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Date: Tue, 27 Nov 2012 16:45:54 +0100
From: Jerome Marchand <jmarchan@...hat.com>
To: Nitin Gupta <ngupta@...are.org>
CC: Minchan Kim <minchan@...nel.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Seth Jennings <sjenning@...ux.vnet.ibm.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 11/27/2012 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@...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?
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
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).
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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