Re: [PATCH 16/16] mm: strictlimit feature

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 29 Jun 2013 21:48:54 +0400 Maxim Patlasov <MPatlasov@parallels.com> wrote:

> From: Miklos Szeredi <mszeredi@suse.cz>
> 
> The feature prevents mistrusted filesystems to grow a large number of dirty
> pages before throttling. For such filesystems balance_dirty_pages always
> check bdi counters against bdi limits. I.e. even if global "nr_dirty" is under
> "freerun", it's not allowed to skip bdi checks. The only use case for now is
> fuse: it sets bdi max_ratio to 1% by default and system administrators are
> supposed to expect that this limit won't be exceeded.
> 
> The feature is on if address space is marked by AS_STRICTLIMIT flag.
> A filesystem may set the flag when it initializes a new inode.
> 

Fengguang, could you please review this patch?

I suggest you await the next version, which hopefully will be more
reviewable...

>
> ...
>
> --- a/include/linux/backing-dev.h
> +++ b/include/linux/backing-dev.h
> @@ -33,6 +33,8 @@ enum bdi_state {
>  	BDI_sync_congested,	/* The sync queue is getting full */
>  	BDI_registered,		/* bdi_register() was done */
>  	BDI_writeback_running,	/* Writeback is in progress */
> +	BDI_idle,		/* No pages under writeback at the moment of
> +				 * last update of write bw */

Why does BDI_idle exist?

>  	BDI_unused,		/* Available bits start here */
>  };
>  
> @@ -43,6 +45,7 @@ enum bdi_stat_item {
>  	BDI_WRITEBACK,
>  	BDI_DIRTIED,
>  	BDI_WRITTEN,
> +	BDI_WRITTEN_BACK,
>  	NR_BDI_STAT_ITEMS
>  };
>  
> @@ -76,6 +79,8 @@ struct backing_dev_info {
>  	unsigned long bw_time_stamp;	/* last time write bw is updated */
>  	unsigned long dirtied_stamp;
>  	unsigned long written_stamp;	/* pages written at bw_time_stamp */
> +	unsigned long writeback_stamp;	/* pages sent to writeback at
> +					 * bw_time_stamp */

Well this sucks.  Some of the "foo_stamp" fields are in units of time
(jiffies?  We aren't told) and some of the "foo_stamp" fields are in
units of number-of-pages.  It would be good to fix the naming here.

>  	unsigned long write_bandwidth;	/* the estimated write bandwidth */
>  	unsigned long avg_write_bandwidth; /* further smoothed write bw */
>  
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index e3dea75..baac702 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -25,6 +25,7 @@ enum mapping_flags {
>  	AS_MM_ALL_LOCKS	= __GFP_BITS_SHIFT + 2,	/* under mm_take_all_locks() */
>  	AS_UNEVICTABLE	= __GFP_BITS_SHIFT + 3,	/* e.g., ramdisk, SHM_LOCK */
>  	AS_BALLOON_MAP  = __GFP_BITS_SHIFT + 4, /* balloon page special map */
> +	AS_STRICTLIMIT	= __GFP_BITS_SHIFT + 5, /* strict dirty limit */

Thing is, "strict dirty limit" isn't documented anywhere, so this
reference is left dangling.

>
> ...
>
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -94,6 +94,7 @@ static int bdi_debug_stats_show(struct seq_file *m, void *v)
>  		   "BackgroundThresh:   %10lu kB\n"
>  		   "BdiDirtied:         %10lu kB\n"
>  		   "BdiWritten:         %10lu kB\n"
> +		   "BdiWrittenBack:     %10lu kB\n"
>  		   "BdiWriteBandwidth:  %10lu kBps\n"
>  		   "b_dirty:            %10lu\n"
>  		   "b_io:               %10lu\n"

I can't imagine what the difference is between BdiWritten and
BdiWrittenBack.

I suggest you document this at the BDI_WRITTEN_BACK definition site in
enum bdi_stat_item.  BDI_WRITTEN (at least) will also need
documentation so people can understand the difference.

>
> ...
>
> @@ -679,29 +711,31 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
>  	if (unlikely(dirty >= limit))
>  		return 0;
>  
> +	if (unlikely(strictlimit)) {
> +		if (bdi_dirty < 8)
> +			return 2 << RATELIMIT_CALC_SHIFT;
> +
> +		if (bdi_dirty >= bdi_thresh)
> +			return 0;
> +
> +		bdi_setpoint = bdi_thresh + bdi_dirty_limit(bdi, bg_thresh);
> +		bdi_setpoint /= 2;
> +
> +		if (bdi_setpoint == 0 || bdi_setpoint == bdi_thresh)
> +			return 0;
> +
> +		pos_ratio = pos_ratio_polynom(bdi_setpoint, bdi_dirty,
> +					      bdi_thresh);
> +		return min_t(long long, pos_ratio, 2 << RATELIMIT_CALC_SHIFT);
> +	}

This would be a suitable site at which to document the strictlimit
feature.  What it is, how it works and most importantly, why it exists.

>
> ...
>
> @@ -994,6 +1029,16 @@ static void bdi_update_dirty_ratelimit(struct backing_dev_info *bdi,
>  	 * keep that period small to reduce time lags).
>  	 */
>  	step = 0;
> +
> +	if (unlikely(strictlimit)) {
> +		dirty = bdi_dirty;
> +		if (bdi_dirty < 8)
> +			setpoint = bdi_dirty + 1;
> +		else
> +			setpoint = (bdi_thresh +
> +				    bdi_dirty_limit(bdi, bg_thresh)) / 2;
> +	}

Explain this to the reader, please.

>
> ...
>

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