[PATCH v2] bcache: avoid oversize memory allocation by small stripe_size

[PATCH v2] bcache: avoid oversize memory allocation by small stripe_size

[Coly Li]

Arraies bcache->stripe_sectors_dirty and bcache->full_dirty_stripes are
used for dirty data writeback, their sizes are decided by backing device
capacity and stripe size. Larger backing device capacity or smaller
stripe size make these two arraies occupies more dynamic memory space.

Currently bcache->stripe_size is directly inherited from
queue->limits.io_opt of underlying storage device. For normal hard
drives, its limits.io_opt is 0, and bcache sets the corresponding
stripe_size to 1TB (1<<31 sectors), it works fine 10+ years. But for
devices do declare value for queue->limits.io_opt, small stripe_size
(comparing to 1TB) becomes an issue for oversize memory allocations of
bcache->stripe_sectors_dirty and bcache->full_dirty_stripes, while the
capacity of hard drives gets much larger in recent decade.

For example a raid5 array assembled by three 20TB hardrives, the raid
device capacity is 40TB with typical 512KB limits.io_opt. After the math
calculation in bcache code, these two arraies will occupy 400MB dynamic
memory. Even worse Andrea Tomassetti reports that a 4KB limits.io_opt is
declared on a new 2TB hard drive, then these two arraies request 2GB and
512MB dynamic memory from kzalloc(). The result is that bcache device
always fails to initialize on his system.

To avoid the oversize memory allocation, bcache->stripe_size should not
directly inherited by queue->limits.io_opt from the underlying device.
This patch defines BCH_MIN_STRIPE_SZ (4MB) as minimal bcache stripe size
and set bcache device's stripe size against the declared limits.io_opt
value from the underlying storage device,
- If the declared limits.io_opt > BCH_MIN_STRIPE_SZ, bcache device will
  set its stripe size directly by this limits.io_opt value.
- If the declared limits.io_opt < BCH_MIN_STRIPE_SZ, bcache device will
  set its stripe size by a value multiplying limits.io_opt and euqal or
  large than BCH_MIN_STRIPE_SZ.

Then the minimal stripe size of a bcache device will always be >= 4MB.
For a 40TB raid5 device with 512KB limits.io_opt, memory occupied by
bcache->stripe_sectors_dirty and bcache->full_dirty_stripes will be 50MB
in total. For a 2TB hard drive with 4KB limits.io_opt, memory occupied
by these two arraies will be 2.5MB in total.

Such mount of memory allocated for bcache->stripe_sectors_dirty and
bcache->full_dirty_stripes is reasonable for most of storage devices.

Reported-by: Andrea Tomassetti <andrea.tomassetti-opensource@devo.com>
Signed-off-by: Coly Li <colyli@suse.de>
Cc: Eric Wheeler <bcache@lists.ewheeler.net>
---
Changelog:
v2: use roundup() to replace round_up() as Eric Wheeler suggested.
v1: the initial version.

 drivers/md/bcache/bcache.h | 1 +
 drivers/md/bcache/super.c  | 2 ++
 2 files changed, 3 insertions(+)

diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index 5a79bb3c272f..83eb7f27db3d 100644
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -265,6 +265,7 @@ struct bcache_device {
 #define BCACHE_DEV_WB_RUNNING		3
 #define BCACHE_DEV_RATE_DW_RUNNING	4
 	int			nr_stripes;
+#define BCH_MIN_STRIPE_SZ		((4 << 20) >> SECTOR_SHIFT)
 	unsigned int		stripe_size;
 	atomic_t		*stripe_sectors_dirty;
 	unsigned long		*full_dirty_stripes;
diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 0ae2b3676293..0eb71543d773 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -905,6 +905,8 @@ static int bcache_device_init(struct bcache_device *d, unsigned int block_size,
 
 	if (!d->stripe_size)
 		d->stripe_size = 1 << 31;
+	else if (d->stripe_size < BCH_MIN_STRIPE_SZ)
+		d->stripe_size = roundup(BCH_MIN_STRIPE_SZ, d->stripe_size);
 
 	n = DIV_ROUND_UP_ULL(sectors, d->stripe_size);
 	if (!n || n > max_stripes) {
-- 
2.35.3

Re: [PATCH v2] bcache: avoid oversize memory allocation by small stripe_size

[Eric Wheeler]

On Thu, 28 Sep 2023, Coly Li wrote:

