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
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