[v7,00/17] zsmalloc/zram: there be preemption

Message

Sergey Senozhatsky Feb. 21, 2025, 9:37 a.m. UTC

Currently zram runs compression and decompression in non-preemptible
sections, e.g.

    zcomp_stream_get()     // grabs CPU local lock
    zcomp_compress()

or

    zram_slot_lock()       // grabs entry spin-lock
    zcomp_stream_get()     // grabs CPU local lock
    zs_map_object()        // grabs rwlock and CPU local lock
    zcomp_decompress()

Potentially a little troublesome for a number of reasons.

For instance, this makes it impossible to use async compression
algorithms or/and H/W compression algorithms, which can wait for OP
completion or resource availability.  This also restricts what
compression algorithms can do internally, for example, zstd can
allocate internal state memory for C/D dictionaries:

do_fsync()
 do_writepages()
  zram_bio_write()
   zram_write_page()                          // become non-preemptible
    zcomp_compress()
     zstd_compress()
      ZSTD_compress_usingCDict()
       ZSTD_compressBegin_usingCDict_internal()
        ZSTD_resetCCtx_usingCDict()
         ZSTD_resetCCtx_internal()
          zstd_custom_alloc()                 // memory allocation

Not to mention that the system can be configured to maximize
compression ratio at a cost of CPU/HW time (e.g. lz4hc or deflate
with very high compression level) so zram can stay in non-preemptible
section (even under spin-lock or/and rwlock) for an extended period
of time.  Aside from compression algorithms, this also restricts what
zram can do.  One particular example is zram_write_page() zsmalloc
handle allocation, which has an optimistic allocation (disallowing
direct reclaim) and a pessimistic fallback path, which then forces
zram to compress the page one more time.

This series changes zram to not directly impose atomicity restrictions
on compression algorithms (and on itself), which makes zram write()
fully preemptible; zram read(), sadly, is not always preemptible yet.
There are still indirect atomicity restrictions imposed by zsmalloc().
One notable example is object mapping API, which returns with:
a) local CPU lock held
b) zspage rwlock held

First, zsmalloc's zspage lock is converted from rwlock to a special
type of RW-lookalike look with some extra guarantees/features.  Second,
a new handle mapping is introduced which doesn't use per-CPU buffers
(and hence no local CPU lock), does fewer memcpy() calls, but requires
users to provide a pointer to temp buffer for object copy-in (when
needed).  Third, zram is converted to the new zsmalloc mapping API
and thus zram read() becomes preemptible.

v6 -> v7:
-- provide dep_map access wrappers to avoid code duplication
   between !lockdep and lockdep builds (Yosry)

Sergey Senozhatsky (17):
  zram: sleepable entry locking
  zram: permit preemption with active compression stream
  zram: remove unused crypto include
  zram: remove max_comp_streams device attr
  zram: remove two-staged handle allocation
  zram: remove writestall zram_stats member
  zram: limit max recompress prio to num_active_comps
  zram: filter out recomp targets based on priority
  zram: rework recompression loop
  zsmalloc: rename pool lock
  zsmalloc: make zspage lock preemptible
  zsmalloc: introduce new object mapping API
  zram: switch to new zsmalloc object mapping API
  zram: permit reclaim in zstd custom allocator
  zram: do not leak page on recompress_store error path
  zram: do not leak page on writeback_store error path
  zram: add might_sleep to zcomp API

 Documentation/ABI/testing/sysfs-block-zram  |   8 -
 Documentation/admin-guide/blockdev/zram.rst |  36 +-
 drivers/block/zram/backend_zstd.c           |  11 +-
 drivers/block/zram/zcomp.c                  |  48 ++-
 drivers/block/zram/zcomp.h                  |   8 +-
 drivers/block/zram/zram_drv.c               | 289 ++++++++--------
 drivers/block/zram/zram_drv.h               |  22 +-
 include/linux/zsmalloc.h                    |   8 +
 mm/zsmalloc.c                               | 356 ++++++++++++++++----
 9 files changed, 498 insertions(+), 288 deletions(-)

--
2.48.1.601.g30ceb7b040-goog