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

[Vlastimil Babka]

On 01/06/2017 03:11 PM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> GFP_NOFS context is used for the following 5 reasons currently
> 	- to prevent from deadlocks when the lock held by the allocation
> 	  context would be needed during the memory reclaim
> 	- to prevent from stack overflows during the reclaim because
> 	  the allocation is performed from a deep context already
> 	- to prevent lockups when the allocation context depends on
> 	  other reclaimers to make a forward progress indirectly
> 	- just in case because this would be safe from the fs POV
> 	- silence lockdep false positives
> 
> Unfortunately overuse of this allocation context brings some problems
> to the MM. Memory reclaim is much weaker (especially during heavy FS
> metadata workloads), OOM killer cannot be invoked because the MM layer
> doesn't have enough information about how much memory is freeable by the
> FS layer.
> 
> In many cases it is far from clear why the weaker context is even used
> and so it might be used unnecessarily. We would like to get rid of
> those as much as possible. One way to do that is to use the flag in
> scopes rather than isolated cases. Such a scope is declared when really
> necessary, tracked per task and all the allocation requests from within
> the context will simply inherit the GFP_NOFS semantic.
> 
> Not only this is easier to understand and maintain because there are
> much less problematic contexts than specific allocation requests, this
> also helps code paths where FS layer interacts with other layers (e.g.
> crypto, security modules, MM etc...) and there is no easy way to convey
> the allocation context between the layers.
> 
> Introduce memalloc_nofs_{save,restore} API to control the scope
> of GFP_NOFS allocation context. This is basically copying
> memalloc_noio_{save,restore} API we have for other restricted allocation
> context GFP_NOIO. The PF_MEMALLOC_NOFS flag already exists and it is
> just an alias for PF_FSTRANS which has been xfs specific until recently.
> There are no more PF_FSTRANS users anymore so let's just drop it.
> 
> PF_MEMALLOC_NOFS is now checked in the MM layer and drops __GFP_FS
> implicitly same as PF_MEMALLOC_NOIO drops __GFP_IO. memalloc_noio_flags
> is renamed to current_gfp_context because it now cares about both
> PF_MEMALLOC_NOFS and PF_MEMALLOC_NOIO contexts. Xfs code paths preserve
> their semantic. kmem_flags_convert() doesn't need to evaluate the flag
> anymore.
> 
> This patch shouldn't introduce any functional changes.
> 
> Let's hope that filesystems will drop direct GFP_NOFS (resp. ~__GFP_FS)
> usage as much as possible and only use a properly documented
> memalloc_nofs_{save,restore} checkpoints where they are appropriate.
> 
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>


[...]

> +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?

> +	return flags;
> +}
> +
> +static inline void memalloc_nofs_restore(unsigned int flags)
> +{
> +	current->flags = (current->flags & ~PF_MEMALLOC_NOFS) | flags;
> +}
> +
>  /* Per-process atomic flags. */
>  #define PFA_NO_NEW_PRIVS 0	/* May not gain new privileges. */
>  #define PFA_SPREAD_PAGE  1      /* Spread page cache over cpuset */

[...]

> @@ -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...

Thanks!

>  				(GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK),
>  		.reclaim_idx = MAX_NR_ZONES - 1,
>  		.target_mem_cgroup = memcg,
> @@ -3723,7 +3723,7 @@ 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 = memalloc_noio_flags(gfp_mask)),
> +		.gfp_mask = (gfp_mask = current_gfp_context(gfp_mask)),
>  		.order = order,
>  		.priority = NODE_RECLAIM_PRIORITY,
>  		.may_writepage = !!(node_reclaim_mode & RECLAIM_WRITE),
>