linux-kernel - Re: [PATCH] mm: strictlimit feature -v2

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Date:	Wed, 3 Jul 2013 15:01:19 +0400
From:	Maxim Patlasov <mpatlasov@...allels.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	<miklos@...redi.hu>, <riel@...hat.com>, <dev@...allels.com>,
	<xemul@...allels.com>, <fuse-devel@...ts.sourceforge.net>,
	<bfoster@...hat.com>, <linux-kernel@...r.kernel.org>,
	<jbottomley@...allels.com>, <linux-mm@...ck.org>,
	<viro@...iv.linux.org.uk>, <linux-fsdevel@...r.kernel.org>,
	<fengguang.wu@...el.com>, <devel@...nvz.org>, <mgorman@...e.de>
Subject: Re: [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@...allels.com> wrote:
>
>> From: Miklos Szeredi <mszeredi@...e.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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