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Date: Mon, 26 Nov 2012 21:13:24 -0800
From: Nitin Gupta <ngupta@...are.org>
To: Minchan Kim <minchan@...nel.org>
CC: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Seth Jennings <sjenning@...ux.vnet.ibm.com>,
Jerome Marchand <jmarchan@...hat.com>,
Dan Magenheimer <dan.magenheimer@...cle.com>,
Konrad Rzeszutek Wilk <konrad@...nok.org>,
Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [PATCH 2/2] zram: allocate metadata when disksize is set up
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@...nel.org>
> ---
> 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?
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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