> Arraies bcache->stripe_sectors_dirty and bcache->full_dirty_stripes are
> used for dirty data writeback, their sizes are decided by backing device
> capacity and stripe size. Larger backing device capacity or smaller
> stripe size make these two arraies occupies more dynamic memory space.
> 
> Currently bcache->stripe_size is directly inherited from
> queue->limits.io_opt of underlying storage device. For normal hard
> drives, its limits.io_opt is 0, and bcache sets the corresponding
> stripe_size to 1TB (1<<31 sectors), it works fine 10+ years. But for
> devices do declare value for queue->limits.io_opt, small stripe_size
> (comparing to 1TB) becomes an issue for oversize memory allocations of
> bcache->stripe_sectors_dirty and bcache->full_dirty_stripes, while the
> capacity of hard drives gets much larger in recent decade.
> 
> For example a raid5 array assembled by three 20TB hardrives, the raid
> device capacity is 40TB with typical 512KB limits.io_opt. After the math
> calculation in bcache code, these two arraies will occupy 400MB dynamic
> memory. Even worse Andrea Tomassetti reports that a 4KB limits.io_opt is
> declared on a new 2TB hard drive, then these two arraies request 2GB and
> 512MB dynamic memory from kzalloc(). The result is that bcache device
> always fails to initialize on his system.
> 
> To avoid the oversize memory allocation, bcache->stripe_size should not
> directly inherited by queue->limits.io_opt from the underlying device.
> This patch defines BCH_MIN_STRIPE_SZ (4MB) as minimal bcache stripe size
> and set bcache device's stripe size against the declared limits.io_opt
> value from the underlying storage device,
> - If the declared limits.io_opt > BCH_MIN_STRIPE_SZ, bcache device will
>   set its stripe size directly by this limits.io_opt value.
> - If the declared limits.io_opt < BCH_MIN_STRIPE_SZ, bcache device will
>   set its stripe size by a value multiplying limits.io_opt and euqal or
>   large than BCH_MIN_STRIPE_SZ.
> 
> Then the minimal stripe size of a bcache device will always be >= 4MB.
> For a 40TB raid5 device with 512KB limits.io_opt, memory occupied by
> bcache->stripe_sectors_dirty and bcache->full_dirty_stripes will be 50MB
> in total. For a 2TB hard drive with 4KB limits.io_opt, memory occupied
> by these two arraies will be 2.5MB in total.
> 
> Such mount of memory allocated for bcache->stripe_sectors_dirty and
> bcache->full_dirty_stripes is reasonable for most of storage devices.
> 
> Reported-by: Andrea Tomassetti <andrea.tomassetti-opensource@devo.com>
> Signed-off-by: Coly Li <colyli@suse.de>
> Cc: Eric Wheeler <bcache@lists.ewheeler.net>
> ---
> Changelog:
> v2: use roundup() to replace round_up() as Eric Wheeler suggested.
> v1: the initial version.
> 
>  drivers/md/bcache/bcache.h | 1 +
>  drivers/md/bcache/super.c  | 2 ++
>  2 files changed, 3 insertions(+)
> 
> diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
> index 5a79bb3c272f..83eb7f27db3d 100644
> --- a/drivers/md/bcache/bcache.h
> +++ b/drivers/md/bcache/bcache.h
> @@ -265,6 +265,7 @@ struct bcache_device {
>  #define BCACHE_DEV_WB_RUNNING		3
>  #define BCACHE_DEV_RATE_DW_RUNNING	4
>  	int			nr_stripes;
> +#define BCH_MIN_STRIPE_SZ		((4 << 20) >> SECTOR_SHIFT)
>  	unsigned int		stripe_size;
>  	atomic_t		*stripe_sectors_dirty;
>  	unsigned long		*full_dirty_stripes;
> diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
> index 0ae2b3676293..0eb71543d773 100644
> --- a/drivers/md/bcache/super.c
> +++ b/drivers/md/bcache/super.c
> @@ -905,6 +905,8 @@ static int bcache_device_init(struct bcache_device *d, unsigned int block_size,
>  
>  	if (!d->stripe_size)
>  		d->stripe_size = 1 << 31;
> +	else if (d->stripe_size < BCH_MIN_STRIPE_SZ)
> +		d->stripe_size = roundup(BCH_MIN_STRIPE_SZ, d->stripe_size);
>  
>  	n = DIV_ROUND_UP_ULL(sectors, d->stripe_size);
>  	if (!n || n > max_stripes) {
> -- 
> 2.35.3
> 

Thanks Coly.  

Reviewed-by: Eric Wheeler <bcache@linux.ewheeler.net>


--
Eric Wheeler

>