On Tue, 02 Jul 2013 21:44:47 +0400 Maxim Patlasov <mpatla...@parallels.com>
wrote:
> From: Miklos Szeredi <mszer...@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.
>
> Changed in v2 (thanks to Andrew Morton):
> - added a few explanatory comments
> - cleaned up the mess in backing_dev_info foo_stamp fields: now it's clearly
> stated that bw_time_stamp is measured in jiffies; renamed other foo_stamp
> fields to reflect that they are in units of number-of-pages.
>
Better, thanks.
The writeback arithemtic makes my head spin - I'd really like Fengguang
to go over this, please.
A quick visit from the spelling police:
>
> ...
>
> @@ -41,8 +43,15 @@ typedef int (congested_fn)(void *, int);
> enum bdi_stat_item {
> BDI_RECLAIMABLE,
> BDI_WRITEBACK,
> - BDI_DIRTIED,
> - BDI_WRITTEN,
> +
> + /*
> + * The three counters below reflects number of events of specific type
> + * happened since bdi_init(). The type is defined in comments below:
"The three counters below reflect the number of events of specific
types since bdi_init()"
> + */
> + BDI_DIRTIED, /* a page was dirtied */
> + BDI_WRITTEN, /* writeout completed for a page */
> + BDI_WRITTEN_BACK, /* a page went to writeback */
> +
> NR_BDI_STAT_ITEMS
> };
>
>
> ...
>
> @@ -680,28 +712,55 @@ static unsigned long bdi_position_ratio(struct
> backing_dev_info *bdi,
> return 0;
>
> /*
> - * global setpoint
> + * The strictlimit feature is a tool preventing mistrusted filesystems
> + * to grow a large number of dirty pages before throttling. For such
"from growing"
> + * filesystems balance_dirty_pages always checks bdi counters against
> + * bdi limits. Even if global "nr_dirty" is under "freerun". This is
> + * especially important for fuse who sets bdi->max_ratio to 1% by
s/who/which/
> + * default. Without strictlimit feature, fuse writeback may consume
> + * arbitrary amount of RAM because it is accounted in
> + * NR_WRITEBACK_TEMP which is not involved in calculating "nr_dirty".
>
> ...
>
> @@ -994,6 +1054,26 @@ static void bdi_update_dirty_ratelimit(struct
> backing_dev_info *bdi,
> * keep that period small to reduce time lags).
> */
> step = 0;
> +
> + /*
> + * For strictlimit case, balanced_dirty_ratelimit was calculated
balance_dirty_ratelimit?
> + * above based on bdi counters and limits (see bdi_position_ratio()).
> + * Hence, to calculate "step" properly, we have to use bdi_dirty as
> + * "dirty" and bdi_setpoint as "setpoint".
> + *
> + * We rampup dirty_ratelimit forcibly if bdi_dirty is low because
> + * it's possible that bdi_thresh is close to zero due to inactivity
> + * of backing device (see the implementation of bdi_dirty_limit()).
> + */
> + if (unlikely(strictlimit)) {
> + dirty = bdi_dirty;
> + if (bdi_dirty < 8)
> + setpoint = bdi_dirty + 1;
> + else
>
> ...
>
> @@ -1057,18 +1140,32 @@ void __bdi_update_bandwidth(struct backing_dev_info
> *bdi,
> if (elapsed > HZ && time_before(bdi->bw_time_stamp, start_time))
> goto snapshot;
>
> + /*
> + * Skip periods when backing dev was idle due to abscence of pages
"absence"
> + * under writeback (when over_bground_thresh() returns false)
> + */
> + if (test_bit(BDI_idle, &bdi->state) &&
> + bdi->writeback_nr_stamp == writeback)
>
> ...
>
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