Hello, Minchan:
2015-01-28 16:15 GMT+08:00 Minchan Kim <minchan@xxxxxxxxxx>:
> Admin could reset zram during I/O operation going on so we have
> used zram->init_lock as read-side lock in I/O path to prevent
> sudden zram meta freeing.
When I/O operation is running, that means the /dev/zram0 is
mounted or swaped on. Then the device could not be reset by
below code:
/* Do not reset an active device! */
if (bdev->bd_holders) {
ret = -EBUSY;
goto out;
}
So the zram->init_lock in I/O path is to check whether the device
has been initialized(echo xxx > /sys/block/zram/disk_size).
Is that right?
>
> However, the init_lock is really troublesome.
> We can't do call zram_meta_alloc under init_lock due to lockdep splat
I do not know much about the lockdep.
enabled the CONFIG_LOCKDEP, But from the test, there is no warning
printed. Could you please tell me how this happened?
Thanks.
> because zram_rw_page is one of the function under reclaim path and
> hold it as read_lock while other places in process context hold it
> as write_lock. So, we have used allocation out of the lock to avoid
> lockdep warn but it's not good for readability and fainally, I met
> another lockdep splat between init_lock and cpu_hotpulug from
> kmem_cache_destroy during wokring zsmalloc compaction. :(
>
> Yes, the ideal is to remove horrible init_lock of zram in rw path.
> This patch removes it in rw path and instead, put init_done bool
> variable to check initialization done with smp_[wmb|rmb] and
> srcu_[un]read_lock to prevent sudden zram meta freeing
> during I/O operation.
>
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> ---
> drivers/block/zram/zram_drv.c | 85 +++++++++++++++++++++++++++++++------------
> drivers/block/zram/zram_drv.h | 5 +++
> 2 files changed, 66 insertions(+), 24 deletions(-)
>
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index a598ada817f0..b33add453027 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -32,6 +32,7 @@
> #include <linux/string.h>
> #include <linux/vmalloc.h>
> #include <linux/err.h>
> +#include <linux/srcu.h>
>
> #include "zram_drv.h"
>
> @@ -53,9 +54,31 @@ static ssize_t name##_show(struct device *d, \
> } \
> static DEVICE_ATTR_RO(name);
>
> -static inline int init_done(struct zram *zram)
> +static inline bool init_done(struct zram *zram)
> {
> - return zram->meta != NULL;
> + /*
> + * init_done can be used without holding zram->init_lock in
> + * read/write handler(ie, zram_make_request) but we should make sure
> + * that zram->init_done should set up after meta initialization is
> + * done. Look at setup_init_done.
> + */
> + bool ret = zram->init_done;
> +
> + if (ret)
> + smp_rmb(); /* pair with setup_init_done */
> +
> + return ret;
> +}
> +
> +static inline void setup_init_done(struct zram *zram, bool flag)
> +{
> + /*
> + * Store operation of struct zram fields should complete
> + * before zram->init_done set up because zram_bvec_rw
> + * doesn't hold an zram->init_lock.
> + */
> + smp_wmb();
> + zram->init_done = flag;
> }
>
> static inline struct zram *dev_to_zram(struct device *dev)
> @@ -326,6 +349,10 @@ static void zram_meta_free(struct zram_meta *meta)
> kfree(meta);
> }
>
> +static void rcu_zram_do_nothing(struct rcu_head *unused)
> +{
> +}
> +
> static struct zram_meta *zram_meta_alloc(int device_id, u64 disksize)
> {
> char pool_name[8];
> @@ -726,11 +753,8 @@ static void zram_reset_device(struct zram *zram, bool reset_capacity)
> return;
> }
>
> - zcomp_destroy(zram->comp);
> zram->max_comp_streams = 1;
>
> - zram_meta_free(zram->meta);
> - zram->meta = NULL;
> /* Reset stats */
> memset(&zram->stats, 0, sizeof(zram->stats));
>
> @@ -738,8 +762,12 @@ static void zram_reset_device(struct zram *zram, bool reset_capacity)
> if (reset_capacity)
> set_capacity(zram->disk, 0);
Should this be after synchronize_srcu()?
>
> + setup_init_done(zram, false);
> + call_srcu(&zram->srcu, &zram->rcu, rcu_zram_do_nothing);
> + synchronize_srcu(&zram->srcu);
> + zram_meta_free(zram->meta);
> + zcomp_destroy(zram->comp);
> up_write(&zram->init_lock);
> -
> /*
> * Revalidate disk out of the init_lock to avoid lockdep splat.
