On 2021/12/1 13:17, Qu Wenruo wrote:
[BACKGROUND]
Currently btrfs never uses bio_split() to split its bio against RAID
stripe boundaries.
Instead inside btrfs we check our stripe boundary everytime we allocate
a new bio, and ensure the new bio never cross stripe boundaries.
[PROBLEMS]
Although this behavior works fine, it's against the common practice used in
stacked drivers, and is making the effort to convert to iomap harder.
There is also an hidden burden, every time we allocate a new bio, we uses
BIO_MAX_BVECS, but since we know the boundaries, for RAID0/RAID10 we can
only fit at most 16 pages (fixed 64K stripe size, and 4K page size),
wasting the 256 slots we allocated.
[CHALLENGES]
To change the situation, this patchset attempts to improve the situation
by moving the bio split into btrfs_map_bio() time, so upper layer should
no longer bother the bio split against RAID stripes or even chunk
boundaries.
But there are several challenges:
- Conflicts in various endio functions
We want the existing granularity, instead of chained endio, thus we
must make the involved endio functions to handle split bios.
Although most endio functions are already doing their works
independent of the bio size, they are not yet fully handling split
bio.
This patch will convert them to use saved bi_iter and only iterate
the split range instead of the whole bio.
This change involved 3 types of IOs:
* Buffered IO
Including both data and metadata
* Direct IO
* Compressed IO
Their endio functions needs different level of updates to handle split
bios.
Furthermore, there is another endio, end_workqueue_bio(), it can't
handle split bios at all, thus we change the timing so that
btrfs_bio_wq_end_io() is only called after the bio being split.
- Checksum verification
Currently we rely on btrfs_bio::csum to contain the checksum for the
whole bio.
If one bio get split, csum will no longer points to the correct
location for the split bio.
This can be solved by introducing btrfs_bio::offset_to_original, and
use that new member to calculate where we should read csum from.
For the parent bio, it still has btrfs_bio::csum for the whole bio,
thus it can still free it correctly.
- Independent endio for each split bio
Unlike stack drivers, for RAID10 btrfs needs to try its best effort to
read every sectors, to handle the following case: (X means bad, either
unable to read or failed to pass checksum verification, V means good)
Dev 1 (missing) | D1 (X) |
Dev 2 (OK) | D1 (V) |
Dev 3 (OK) | D2 (V) |
Dev 4 (OK) | D2 (X) |
In the above RAID10 case, dev1 is missing, and although dev4 is fine,
its D2 sector is corrupted (by bit rot or whatever).
If we use bio_chain(), read bio for both D1 and D2 will be split, and
since D1 is missing, the whole D1 and D2 read will be marked as error,
thus we will try to read from dev2 and dev4.
But D2 in dev4 has csum mismatch, we can only rebuild D1 and D2
correctly from dev2:D1 and dev3:D2.
This patchset resolve this by saving bi_iter into btrfs_bio::iter, and
uses that at endio to iterate only the split part of an bio.
Other than this, existing read/write page endio functions can handle
them properly without problem.
- Bad RAID56 naming/functionality
There are quite some RAID56 call sites relies on specific behavior on
__btrfs_map_block(), like returning @map_length as stripe_len other
than real mapped length.
This is handled by some small cleanups specific for RAID56.
[NEED FEEDBACK]
In this refactor, btrfs is utilizing a lot of call sites like:
btrfs_bio_save_iter(); // Save bi_iter into some other location
__bio_for_each_segment(bvec, bio, iter, btrfs_bio->iter) {
/* Doing endio for each bvec */
}
And manually implementing an endio which does some work of
__bio_chain_endio() but with extra btrfs specific workload.
I'm wondering if block layer is fine to provide some *enhanced* chain
bio facilities?
[CHANGELOG]
RFC->v1:
- Better patch split
Now patch 01~06 are refactors/cleanups/preparations.
While 07~13 are the patches that doing the conversion while can handle
both old and new bio split timings.
Finally patch 14~16 convert the bio split call sites one by one to
newer facility.
The final patch is just a small clean up.
- Various bug fixes
During the full fstests run, various stupid bugs are exposed and
fixed.
The latest version can be fetched from this branch:
https://github.com/adam900710/linux/tree/refactor_chunk_map
Which already has the fix for the btrfs/187 crash, which is caused by a
missing modification for scrub_stripe_index_and_offset().
Thanks,
Qu
Qu Wenruo (17):
btrfs: update an stale comment on btrfs_submit_bio_hook()
btrfs: save bio::bi_iter into btrfs_bio::iter before submitting
btrfs: use correct bio size for error message in btrfs_end_dio_bio()
btrfs: refactor btrfs_map_bio()
btrfs: move btrfs_bio_wq_end_io() calls into submit_stripe_bio()
btrfs: replace btrfs_dio_private::refs with
btrfs_dio_private::pending_bytes
btrfs: introduce btrfs_bio_split() helper
btrfs: make data buffered read path to handle split bio properly
btrfs: make data buffered write endio function to be split bio
compatible
btrfs: make metadata write endio functions to be split bio compatible
btrfs: make dec_and_test_compressed_bio() to be split bio compatible
btrfs: return proper mapped length for RAID56 profiles in
__btrfs_map_block()
btrfs: allow btrfs_map_bio() to split bio according to chunk stripe
boundaries
btrfs: remove buffered IO stripe boundary calculation
btrfs: remove stripe boundary calculation for compressed IO
btrfs: remove the stripe boundary calculation for direct IO
btrfs: unexport btrfs_get_io_geometry()
fs/btrfs/btrfs_inode.h | 10 +-
fs/btrfs/compression.c | 70 +++-----------
fs/btrfs/disk-io.c | 9 +-
fs/btrfs/extent_io.c | 189 +++++++++++++++++++++++++------------
fs/btrfs/extent_io.h | 2 +
fs/btrfs/inode.c | 210 ++++++++++++++++-------------------------
fs/btrfs/raid56.c | 14 ++-
fs/btrfs/raid56.h | 2 +-
fs/btrfs/scrub.c | 4 +-
fs/btrfs/volumes.c | 157 ++++++++++++++++++++++--------
fs/btrfs/volumes.h | 75 +++++++++++++--
11 files changed, 435 insertions(+), 307 deletions(-)
