Re: [fuse-devel] [PATCH 14/14] mm: Account for WRITEBACK_TEMP in balance_dirty_pages

["Maxim V. Patlasov" <mpatlasov@parallels.com>]

Miklos, MM folks,

04/26/2013 06:02 PM, Miklos Szeredi пишет:
> On Fri, Apr 26, 2013 at 12:32:24PM +0400, Maxim V. Patlasov wrote:
>
>>> The idea is that fuse filesystems should not go over the bdi limit even if
>>> the global limit hasn't been reached.
>> This might work, but kicking flusher every time someone write to
>> fuse mount and dives into balance_dirty_pages looks fishy.
> Yeah.  Fixed patch attached.

The patch didn't work for me. I'll investigate what's wrong and get back 
to you later.

>
>> Let's combine
>> our suggestions: mark fuse inodes with AS_FUSE_WRITEBACK flag and
>> convert what you strongly dislike above to:
>>
>> if (test_bit(AS_FUSE_WRITEBACK, &mapping->flags))
>> nr_dirty += global_page_state(NR_WRITEBACK_TEMP);
> I don't think this is right.  The fuse daemon could itself be writing to another
> fuse filesystem, in which case blocking because of NR_WRITEBACK_TEMP being high
> isn't a smart strategy.

Please don't say 'blocking'. Per-bdi checks will decide whether to block 
or not. In the case you set forth, judging on per-bdi checks would be 
completely fine for upper fuse: it may and should block for a while if 
lower fuse doesn't catch up.

>
> Furthermore it isn't enough.  Becuase the root problem, I think, is that we
> allow fuse filesystems to grow a large number of dirty pages before throttling.
> This was never intended and it may actually have worked properly at a point in
> time but broke by some change to the dirty throttling algorithm.

Could someone from mm list step in and comment on this point? Which 
approach is better to follow: account NR_WRITEBACK_TEMP in 
balance_dirty_pages accurately (as we discussed in LSF/MM) or re-work 
balance_dirty_pages in direction suggested by Miklos (fuse should never 
go over the bdi limit even if the global limit hasn't been reached)?

I'm for accounting NR_WRITEBACK_TEMP because balance_dirty_pages is 
already overcomplicated (imho) and adding new clauses for FUSE makes me 
sick.

Thanks,
Maxim

>
> Thanks,
> Miklos
>
>
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index 137185c..195ee45 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -291,6 +291,7 @@ struct inode *fuse_iget(struct super_block *sb, u64 nodeid,
>   		inode->i_flags |= S_NOATIME|S_NOCMTIME;
>   		inode->i_generation = generation;
>   		inode->i_data.backing_dev_info = &fc->bdi;
> +		set_bit(AS_STRICTLIMIT, &inode->i_data.flags);
>   		fuse_init_inode(inode, attr);
>   		unlock_new_inode(inode);
>   	} else if ((inode->i_mode ^ attr->mode) & S_IFMT) {
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 0e38e13..97f6a0c 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 */
>   };
>   
>   static inline void mapping_set_error(struct address_space *mapping, int error)
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index efe6814..b6db421 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -1226,6 +1226,7 @@ static void balance_dirty_pages(struct address_space *mapping,
>   	unsigned long dirty_ratelimit;
>   	unsigned long pos_ratio;
>   	struct backing_dev_info *bdi = mapping->backing_dev_info;
> +	int strictlimit = test_bit(AS_STRICTLIMIT, &mapping->flags);
>   	unsigned long start_time = jiffies;
>   
>   	for (;;) {
> @@ -1250,7 +1251,7 @@ static void balance_dirty_pages(struct address_space *mapping,
>   		 */
>   		freerun = dirty_freerun_ceiling(dirty_thresh,
>   						background_thresh);
> -		if (nr_dirty <= freerun) {
> +		if (nr_dirty <= freerun && !strictlimit) {
>   			current->dirty_paused_when = now;
>   			current->nr_dirtied = 0;
>   			current->nr_dirtied_pause =
> @@ -1258,7 +1259,7 @@ static void balance_dirty_pages(struct address_space *mapping,
>   			break;
>   		}
>   
> -		if (unlikely(!writeback_in_progress(bdi)))
> +		if (unlikely(!writeback_in_progress(bdi)) && !strictlimit)
>   			bdi_start_background_writeback(bdi);
>   
>   		/*
> @@ -1296,8 +1297,12 @@ static void balance_dirty_pages(struct address_space *mapping,
>   				    bdi_stat(bdi, BDI_WRITEBACK);
>   		}
>   
> +		if (unlikely(!writeback_in_progress(bdi)) &&
> +		    bdi_dirty > bdi_thresh / 2)
> +			bdi_start_background_writeback(bdi);
> +
>   		dirty_exceeded = (bdi_dirty > bdi_thresh) &&
> -				  (nr_dirty > dirty_thresh);
> +				  ((nr_dirty > dirty_thresh) || strictlimit);
>   		if (dirty_exceeded && !bdi->dirty_exceeded)
>   			bdi->dirty_exceeded = 1;
>   
>
>