))".
This is fixed with reworked file extent iteration code.

- Address merged extent maps properly
With fixed defrag_get_extent(), we can rely on it to get original em
from disk.
So what we need to do is just to ignore merged extents which meets
our generation requirement.

v3:
- Rebased to latest misc-next

- Fix several generation spell typo

- Fix a case where btrfs_search_slot() can lead to path->slots[0] >=
nritems

- Fix the commit message on modified extent map
Now that part mentioning fsync() doesn't help on the autodefrag bug.

- Update the wording on extent map read from subvolume trees

Qu Wenruo (2):
btrfs: defrag: bring back the old file extent search behavior
btrfs: defrag: don't use merged extent map for their generation check

fs/btrfs/extent_map.c | 2 +
fs/btrfs/extent_map.h | 8 ++
fs/btrfs/ioctl.c | 174 +++++++++++++++++++++++++++++++++++++++++-
3 files changed, 180 insertions(+), 4 deletions(-)

--
2.35.0