On Mon, 22 Sep 2014 09:03:10 +0900 Minchan Kim <minchan@xxxxxxxxxx> wrote:
> This patch implement SWAP_FULL handler in zram so that VM can
> know whether zram is full or not and use it to stop anonymous
> page reclaim.
>
> How to judge fullness is below,
>
> fullness = (100 * used space / total space)
>
> It means the higher fullness is, the slower we reach zram full.
> Now, default of fullness is 80 so that it biased more momory
> consumption rather than early OOM kill.
It's unclear to me why this is being done. What's wrong with "use it
until it's full then stop", which is what I assume the current code
does? Why add this stuff? What goes wrong with the current code and
how does this fix it?
ie: better explanation and justification in the chagnelogs, please.
> Above logic works only when used space of zram hit over the limit
> but zram also pretend to be full once 32 consecutive allocation
> fail happens. It's safe guard to prevent system hang caused by
> fragment uncertainty.
So allocation requests are of variable size, yes? If so, the above
statement should read "32 consecutive allocation attempts for regions
or size 2 or more slots". Because a failure of a single-slot
allocation attempt is an immediate failure.
The 32-in-a-row thing sounds like a hack. Why can't we do this
deterministically? If one request for four slots fails then the next
one will as well, so why bother retrying?
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -43,6 +43,20 @@ static const char *default_compressor = "lzo";
> /* Module params (documentation at end) */
> static unsigned int num_devices = 1;
>
> +/*
> + * If (100 * used_pages / total_pages) >= ZRAM_FULLNESS_PERCENT),
> + * we regards it as zram-full. It means that the higher
> + * ZRAM_FULLNESS_PERCENT is, the slower we reach zram full.
> + */
I just don't understand this patch :( To me, the above implies that the
user who sets 80% has elected to never use 20% of the zram capacity.
Why on earth would anyone do that? This chagnelog doesn't tell me.
> +#define ZRAM_FULLNESS_PERCENT 80
We've had problems in the past where 1% is just too large an increment
for large systems.
> @@ -597,10 +613,15 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
> }
>
> alloced_pages = zs_get_total_pages(meta->mem_pool);
> - if (zram->limit_pages && alloced_pages > zram->limit_pages) {
> - zs_free(meta->mem_pool, handle);
> - ret = -ENOMEM;
> - goto out;
> + if (zram->limit_pages) {
> + if (alloced_pages > zram->limit_pages) {
This is all a bit racy, isn't it? pool->pages_allocated and
zram->limit_pages could be changing under our feet.
> + zs_free(meta->mem_pool, handle);
> + atomic_inc(&zram->alloc_fail);
> + ret = -ENOMEM;
> + goto out;
> + } else {
> + atomic_set(&zram->alloc_fail, 0);
> + }
}
update_used_max(zram, alloced_pages);
> @@ -711,6 +732,7 @@ static void zram_reset_device(struct zram *zram, bool reset_capacity)
> down_write(&zram->init_lock);
>
> zram->limit_pages = 0;
> + atomic_set(&zram->alloc_fail, 0);
>
> if (!init_done(zram)) {
> up_write(&zram->init_lock);
> @@ -944,6 +966,34 @@ static int zram_slot_free_notify(struct block_device *bdev,
> return 0;
> }
>
> +static int zram_full(struct block_device *bdev, void *arg)
This could return a bool. That implies that zram_swap_hint should
return bool too, but as we haven't been told what the zram_swap_hint
return value does, I'm a bit stumped.
And why include the unusefully-named "void *arg"? It doesn't get used here.
> +{
> + struct zram *zram;
> + struct zram_meta *meta;
> + unsigned long total_pages, compr_pages;
> +
> + zram = bdev->bd_disk->private_data;
> + if (!zram->limit_pages)
> + return 0;
> +
> + meta = zram->meta;
> + total_pages = zs_get_total_pages(meta->mem_pool);
> +
> + if (total_pages >= zram->limit_pages) {
> +
> + compr_pages = atomic64_read(&zram->stats.compr_data_size)
> + >> PAGE_SHIFT;
> + if ((100 * compr_pages / total_pages)
> + >= ZRAM_FULLNESS_PERCENT)
> + return 1;
> + }
> +
> + if (atomic_read(&zram->alloc_fail) > ALLOC_FAIL_MAX)
> + return 1;
> +
> + return 0;
> +}
> +
> static int zram_swap_hint(struct block_device *bdev,
> unsigned int hint, void *arg)
> {
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