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    SubjectRe: [PATCH 1/2] zram: free meta out of init_lock
    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;
    }


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