Re: [PATCH 4/6] xfs: use memalloc_nofs_{save,restore} instead of memalloc_noio*

[Matthew Wilcox]

On Mon, Feb 06, 2017 at 03:07:16PM +0100, Michal Hocko wrote:
> +++ b/fs/xfs/xfs_buf.c
> @@ -442,17 +442,17 @@ _xfs_buf_map_pages(
>  		bp->b_addr = NULL;
>  	} else {
>  		int retried = 0;
> -		unsigned noio_flag;
> +		unsigned nofs_flag;
>  
>  		/*
>  		 * vm_map_ram() will allocate auxillary structures (e.g.
>  		 * pagetables) with GFP_KERNEL, yet we are likely to be under
>  		 * GFP_NOFS context here. Hence we need to tell memory reclaim
> -		 * that we are in such a context via PF_MEMALLOC_NOIO to prevent
> +		 * that we are in such a context via PF_MEMALLOC_NOFS to prevent
>  		 * memory reclaim re-entering the filesystem here and
>  		 * potentially deadlocking.
>  		 */

This comment feels out of date ... how about:

		/*
		 * vm_map_ram will allocate auxiliary structures (eg page
		 * tables) with GFP_KERNEL.  If that tries to reclaim memory
		 * by calling back into this filesystem, we may deadlock.
		 * Prevent that by setting the NOFS flag.
		 */

> -		noio_flag = memalloc_noio_save();
> +		nofs_flag = memalloc_nofs_save();
>  		do {
>  			bp->b_addr = vm_map_ram(bp->b_pages, bp->b_page_count,
>  						-1, PAGE_KERNEL);

Also, I think it shows that this is the wrong place in XFS to be calling
memalloc_nofs_save().  I'm not arguing against including this patch;
it's a step towards where we want to be.  I also don't know XFS well
enough to know where to set that flag ;-)  Presumably when we start a
transaction ... ?