On (09/15/15 01:42), Dan Streetman wrote:

Well, zram explicitly expects to be able to store PAGE_SIZE'd objects:

if (unlikely(clen > max_zpage_size))
    clen = PAGE_SIZE;
handle = zs_malloc(meta->mem_pool, clen);

so the max_zpage_size doesn't prevent zram from trying to store the
page in zsmalloc/zbud/whatever; instead, if the compressed page is
larger than max_zpage_size, it just stores it uncompressed (as a side
note, I'm not quite sure what the benefit of not storing in compressed
form any pages that compress to between 3/4 and 1 page is...I suppose
the decompression time is skipped, but it also wastes space...i would
just make max_zpage_size == PAGE_SIZE).

correct, to avoid decompression of something that doesn't really
save any memory. compressed buffer may be very close to PAGE_SIZE,
so it sort of makes sense. I agree, that (3/4, 1 page] looks like
a magically picked range, though.

but zbud can't store a PAGE_SIZE'd object.  so the behavior would
change.  The current behavior is:

compressed page <= max_zpage_size : stored compressed
compressed page > max_zpage_size : stored uncompressed

new behavior:

compressed page <= max_zpage_size : stored compressed
compressed page > max_zpage_size : zram write fails

yes. and per my observation, `compressed page > max_zpage_size'
happens "quite often".

	-ss

to do this right, I think you have to change zbud to be able to store
PAGE_SIZE'd objects.  That should be doable, I think you can just the
page->lru to store it in the zbud lru, and use a page flag to indicate
it's uncompressed, full PAGE_SIZE page, or something like that.  But
without the ability to store full pages, zbud won't work well with
zram.

quoted

So, let's have max_zpage_size configurable as a module parameter.

Signed-off-by: Vitaly Wool <redacted>
---
 drivers/block/zram/zram_drv.c | 13 +++++++++++++
 drivers/block/zram/zram_drv.h | 16 ----------------
 2 files changed, 13 insertions(+), 16 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 9fa15bb..6d9f1d1 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c

@@ -42,6 +42,7 @@ static const char *default_compressor = "lzo";

 /* Module params (documentation at end) */
 static unsigned int num_devices = 1;
+static size_t max_zpage_size = PAGE_SIZE / 4 * 3;

 static inline void deprecated_attr_warn(const char *name)
 {

@@ -1411,6 +1412,16 @@ static int __init zram_init(void)
                return ret;
        }

+       /*
+        * max_zpage_size must be less than or equal to:
+        * ZS_MAX_ALLOC_SIZE. Otherwise, zs_malloc() would
+        * always return failure.
+        */
+       if (max_zpage_size > PAGE_SIZE) {
+               pr_err("Invalid max_zpage_size %ld\n", max_zpage_size);
+               return -EINVAL;
+       }
+
        zram_major = register_blkdev(0, "zram");
        if (zram_major <= 0) {
                pr_err("Unable to get major number\n");

@@ -1444,6 +1455,8 @@ module_exit(zram_exit);

 module_param(num_devices, uint, 0);
 MODULE_PARM_DESC(num_devices, "Number of pre-created zram devices");
+module_param(max_zpage_size, ulong, 0);
+MODULE_PARM_DESC(max_zpage_size, "Threshold for storing compressed pages");

 MODULE_LICENSE("Dual BSD/GPL");
 MODULE_AUTHOR("Nitin Gupta <ngupta@vflare.org>");

diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
index 8e92339..3a29c33 100644
--- a/drivers/block/zram/zram_drv.h
+++ b/drivers/block/zram/zram_drv.h

@@ -20,22 +20,6 @@

 #include "zcomp.h"

-/*-- Configurable parameters */
-
-/*
- * Pages that compress to size greater than this are stored
- * uncompressed in memory.
- */
-static const size_t max_zpage_size = PAGE_SIZE / 4 * 3;
-
-/*
- * NOTE: max_zpage_size must be less than or equal to:
- *   ZS_MAX_ALLOC_SIZE. Otherwise, zs_malloc() would
- * always return failure.
- */
-
-/*-- End of configurable params */
-
 #define SECTOR_SHIFT           9
 #define SECTORS_PER_PAGE_SHIFT (PAGE_SHIFT - SECTOR_SHIFT)
 #define SECTORS_PER_PAGE       (1 << SECTORS_PER_PAGE_SHIFT)
--

1.9.1

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