Re: [PATCH v5 2/2] mm: vmscan: refactor reclaim_state helpers

[Tim Chen]

On Wed, 2023-04-05 at 18:54 +0000, Yosry Ahmed wrote:
> During reclaim, we keep track of pages reclaimed from other means than
> LRU-based reclaim through scan_control->reclaim_state->reclaimed_slab,
> which we stash a pointer to in current task_struct.
> 
> However, we keep track of more than just reclaimed slab pages through
> this. We also use it for clean file pages dropped through pruned inodes,
> and xfs buffer pages freed. Rename reclaimed_slab to reclaimed, and add
> a helper function that wraps updating it through current, so that future
> changes to this logic are contained within mm/vmscan.c.
> 
> Additionally, add a flush_reclaim_state() helper to wrap using
> reclaim_state->reclaimed to updated sc->nr_reclaimed, and use that
> helper to add an elaborate comment about why we only do the update for
> global reclaim.
> 
> Finally, move set_task_reclaim_state() next to flush_reclaim_state() so
> that all reclaim_state helpers are in close proximity for easier
> readability.
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> ---
>  fs/inode.c           |  3 +-
>  fs/xfs/xfs_buf.c     |  3 +-
>  include/linux/swap.h | 17 +++++++++-
>  mm/slab.c            |  3 +-
>  mm/slob.c            |  6 ++--
>  mm/slub.c            |  5 ++-
>  mm/vmscan.c          | 75 ++++++++++++++++++++++++++++++++------------
>  7 files changed, 78 insertions(+), 34 deletions(-)
> 
> diff --git a/fs/inode.c b/fs/inode.c
> index 4558dc2f13557..e60fcc41faf17 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -864,8 +864,7 @@ static enum lru_status inode_lru_isolate(struct list_head *item,
>  				__count_vm_events(KSWAPD_INODESTEAL, reap);
>  			else
>  				__count_vm_events(PGINODESTEAL, reap);
> -			if (current->reclaim_state)
> -				current->reclaim_state->reclaimed_slab += reap;
> +			mm_account_reclaimed_pages(reap);
>  		}
>  		iput(inode);
>  		spin_lock(lru_lock);
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index 54c774af6e1c6..15d1e5a7c2d34 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -286,8 +286,7 @@ xfs_buf_free_pages(
>  		if (bp->b_pages[i])
>  			__free_page(bp->b_pages[i]);
>  	}
> -	if (current->reclaim_state)
> -		current->reclaim_state->reclaimed_slab += bp->b_page_count;
> +	mm_account_reclaimed_pages(bp->b_page_count);
>  
>  	if (bp->b_pages != bp->b_page_array)
>  		kmem_free(bp->b_pages);
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 209a425739a9f..e131ac155fb95 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -153,13 +153,28 @@ union swap_header {
>   * memory reclaim
>   */
>  struct reclaim_state {
> -	unsigned long reclaimed_slab;
> +	/* pages reclaimed outside of LRU-based reclaim */
> +	unsigned long reclaimed;
>  #ifdef CONFIG_LRU_GEN
>  	/* per-thread mm walk data */
>  	struct lru_gen_mm_walk *mm_walk;
>  #endif
>  };
>  
> +/*
> + * mm_account_reclaimed_pages(): account reclaimed pages outside of LRU-based
> + * reclaim
> + * @pages: number of pages reclaimed
> + *
> + * If the current process is undergoing a reclaim operation, increment the
> + * number of reclaimed pages by @pages.
