Re: [PATCH 3/8] mm: introduce memalloc_nofs_{save,restore} API

[Michal Hocko <mhocko@kernel.org>]

On Mon 09-01-17 14:04:21, Vlastimil Babka wrote:
[...]
> > +static inline unsigned int memalloc_nofs_save(void)
> > +{
> > +	unsigned int flags = current->flags & PF_MEMALLOC_NOFS;
> > +	current->flags |= PF_MEMALLOC_NOFS;
> 
> So this is not new, as same goes for memalloc_noio_save, but I've
> noticed that e.g. exit_signal() does tsk->flags |= PF_EXITING;
> So is it possible that there's a r-m-w hazard here?

exit_signals operates on current and all task_struct::flags should be
used only on the current.
[...]

> > @@ -3029,7 +3029,7 @@ 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 = (gfp_mask & GFP_RECLAIM_MASK) |
> > +		.gfp_mask = (current_gfp_context(gfp_mask) & GFP_RECLAIM_MASK) |
> 
> So this function didn't do memalloc_noio_flags() before? Is it a bug
> that should be fixed separately or at least mentioned? Because that
> looks like a functional change...

We didn't need it. Kmem charges are opt-in and current all of them
support GFP_IO. The LRU pages are not charged in NOIO context either.
We need it now because there will be callers to charge GFP_KERNEL while
being inside the NOFS scope.

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. 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
---
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