linux-kernel - Re: [PATCH 4/6] xfs: use memalloc_nofs_{save,restore} instead of memalloc

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Message-ID: <20170206153923.GL2267@bombadil.infradead.org>
Date:   Mon, 6 Feb 2017 07:39:23 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        Dave Chinner <david@...morbit.com>, djwong@...nel.org,
        Theodore Ts'o <tytso@....edu>, Chris Mason <clm@...com>,
        David Sterba <dsterba@...e.cz>, Jan Kara <jack@...e.cz>,
        ceph-devel@...r.kernel.org, cluster-devel@...hat.com,
        linux-nfs@...r.kernel.org, logfs@...fs.org,
        linux-xfs@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-btrfs@...r.kernel.org, linux-mtd@...ts.infradead.org,
        reiserfs-devel@...r.kernel.org,
        linux-ntfs-dev@...ts.sourceforge.net,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-afs@...ts.infradead.org, LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 4/6] xfs: use memalloc_nofs_{save,restore} instead of
 memalloc_noio*

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This comment feels out of date ...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 ... ?