zram: introduce compressed data writeback #411
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zram stores all written back slots raw, which implies that during writeback zram first has to decompress slots (except for ZRAM_HUGE slots, which are raw already). The problem with this approach is that not every written back page gets read back (either via read() or via page-fault), which means that zram basically wastes CPU cycles and battery decompressing such slots. This changes with introduction of decompression on demand, in other words decompression on read()/page-fault. One caveat of decompression on demand is that async read is completed in IRQ context, while zram decompression is sleepable. To workaround this, read-back decompression is offloaded to a preemptible context - system high-prio work-queue. At this point compressed writeback is still disabled, a follow up patch will introduce a new device attribute which will make it possible to toggle compressed writeback per-device. [senozhatsky: rewrote original implementation] Signed-off-by: Richard Chang <richardycc@google.com> Co-developed-by: Sergey Senozhatsky <senozhatsky@chromium.org> Suggested-by: Minchan Kim <minchan@google.com> Suggested-by: Brian Geffon <bgeffon@google.com>
Introduce witeback_compressed device attribute to toggle compressed writeback (decompression on demand) feature. [senozhatsky: rewrote original patch, added documentation] Signed-off-by: Richard Chang <richardycc@google.com> Co-developed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Add missing writeback_batch_size documentation. Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Move bd_stat function and attribute declaration to existing CONFIG_WRITEBACK ifdef-sections. Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
We don't free page in zram_free_page(), not all slots even have any memory associated with them (e.g. ZRAM_SAME). We free the slot (or reset it), rename the function accordingly. Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Use init_lock guard() in sysfs store/show handlers, in order to simplify and, more importantly, to modernize the code. While at it, fix up more coding styles. Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Do not spread device attributes declarations across the file, move io_stat, mm_stat, debug_stat to a common device-attr section. Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
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Upstream branch: c2f2b01 |
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Pull request for series with
subject: zram: introduce compressed data writeback
version: 1
url: https://patchwork.kernel.org/project/linux-block/list/?series=1029165