Re: dirty balancing deadlock

[Andrew Morton]

On Mon, 19 Feb 2007 18:11:55 +0100
Miklos Szeredi <miklos@xxxxxxxxxx> wrote:

> How about this?

I still don't understand this bug.

> Solves the FUSE deadlock, but not the throttle_vm_writeout() one.
> I'll try to tackle that one as well.
> 
> If the per-bdi dirty counter goes below 16, balance_dirty_pages()
> returns.
> 
> Does the constant need to tunable?  If it's too large, then the global
> threshold is more easily exceeded.  If it's too small, then in a tight
> situation progress will be slower.
> 
> Thanks,
> Miklos
> 
> Index: linux/mm/page-writeback.c
> ===================================================================
> --- linux.orig/mm/page-writeback.c	2007-02-19 17:32:41.000000000 +0100
> +++ linux/mm/page-writeback.c	2007-02-19 18:05:28.000000000 +0100
> @@ -198,6 +198,25 @@ static void balance_dirty_pages(struct a
>  			dirty_thresh)
>  				break;
>  
> +		/*
> +		 * Acquit this producer if there's little or nothing
> +		 * to write back to this particular queue
> +		 *
> +		 * Without this check a deadlock is possible in the
> +		 * following case:
> +		 *
> +		 * - filesystem A writes data through filesystem B
> +		 * - filesystem A has dirty pages over dirty_thresh
> +		 * - writeback is started, this triggers a write in B
> +		 * - balance_dirty_pages() is called synchronously
> +		 * - the write to B blocks
> +		 * - the writeback completes, but dirty is still over threshold
> +		 * - the blocking write prevents futher writes from happening
> +		 */
> +		if (atomic_long_read(&bdi->nr_dirty) +
> +		    atomic_long_read(&bdi->nr_writeback) < 16)
> +			break;
> +

The problem seems to that little "- the write to B blocks".

How come it blocks?  I mean, if we cannot retire writes to that filesystem
then we're screwed anyway.

Anyway, I think I'll think about this issue a little later on.  You might
as well prepare full changelogs for your proposed changes, because we'll be
needing them anyway.

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