Re: [PATCH 2/4] mm: introduce memalloc_noreclaim_{save, restore}

Michal Hocko
2017-04-05T11:28:18Z

On Wed 05-04-17 09:46:58, Vlastimil Babka wrote:
> The previous patch has shown that simply setting and clearing PF_MEMALLOC in
> current->flags can result in wrongly clearing a pre-existing PF_MEMALLOC flag
> and potentially lead to recursive reclaim. Let's introduce helpers that support
> proper nesting by saving the previous stat of the flag, similar to the existing
> memalloc_noio_* and memalloc_nofs_* helpers. Convert existing setting/clearing
> of PF_MEMALLOC within mm to the new helpers.
>
> There are no known issues with the converted code, but the change makes it more
> robust.
>
> Suggested-by: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>
> Signed-off-by: Vlastimil Babka <vbabka-AlSwsSmVLrQ@public.gmane.org>

One could argue that tsk_restore_flags() could be extended to provide
tsk_set_flags() and use it for all allocation related PF flags. I do not
have a strong opinion on that but explicit API sounds a bit better to me
because is easier to follow (at least for me). If others think that
generic API would be better then I won't have any objections. Anyway
this looks good to me.
Acked-by: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>

> ---
> include/linux/sched/mm.h | 12 ++++++++++++
> mm/page_alloc.c | 11 ++++++-----
> mm/vmscan.c | 17 +++++++++++------
> 3 files changed, 29 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index 9daabe138c99..2b24a6974847 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -191,4 +191,16 @@ static inline void memalloc_nofs_restore(unsigned int flags)
> current->flags = (current->flags & ~PF_MEMALLOC_NOFS) | flags;
> }
>
> +static inline unsigned int memalloc_noreclaim_save(void)
> +{
> + unsigned int flags = current->flags & PF_MEMALLOC;
> + current->flags |= PF_MEMALLOC;
> + return flags;
> +}
> +
> +static inline void memalloc_noreclaim_restore(unsigned int flags)
> +{
> + current->flags = (current->flags & ~PF_MEMALLOC) | flags;
> +}
> +
> #endif /* _LINUX_SCHED_MM_H */
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index b84e6ffbe756..037e32dccd7a 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3288,15 +3288,15 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
> enum compact_priority prio, enum compact_result *compact_result)
> {
> struct page *page;
> - unsigned int noreclaim_flag = current->flags & PF_MEMALLOC;
> + unsigned int noreclaim_flag;
>
> if (!order)
> return NULL;
>
> - current->flags |= PF_MEMALLOC;
> + noreclaim_flag = memalloc_noreclaim_save();
> *compact_result = try_to_compact_pages(gfp_mask, order, alloc_flags, ac,
> prio);
> - current->flags = (current->flags & ~PF_MEMALLOC) | noreclaim_flag;
> + memalloc_noreclaim_restore(noreclaim_flag);
>
> if (*compact_result <= COMPACT_INACTIVE)
> return NULL;
> @@ -3443,12 +3443,13 @@ __perform_reclaim(gfp_t gfp_mask, unsigned int order,
> {
> struct reclaim_state reclaim_state;
> int progress;
> + unsigned int noreclaim_flag;
>
> cond_resched();
>
> /* We now go into synchronous reclaim */
> cpuset_memory_pressure_bump();
> - current->flags |= PF_MEMALLOC;
> + noreclaim_flag = memalloc_noreclaim_save();
> lockdep_set_current_reclaim_state(gfp_mask);
> reclaim_state.reclaimed_slab = 0;
> current->reclaim_state = &reclaim_state;
> @@ -3458,7 +3459,7 @@ __perform_reclaim(gfp_t gfp_mask, unsigned int order,
>
> current->reclaim_state = NULL;
> lockdep_clear_current_reclaim_state();
> - current->flags &= ~PF_MEMALLOC;
> + memalloc_noreclaim_restore(noreclaim_flag);
>
> cond_resched();
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 58615bb27f2f..ff63b91a0f48 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2992,6 +2992,7 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
> struct zonelist *zonelist;
> unsigned long nr_reclaimed;
> int nid;
> + unsigned int noreclaim_flag;
> struct scan_control sc = {
> .nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX),
> .gfp_mask = (current_gfp_context(gfp_mask) & GFP_RECLAIM_MASK) |
> @@ -3018,9 +3019,9 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
> sc.gfp_mask,
> sc.reclaim_idx);
>
> - current->flags |= PF_MEMALLOC;
> + noreclaim_flag = memalloc_noreclaim_save();
> nr_reclaimed = do_try_to_free_pages(zonelist, &sc);
> - current->flags &= ~PF_MEMALLOC;
> + memalloc_noreclaim_restore(noreclaim_flag);
>
> trace_mm_vmscan_memcg_reclaim_end(nr_reclaimed);
>
> @@ -3544,8 +3545,9 @@ unsigned long shrink_all_memory(unsigned long nr_to_reclaim)
> struct zonelist *zonelist = node_zonelist(numa_node_id(), sc.gfp_mask);
> struct task_struct *p = current;
> unsigned long nr_reclaimed;
> + unsigned int noreclaim_flag;
>
> - p->flags |= PF_MEMALLOC;
> + noreclaim_flag = memalloc_noreclaim_save();
> lockdep_set_current_reclaim_state(sc.gfp_mask);
> reclaim_state.reclaimed_slab = 0;
> p->reclaim_state = &reclaim_state;
> @@ -3554,7 +3556,7 @@ unsigned long shrink_all_memory(unsigned long nr_to_reclaim)
>
> p->reclaim_state = NULL;
> lockdep_clear_current_reclaim_state();
> - p->flags &= ~PF_MEMALLOC;
> + memalloc_noreclaim_restore(noreclaim_flag);
>
> return nr_reclaimed;
> }
> @@ -3719,6 +3721,7 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
> struct task_struct *p = current;
> struct reclaim_state reclaim_state;
> int classzone_idx = gfp_zone(gfp_mask);
> + unsigned int noreclaim_flag;
> struct scan_control sc = {
> .nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX),
> .gfp_mask = (gfp_mask = current_gfp_context(gfp_mask)),
> @@ -3736,7 +3739,8 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
> * and we also need to be able to write out pages for RECLAIM_WRITE
> * and RECLAIM_UNMAP.
> */
> - p->flags |= PF_MEMALLOC | PF_SWAPWRITE;
> + noreclaim_flag = memalloc_noreclaim_save();
> + p->flags |= PF_SWAPWRITE;
> lockdep_set_current_reclaim_state(gfp_mask);
> reclaim_state.reclaimed_slab = 0;
> p->reclaim_state = &reclaim_state;
> @@ -3752,7 +3756,8 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
> }
>
> p->reclaim_state = NULL;
> - current->flags &= ~(PF_MEMALLOC | PF_SWAPWRITE);
> + current->flags &= ~PF_SWAPWRITE;
> + memalloc_noreclaim_restore(noreclaim_flag);
> lockdep_clear_current_reclaim_state();
> return sc.nr_reclaimed >= nr_pages;
> }
> --
> 2.12.2

--
Michal Hocko
SUSE Labs

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