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    SubjectRe: [PATCH 1/2] zram: free meta out of init_lock
    Hello,

    On (01/26/15 10:33), Minchan Kim wrote:
    > Hello,
    >
    > On Sat, Jan 24, 2015 at 12:47:07AM +0900, Sergey Senozhatsky wrote:
    > > On (01/23/15 15:48), Jerome Marchand wrote:
    > > > 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;
    > > }
    > >
    >
    > The init_lock is really troublesome. We can't do call zram_meta_alloc
    > under init_lock due to lockdep report. Please keep in mind.
    >

    ah... I do recall it, thanks for your reminder.


    > The zram_rw_page is one of the function under reclaim path and hold it
    > as read_lock while here holds it as write_lock.
    > It's a false positive so that we might could make shut lockdep up
    > by annotation but I don't want it but want to work with lockdep rather
    > than disable. As well, there are other pathes to use init_lock to
    > protect other data where would be victims of lockdep.
    >
    > I didn't tell the motivation of this patch because it made you busy
    > guys wasted. Let me tell it now. It was another lockdep report by
    > kmem_cache_destroy for zsmalloc compaction about init_lock. That's why
    > the patchset was one of the patch in compaction.
    >
    > Yes, the ideal is to remove horrible init_lock of zram in this phase and
    > make code more simple and clear but I don't want to stuck zsmalloc
    > compaction by the work.


    > Having said that, I feel it's time to revisit
    > to remove init_lock.
    > At least, I will think over to find a solution to kill init_lock.

    hm, can't think of anything quick...

    -ss


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