linux-kernel - Re: [PATCH 3/8] mm: introduce memalloc_nofs

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Message-ID: <20170109135901.GJ7495@dhcp22.suse.cz>
Date:   Mon, 9 Jan 2017 14:59:01 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.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>
Subject: Re: [PATCH 3/8] mm: introduce memalloc_nofs_{save,restore} API

On Mon 09-01-17 14:42:10, Michal Hocko wrote:
> On Mon 09-01-17 14:04:21, Vlastimil Babka wrote:
[...]
> Now that you have opened this I have noticed that the code is wrong
> here because GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK would overwrite
> the removed GFP_FS.

Blee, it wouldn't because ~GFP_RECLAIM_MASK will not contain neither
GFP_FS nor GFP_IO. So all is good here.

> I guess it would be better and less error prone
> to move the current_gfp_context part into the direct reclaim entry -
> do_try_to_free_pages - and put the comment like this

well, after more thinking about we, should probably keep it where it is.
If for nothing else try_to_free_mem_cgroup_pages has a tracepoint which
prints the gfp mask so we should use the filtered one. So let's just
scratch this follow up fix.

> ---
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 4ea6b610f20e..df7975185f11 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2756,6 +2756,13 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
>  	int initial_priority = sc->priority;
>  	unsigned long total_scanned = 0;
>  	unsigned long writeback_threshold;
> +
> +	/*
> +	 * Make sure that the gfp context properly handles scope gfp mask.
> +	 * This might weaken the reclaim context (e.g. make it GFP_NOFS or
> +	 * GFP_NOIO).
> +	 */
> +	sc->gfp_mask = current_gfp_context(sc->gfp_mask);
>  retry:
>  	delayacct_freepages_start();
>  
> @@ -2949,7 +2956,7 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
>  	unsigned long nr_reclaimed;
>  	struct scan_control sc = {
>  		.nr_to_reclaim = SWAP_CLUSTER_MAX,
> -		.gfp_mask = (gfp_mask = current_gfp_context(gfp_mask)),
> +		.gfp_mask = gfp_mask,
>  		.reclaim_idx = gfp_zone(gfp_mask),
>  		.order = order,
>  		.nodemask = nodemask,
> @@ -3029,8 +3036,7 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
>  	int nid;
>  	struct scan_control sc = {
>  		.nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX),
> -		.gfp_mask = (current_gfp_context(gfp_mask) & GFP_RECLAIM_MASK) |
> -				(GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK),
> +		.gfp_mask = GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK,
>  		.reclaim_idx = MAX_NR_ZONES - 1,
>  		.target_mem_cgroup = memcg,
>  		.priority = DEF_PRIORITY,
> @@ -3723,7 +3729,7 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
>  	int classzone_idx = gfp_zone(gfp_mask);
>  	struct scan_control sc = {
>  		.nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX),
> -		.gfp_mask = (gfp_mask = current_gfp_context(gfp_mask)),
> +		.gfp_mask = gfp_mask,
>  		.order = order,
>  		.priority = NODE_RECLAIM_PRIORITY,
>  		.may_writepage = !!(node_reclaim_mode & RECLAIM_WRITE),
> -- 
> Michal Hocko
> SUSE Labs

-- 
Michal Hocko
SUSE Labs