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    SubjectRe: [PATCH 1/3] zram: make max_zpage_size configurable
    On Mon, Sep 14, 2015 at 9:50 AM, Vitaly Wool <vitalywool@gmail.com> wrote:
    > It makes sense to have control over what compression ratios are
    > ok to store pages uncompressed and what not. Moreover, if we end
    > up using zbud allocator for zram, any attempt to allocate a whole
    > page will fail, so we may want to avoid this as much as possible.

    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).

    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

    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.

    >
    > So, let's have max_zpage_size configurable as a module parameter.
    >
    > Signed-off-by: Vitaly Wool <vitalywool@gmail.com>
    > ---
    > 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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