[PATCH 0/2] btrfs: prevent direct reclaim during compressed readahead

["JP Kobryn (Meta)" <jp.kobryn@linux.dev>]

We're finding that while under memory pressure, direct reclaim is kicking
in during compressed readahead. This puts the associated task into D-state.
Then shrink_lruvec() disables interrupts when acquiring the LRU lock. Under
heavy pressure, reclaim can run long enough that the CPU becomes prone to
CSD lock stalls since it cannot service incoming IPIs. Although the CSD
lock stalls are the worst case scenario, we have found many more subtle
occurrences of this latency on the order of seconds, over a minute in some
cases.

Prevent direct reclaim during compressed readahead. This is achieved by
using different GFP flags whenever the bio is marked for readahead. The
flags are similar to GFP_NOFS but stripped of __GFP_DIRECT_RECLAIM.  Also,
__GFP_NOWARN is added since these allocations are allowed to fail. Demand
reads still use full GFP_NOFS and will enter reclaim if needed.

There has been some previous work done to reduce the frequency of calling
add_ra_bio_pages() [0]. This patch is complementary in that it reduces the
latency associated with those calls.

[0] https://lore.kernel.org/linux-btrfs/656838ec1232314a2657716e59f4f15a8eadba64.1751492111.git.boris@bur.io/

JP Kobryn (Meta) (2):
  btrfs: additional gfp api for allocating compressed folios
  btrfs: prevent direct reclaim during compressed readahead

 fs/btrfs/compression.c | 44 ++++++++++++++++++++++++++++++++++--------
 fs/btrfs/compression.h |  1 +
 2 files changed, 37 insertions(+), 8 deletions(-)

-- 
2.52.0