f2fs: introduce max_atc_write_bio_entry_cnt

Commit 3de6b8094115 ("f2fs: Run f2fs_write_end_io() asynchronously")
introduced max_atc_write_bio_size to offload write bio completion to a
workqueue when f2fs_write_end_io() is called in atomic context.

However, using the bio byte size (bi_iter.bi_size) as the threshold has
limitations once large folios or multi-page bvecs are involved:

1. In f2fs_write_end_bio(), execution time is dominated by the
   completion loop (bio_for_each_folio_all() for pagecache folios,
   or while(entry) for cached metadata blocks), where per-entry
   locking, status clearing, sanity checks, and writeback end are
   performed. The CPU latency in atomic context scales with the number
   of entries in the bio, not its byte size.
2. For large folios, a large bio (e.g. 1MB comprised of single large
   folios) only iterates a few times through the loop. Throttling solely
   by max_atc_write_bio_size unnecessarily offloads such low-latency
   bios to the workqueue, adding context-switch overhead.
3. Conversely, relying on bio->bi_vcnt is insufficient because the block
   layer (bvec_try_merge_page) merges physically consecutive pages/folios
   into the same bio_vec segment. Thus, a bio with bi_vcnt == 1 could
   still contain multiple entries and iterate many times in end_io.

To address this, track the exact number of entries (folios or cached
blocks) queued to the write bio via entry_cnt in struct f2fs_bio, and
introduce a sysfs attribute max_atc_write_bio_entry_cnt. This enables
precise control over atomic context latency without penalizing large
folios.

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

0 files changed, 0 insertions, 0 deletions