Re: [PATCH] reserved buffers only for PF_MEMALLOC

[Marcelo Tosatti]

On Tue, Aug 10, 2004 at 01:20:24PM -0400, Rik van Riel wrote:
> 
> The buffer allocation path in 2.4 has a long standing bug,
> where non-PF_MEMALLOC tasks can dig into the reserved pool
> in get_unused_buffer_head().  The following patch makes the
> reserved pool only accessible to PF_MEMALLOC tasks.
> 
> Other processes will loop in create_buffers() - the only
> function that calls get_unused_buffer_head() - and will call
> try_to_free_pages(GFP_NOIO), freeing any buffer heads that
> have become freeable due to IO completion.
> 
> Note that PF_MEMALLOC tasks will NOT do anything inside
> try_to_free_pages(), so it is needed that they are able to
> dig into the reserved buffer heads while other tasks are
> not.

Sounds the correct thing to do, thanks.

Out of curiosity: Do you actually seen any practical problem due to 
get_unused_buffer_head() calls eating into the reserved pool?

Or have any testcase which would trigger a problem (OOM) due to it? 
> 
> Signed-off-by:  Rik van Riel <riel@xxxxxxxxxx>
> 
> --- linux/fs/buffer.c.deadlock	2004-08-10 11:33:08.000000000 -0400
> +++ linux/fs/buffer.c	2004-08-10 11:34:54.000000000 -0400
> @@ -1260,8 +1260,9 @@ struct buffer_head * get_unused_buffer_h
>  
>  	/*
>  	 * If we need an async buffer, use the reserved buffer heads.
> +	 * Non-PF_MEMALLOC tasks can just loop in create_buffers().
>  	 */
> -	if (async) {
> +	if (async && (current->flags & PF_MEMALLOC)) {
>  		spin_lock(&unused_list_lock);
>  		if (unused_list) {
>  			bh = unused_list;
-
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