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

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
---

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

On 01/09/2017 02:42 PM, Michal Hocko wrote:
> 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.
> [...]

Ah, good to know.

> 
>>> @@ -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.

I see.

> 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

Agree with your "So let's just scratch this follow up fix in the next
e-mail.

So for the unchanged patch.

Acked-by: Vlastimil Babka <vbabka@suse.cz>

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