btrfs: enable cross-folio readahead for bs < ps and large folio cases

[BACKGROUND]
When bs < ps support was initially introduced, the compressed data
readahead was disabled as at that time the target page size was 64K.
This means a compressed data extent can span at most 3 64K pages (the
head and tail parts are not aligned to 64K), meaning the benefit is
pretty minimal.

[UNEXPECTED WORKING SITUATION]
But with the already merged large folio support, we're already enabling
readahead with subpage routine unintentionally, e.g.:

   0      4K      8K      12K      16K
   |   Folio 0    |    Folio 8K    |
   |<----- Compressed data ------->|

We have 2 8K sized folios, all backed by a single compressed data.

In that case add_ra_bio_pages() will continue to add folio 8K into the
read bio, as the condition to skip is only (bs < ps), not taking the
newer large folio support into consideration at all.

So for folio 8K, it is added to the read bio, but without subpage lock
bitmap populated.

Then at end_bbio_data_read(), folio 0 has proper locked bitmap set, but
folio 8K does not.
This inconsistency is handled by the extra safety net at
btrfs_subpage_end_and_test_lock() where if a folio has no @nr_locked, it
will just be unlocked without touching the locked bitmap.

[ENHANCEMENT]
Make add_ra_bio_pages() support bs < ps and large folio cases, by
removing the check and calling btrfs_folio_set_lock() unconditionally.

This won't make any difference on 4K page sized systems with large
folios, as the readahead is already working, although unexpectedly.

But this will enable true compressed data readahead for bs < ps cases
properly.

Please note that such readahead will only work if the compressed extent is
crossing folio boundaries, which is also the existing limitation.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

0 files changed, 0 insertions, 0 deletions