From: Brian Geffon Subject: zram: add a huge_idle writeback mode Today it's only possible to write back as a page, idle, or huge. A user might want to writeback pages which are huge and idle first as these idle pages do not require decompression and make a good first pass for writeback. Idle writeback specifically has the advantage that a refault is unlikely given that the page has been swapped for some amount of time without being refaulted. Huge writeback has the advantage that you're guaranteed to get the maximum benefit from a single page writeback, that is, you're reclaiming one full page of memory. Pages which are compressed in zram being written back result in some benefit which is always less than a page size because of the fact that it was compressed. The primary use of this is for minimizing refaults in situations where the device has to be sensitive to storage endurance. On ChromeOS we have devices with slow eMMC and repeated writes and refaults can negatively affect performance and endurance. Link: https://lkml.kernel.org/r/20220322215821.1196994-1-bgeffon@google.com Signed-off-by: Brian Geffon Acked-by: Minchan Kim Cc: Nitin Gupta Cc: Sergey Senozhatsky Signed-off-by: Andrew Morton --- Documentation/admin-guide/blockdev/zram.rst | 5 +++++ drivers/block/zram/zram_drv.c | 10 ++++++---- 2 files changed, 11 insertions(+), 4 deletions(-) --- a/Documentation/admin-guide/blockdev/zram.rst~zram-add-a-huge_idle-writeback-mode +++ a/Documentation/admin-guide/blockdev/zram.rst @@ -343,6 +343,11 @@ Admin can request writeback of those idl With the command, zram will writeback idle pages from memory to the storage. +Additionally, if a user choose to writeback only huge and idle pages +this can be accomplished with:: + + echo huge_idle > /sys/block/zramX/writeback + If an admin wants to write a specific page in zram device to the backing device, they could write a page index into the interface. --- a/drivers/block/zram/zram_drv.c~zram-add-a-huge_idle-writeback-mode +++ a/drivers/block/zram/zram_drv.c @@ -639,8 +639,8 @@ static int read_from_bdev_async(struct z #define PAGE_WB_SIG "page_index=" #define PAGE_WRITEBACK 0 -#define HUGE_WRITEBACK 1 -#define IDLE_WRITEBACK 2 +#define HUGE_WRITEBACK (1<<0) +#define IDLE_WRITEBACK (1<<1) static ssize_t writeback_store(struct device *dev, @@ -660,6 +660,8 @@ static ssize_t writeback_store(struct de mode = IDLE_WRITEBACK; else if (sysfs_streq(buf, "huge")) mode = HUGE_WRITEBACK; + else if (sysfs_streq(buf, "huge_idle")) + mode = IDLE_WRITEBACK | HUGE_WRITEBACK; else { if (strncmp(buf, PAGE_WB_SIG, sizeof(PAGE_WB_SIG) - 1)) return -EINVAL; @@ -721,10 +723,10 @@ static ssize_t writeback_store(struct de zram_test_flag(zram, index, ZRAM_UNDER_WB)) goto next; - if (mode == IDLE_WRITEBACK && + if (mode & IDLE_WRITEBACK && !zram_test_flag(zram, index, ZRAM_IDLE)) goto next; - if (mode == HUGE_WRITEBACK && + if (mode & HUGE_WRITEBACK && !zram_test_flag(zram, index, ZRAM_HUGE)) goto next; /* _