Re: [PATCH 2/2] zram: allocate metadata when disksize is set up

[Minchan Kim]

On Tue, Nov 27, 2012 at 04:45:54PM +0100, Jerome Marchand wrote:
> On 11/27/2012 06:13 AM, Nitin Gupta wrote:
> > On 11/22/2012 06:42 PM, Minchan Kim wrote:
> >> Lockdep complains about recursive deadlock of zram->init_lock.
> >> Because zram_init_device could be called in reclaim context and
> >> it requires a page with GFP_KERNEL.
> >>
> >> We can fix it via replacing GFP_KERNEL with GFP_NOIO.
> >> But more big problem is vzalloc in zram_init_device which calls GFP_KERNEL.
> >> We can change it with __vmalloc which can receive gfp_t.
> >> But still we have a problem. Although __vmalloc can handle gfp_t, it calls
> >> allocation of GFP_KERNEL. That's why I sent the patch.
> >> https://lkml.org/lkml/2012/4/23/77
> >>
> >> Yes. Fundamental problem is utter crap API vmalloc.
> >> If we can fix it, everyone would be happy. But life isn't simple
> >> like seeing my thread of the patch.
> >>
> >> So next option is to give up lazy initialization and initialize it at the
> >> very disksize setting time. But it makes unnecessary metadata waste until
> >> zram is really used. But let's think about it.
> >>
> >> 1) User of zram normally do mkfs.xxx or mkswap before using
> >>     the zram block device(ex, normally, do it at booting time)
> >>     It ends up allocating such metadata of zram before real usage so
> >>     benefit of lazy initialzation would be mitigated.
> >>
> >> 2) Some user want to use zram when memory pressure is high.(ie, load zram
> >>     dynamically, NOT booting time). It does make sense because people don't
> >>     want to waste memory until memory pressure is high(ie, where zram is really
> >>     helpful time). In this case, lazy initialzation could be failed easily
> >>     because we will use GFP_NOIO instead of GFP_KERNEL for avoiding deadlock.
> >>     So the benefit of lazy initialzation would be mitigated, too.
> >>
> >> 3) Metadata overhead is not critical and Nitin has a plan to diet it.
> >>     4K : 12 byte(64bit machine) -> 64G : 192M so 0.3% isn't big overhead
> >>     If insane user use such big zram device up to 20, it could consume 6% of ram
> >>     but efficieny of zram will cover the waste.
> >>
> >> So this patch gives up lazy initialization and instead we initialize metadata
> >> at disksize setting time.
> >>
> >> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> >> ---
> >>   drivers/staging/zram/zram_drv.c   |   21 ++++-----------------
> >>   drivers/staging/zram/zram_sysfs.c |    1 +
> >>   2 files changed, 5 insertions(+), 17 deletions(-)
> >>
> >> diff --git a/drivers/staging/zram/zram_drv.c b/drivers/staging/zram/zram_drv.c
> >> index 9ef1eca..f364fb5 100644
> >> --- a/drivers/staging/zram/zram_drv.c
> >> +++ b/drivers/staging/zram/zram_drv.c
> >> @@ -441,16 +441,13 @@ static void zram_make_request(struct request_queue *queue, struct bio *bio)
> >>   {
> >>   	struct zram *zram = queue->queuedata;
> >>
> >> -	if (unlikely(!zram->init_done) && zram_init_device(zram))
> >> -		goto error;
> >> -
> >>   	down_read(&zram->init_lock);
> >>   	if (unlikely(!zram->init_done))
> >> -		goto error_unlock;
> >> +		goto error;
> >>
> >>   	if (!valid_io_request(zram, bio)) {
> >>   		zram_stat64_inc(zram, &zram->stats.invalid_io);
> >> -		goto error_unlock;
> >> +		goto error;
> >>   	}
> >>
> >>   	__zram_make_request(zram, bio, bio_data_dir(bio));
> >> @@ -458,9 +455,8 @@ static void zram_make_request(struct request_queue *queue, struct bio *bio)
> >>
> >>   	return;
> >>
> >> -error_unlock:
> >> -	up_read(&zram->init_lock);
> >>   error:
> >> +	up_read(&zram->init_lock);
> >>   	bio_io_error(bio);
> >>   }
> >>
> >> @@ -509,19 +505,12 @@ void zram_reset_device(struct zram *zram)
> >>   	up_write(&zram->init_lock);
> >>   }
> >>
> >> +/* zram->init_lock should be hold */
> > 
> > s/hold/held
> > 
> > btw, shouldn't we also change GFP_KERNEL to GFP_NOIO in is_partial_io() 
> > case in both read/write handlers?
> 
> Good point. Actually, the one in zram_bvec_read() should actually be
> GFP_ATOMIC because of the kmap_atomic() above (or be moved out of

Right.

> kmap_atomic/kunmap_atomic nest).
> Another solution would be to allocate some working buffer at device
> init as it's done for compress_buffer/workmem. It would make
> zram_bvec_read/write look simpler (no need to free memory or manage 
> kmalloc failure).

Fair enough.
I sent a patch which replace GFP_KERNEL with GFP_ATOMIC but your suggestion
would be better. It could be a separate patch. I will send it.

Thanks.

> 
> Jerome
> 
> > 
> > Rest of the patch looks good.
> > 
> > 
> > Thanks,
> > Nitin
> > 
> >>   int zram_init_device(struct zram *zram)
> >>   {
> >>   	int ret;
> >>   	size_t num_pages;
> >>
> >> -	down_write(&zram->init_lock);
> >> -	if (zram->init_done) {
> >> -		up_write(&zram->init_lock);
> >> -		return 0;
> >> -	}
> >> -
> >> -	BUG_ON(!zram->disksize);
> >> -
> >>   	if (zram->disksize > 2 * (totalram_pages << PAGE_SHIFT)) {
> >>   		pr_info(
> >>   		"There is little point creating a zram of greater than "
> >> @@ -570,7 +559,6 @@ int zram_init_device(struct zram *zram)
> >>   	}
> >>
> >>   	zram->init_done = 1;
> >> -	up_write(&zram->init_lock);
> >>
> >>   	pr_debug("Initialization done!\n");
> >>   	return 0;
> >> @@ -580,7 +568,6 @@ fail_no_table:
> >>   	zram->disksize = 0;
> >>   fail:
> >>   	__zram_reset_device(zram);
> >> -	up_write(&zram->init_lock);
> >>   	pr_err("Initialization failed: err=%d\n", ret);
> >>   	return ret;
> >>   }
> >> diff --git a/drivers/staging/zram/zram_sysfs.c b/drivers/staging/zram/zram_sysfs.c
> >> index 4143af9..369db12 100644
> >> --- a/drivers/staging/zram/zram_sysfs.c
> >> +++ b/drivers/staging/zram/zram_sysfs.c
> >> @@ -71,6 +71,7 @@ static ssize_t disksize_store(struct device *dev,
> >>
> >>   	zram->disksize = PAGE_ALIGN(disksize);
> >>   	set_capacity(zram->disk, zram->disksize >> SECTOR_SHIFT);
> >> +	zram_init_device(zram);
> >>   	up_write(&zram->init_lock);
> >>
> >>   	return len;
> >>
> > 
> 
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-- 
Kind regards,
Minchan Kim
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