Re: [PATCH 1/2] zram: free meta out of init_lock

Sergey Senozhatsky
2015-01-23T15:46:38Z

On (01/23/15 15:48), Jerome Marchand wrote:
> Date: Fri, 23 Jan 2015 15:48:05 +0100
> From: Jerome Marchand <jmarchan@redhat.com>
> To: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>, Minchan Kim
> <minchan@kernel.org>
> CC: Andrew Morton <akpm@linux-foundation.org>,
> linux-kernel@vger.kernel.org, linux-mm@kvack.org, Nitin Gupta
> <ngupta@vflare.org>
> Subject: Re: [PATCH 1/2] zram: free meta out of init_lock
> User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101
> Thunderbird/31.3.0
>
> On 01/23/2015 03:24 PM, Sergey Senozhatsky wrote:
> > On (01/23/15 14:58), Minchan Kim wrote:
> >> We don't need to call zram_meta_free, zcomp_destroy and zs_free
> >> under init_lock. What we need to prevent race with init_lock
> >> in reset is setting NULL into zram->meta (ie, init_done).
> >> This patch does it.
> >>
> >> Signed-off-by: Minchan Kim <minchan@kernel.org>
> >> ---
> >> drivers/block/zram/zram_drv.c | 28 ++++++++++++++++------------
> >> 1 file changed, 16 insertions(+), 12 deletions(-)
> >>
> >> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> >> index 9250b3f54a8f..0299d82275e7 100644
> >> --- a/drivers/block/zram/zram_drv.c
> >> +++ b/drivers/block/zram/zram_drv.c
> >> @@ -708,6 +708,7 @@ static void zram_reset_device(struct zram *zram, bool reset_capacity)
> >> {
> >> size_t index;
> >> struct zram_meta *meta;
> >> + struct zcomp *comp;
> >>
> >> down_write(&zram->init_lock);
> >>
> >> @@ -719,20 +720,10 @@ static void zram_reset_device(struct zram *zram, bool reset_capacity)
> >> }
> >>
> >> meta = zram->meta;
> >> - /* Free all pages that are still in this zram device */
> >> - for (index = 0; index < zram->disksize >> PAGE_SHIFT; index++) {
> >> - unsigned long handle = meta->table[index].handle;
> >> - if (!handle)
> >> - continue;
> >> -
> >> - zs_free(meta->mem_pool, handle);
> >> - }
> >> -
> >> - zcomp_destroy(zram->comp);
> >
> > I'm not so sure about moving zcomp destruction. if we would have detached it
> > from zram, then yes. otherwise, think of zram ->destoy vs ->init race.
> >
> > suppose,
> > CPU1 waits for down_write() init lock in disksize_store() with new comp already allocated;
> > CPU0 detaches ->meta and releases write init lock;
> > CPU1 grabs the lock and does zram->comp = comp;
> > CPU0 reaches the point of zcomp_destroy(zram->comp);
>
> I don't see your point: this patch does not call
> zcomp_destroy(zram->comp) anymore, but zram_destroy(comp), where comp is
> the old zram->comp.

oh... yes. sorry! my bad.

anyway, on a second thought, do we even want to destoy meta out of init_lock?

I mean, it will let you init new device quicker. but... assume, you have
30G zram (or any other bad-enough number). on CPU0 you reset device -- iterate
over 30G meta->table, etc. out of init_lock.
on CPU1 you concurrently re-init device and request again 30G.

how bad that can be?

diskstore called on already initialised device is also not so perfect.
we first will try to allocate ->meta (vmalloc pages for another 30G),
then allocate comp, then down_write() init lock to find out that device
is initialised and we need to release allocated memory.

may be we better keep ->meta destruction under init_lock and additionally
move ->meta and ->comp allocation under init_lock in disksize_store()?

like the following one:

---

drivers/block/zram/zram_drv.c | 25 +++++++++++++------------
1 file changed, 13 insertions(+), 12 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 9250b3f..827ab21 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -765,9 +765,18 @@ static ssize_t disksize_store(struct device *dev,
return -EINVAL;

disksize = PAGE_ALIGN(disksize);
+ down_write(&zram->init_lock);
+ if (init_done(zram)) {
+ up_write(&zram->init_lock);
+ pr_info("Cannot change disksize for initialized device\n");
+ return -EBUSY;
+ }
+
meta = zram_meta_alloc(zram->disk->first_minor, disksize);
- if (!meta)
- return -ENOMEM;
+ if (!meta) {
+ err = -ENOMEM;
+ goto out_unlock;
+ }

comp = zcomp_create(zram->compressor, zram->max_comp_streams);
if (IS_ERR(comp)) {
@@ -777,13 +786,6 @@ static ssize_t disksize_store(struct device *dev,
goto out_free_meta;
}

- down_write(&zram->init_lock);
- if (init_done(zram)) {
- pr_info("Cannot change disksize for initialized device\n");
- err = -EBUSY;
- goto out_destroy_comp;
- }
-
zram->meta = meta;
zram->comp = comp;
zram->disksize = disksize;
@@ -799,11 +801,10 @@ static ssize_t disksize_store(struct device *dev,

return len;

-out_destroy_comp:
- up_write(&zram->init_lock);
- zcomp_destroy(comp);
out_free_meta:
zram_meta_free(meta);
+out_unlock:
+ up_write(&zram->init_lock);
return err;
}