> * It's okay because disk's capacity is protected by init_lock
> @@ -762,10 +790,19 @@ static ssize_t disksize_store(struct device *dev,
> if (!disksize)
> return -EINVAL;
>
> + down_write(&zram->init_lock);
> + if (init_done(zram)) {
> + pr_info("Cannot change disksize for initialized device\n");
> + up_write(&zram->init_lock);
> + return -EBUSY;
> + }
> +
> disksize = PAGE_ALIGN(disksize);
> meta = zram_meta_alloc(zram->disk->first_minor, disksize);
> - if (!meta)
> + if (!meta) {
> + up_write(&zram->init_lock);
> return -ENOMEM;
> + }
>
> comp = zcomp_create(zram->compressor, zram->max_comp_streams);
> if (IS_ERR(comp)) {
> @@ -775,17 +812,11 @@ 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;
> set_capacity(zram->disk, zram->disksize >> SECTOR_SHIFT);
> + setup_init_done(zram, true);
> up_write(&zram->init_lock);
>
> /*
> @@ -797,10 +828,8 @@ static ssize_t disksize_store(struct device *dev,
>
> return len;
>
> -out_destroy_comp:
> - up_write(&zram->init_lock);
> - zcomp_destroy(comp);
> out_free_meta:
> + up_write(&zram->init_lock);
> zram_meta_free(meta);
> return err;
> }
> @@ -905,9 +934,10 @@ out:
> */
> static void zram_make_request(struct request_queue *queue, struct bio *bio)
> {
> + int idx;
> struct zram *zram = queue->queuedata;
>
> - down_read(&zram->init_lock);
> + idx = srcu_read_lock(&zram->srcu);
> if (unlikely(!init_done(zram)))
> goto error;
>
> @@ -918,12 +948,12 @@ static void zram_make_request(struct request_queue *queue, struct bio *bio)
> }
>
> __zram_make_request(zram, bio);
> - up_read(&zram->init_lock);
> + srcu_read_unlock(&zram->srcu, idx);
>
> return;
>
> error:
> - up_read(&zram->init_lock);
> + srcu_read_unlock(&zram->srcu, idx);
> bio_io_error(bio);
> }
>
> @@ -945,18 +975,20 @@ static void zram_slot_free_notify(struct block_device *bdev,
> static int zram_rw_page(struct block_device *bdev, sector_t sector,
> struct page *page, int rw)
> {
> - int offset, err;
> + int offset, err, idx;
> u32 index;
> struct zram *zram;
> struct bio_vec bv;
>
> zram = bdev->bd_disk->private_data;
> + idx = srcu_read_lock(&zram->srcu);
> +
> if (!valid_io_request(zram, sector, PAGE_SIZE)) {
> atomic64_inc(&zram->stats.invalid_io);
> + srcu_read_unlock(&zram->srcu, idx);
> return -EINVAL;
> }
>
> - down_read(&zram->init_lock);
> if (unlikely(!init_done(zram))) {
> err = -EIO;
> goto out_unlock;
> @@ -971,7 +1003,7 @@ static int zram_rw_page(struct block_device *bdev, sector_t sector,
>
> err = zram_bvec_rw(zram, &bv, index, offset, rw);
> out_unlock:
> - up_read(&zram->init_lock);
> + srcu_read_unlock(&zram->srcu, idx);
> /*
> * If I/O fails, just return error(ie, non-zero) without
> * calling page_endio.
> @@ -1041,6 +1073,11 @@ static int create_device(struct zram *zram, int device_id)
>
> init_rwsem(&zram->init_lock);
>
> + if (init_srcu_struct(&zram->srcu)) {
> + pr_err("Error initialize srcu for device %d\n", device_id);
> + goto out;
> + }
> +
> zram->queue = blk_alloc_queue(GFP_KERNEL);
> if (!zram->queue) {
> pr_err("Error allocating disk queue for device %d\n",
> @@ -1125,8 +1162,8 @@ static void destroy_device(struct zram *zram)
>
> del_gendisk(zram->disk);
> put_disk(zram->disk);
> -
> blk_cleanup_queue(zram->queue);
> + cleanup_srcu_struct(&zram->srcu);
> }
>
> static int __init zram_init(void)
> diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> index e492f6bf11f1..2042c310aea8 100644
> --- a/drivers/block/zram/zram_drv.h
> +++ b/drivers/block/zram/zram_drv.h
> @@ -105,8 +105,13 @@ struct zram {
> struct gendisk *disk;
> struct zcomp *comp;
>
> + struct srcu_struct srcu;
> + struct rcu_head rcu;
> +
> /* Prevent concurrent execution of device init, reset and R/W request */
> struct rw_semaphore init_lock;
> + bool init_done;
> +
> /*
> * This is the limit on amount of *uncompressed* worth of data
> * we can store in a disk.
> --
> 1.9.1
>
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