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SubjectRe: [PATCH] mm: strictlimit feature -v2
07/02/2013 11:38 PM, Andrew Morton пишет:
> On Tue, 02 Jul 2013 21:44:47 +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.
>>
>> 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:

Great! Thank you, Andrew. I'll wait for Fengguang' feedback for a while
before respin.

>
>> ...
>>
>> @@ -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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