> + */
> +static inline void mm_account_reclaimed_pages(unsigned long pages)
> +{
> +	if (current->reclaim_state)
> +		current->reclaim_state->reclaimed += pages;
> +}
> +
>  #ifdef __KERNEL__
>  
>  struct address_space;
> diff --git a/mm/slab.c b/mm/slab.c
> index dabc2a671fc6f..64bf1de817b24 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -1392,8 +1392,7 @@ static void kmem_freepages(struct kmem_cache *cachep, struct slab *slab)
>  	smp_wmb();
>  	__folio_clear_slab(folio);
>  
> -	if (current->reclaim_state)
> -		current->reclaim_state->reclaimed_slab += 1 << order;
> +	mm_account_reclaimed_pages(1 << order);
>  	unaccount_slab(slab, order, cachep);
>  	__free_pages(&folio->page, order);
>  }
> diff --git a/mm/slob.c b/mm/slob.c
> index fe567fcfa3a39..79cc8680c973c 100644
> --- a/mm/slob.c
> +++ b/mm/slob.c
> @@ -61,7 +61,7 @@
>  #include <linux/slab.h>
>  
>  #include <linux/mm.h>
> -#include <linux/swap.h> /* struct reclaim_state */
> +#include <linux/swap.h> /* mm_account_reclaimed_pages() */
>  #include <linux/cache.h>
>  #include <linux/init.h>
>  #include <linux/export.h>
> @@ -211,9 +211,7 @@ static void slob_free_pages(void *b, int order)
>  {
>  	struct page *sp = virt_to_page(b);
>  
> -	if (current->reclaim_state)
> -		current->reclaim_state->reclaimed_slab += 1 << order;
> -
> +	mm_account_reclaimed_pages(1 << order);
>  	mod_node_page_state(page_pgdat(sp), NR_SLAB_UNRECLAIMABLE_B,
>  			    -(PAGE_SIZE << order));
>  	__free_pages(sp, order);
> diff --git a/mm/slub.c b/mm/slub.c
> index 39327e98fce34..7aa30eef82350 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -11,7 +11,7 @@
>   */
>  
>  #include <linux/mm.h>
> -#include <linux/swap.h> /* struct reclaim_state */
> +#include <linux/swap.h> /* mm_account_reclaimed_pages() */
>  #include <linux/module.h>
>  #include <linux/bit_spinlock.h>
>  #include <linux/interrupt.h>
> @@ -2063,8 +2063,7 @@ static void __free_slab(struct kmem_cache *s, struct slab *slab)
>  	/* Make the mapping reset visible before clearing the flag */
>  	smp_wmb();
>  	__folio_clear_slab(folio);
> -	if (current->reclaim_state)
> -		current->reclaim_state->reclaimed_slab += pages;
> +	mm_account_reclaimed_pages(pages);
>  	unaccount_slab(slab, order, s);
>  	__free_pages(&folio->page, order);
>  }
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index c82bd89f90364..049e39202e6ce 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -188,18 +188,6 @@ struct scan_control {
>   */
>  int vm_swappiness = 60;
>  
> -static void set_task_reclaim_state(struct task_struct *task,
> -				   struct reclaim_state *rs)
> -{
> -	/* Check for an overwrite */
> -	WARN_ON_ONCE(rs && task->reclaim_state);
> -
> -	/* Check for the nulling of an already-nulled member */
> -	WARN_ON_ONCE(!rs && !task->reclaim_state);
> -
> -	task->reclaim_state = rs;
> -}
> -
>  LIST_HEAD(shrinker_list);
>  DECLARE_RWSEM(shrinker_rwsem);
>  
> @@ -511,6 +499,59 @@ static bool writeback_throttling_sane(struct scan_control *sc)
>  }
>  #endif
>  
> +static void set_task_reclaim_state(struct task_struct *task,
> +				   struct reclaim_state *rs)
> +{
> +	/* Check for an overwrite */
> +	WARN_ON_ONCE(rs && task->reclaim_state);
> +
> +	/* Check for the nulling of an already-nulled member */
> +	WARN_ON_ONCE(!rs && !task->reclaim_state);
> +
> +	task->reclaim_state = rs;
> +}
> +
> +/*
> + * flush_reclaim_state(): add pages reclaimed outside of LRU-based reclaim to
> + * scan_control->nr_reclaimed.
> + */
> +static void flush_reclaim_state(struct scan_control *sc,
> +				struct reclaim_state *rs)
> +{
> +	/*
> +	 * Currently, reclaim_state->reclaimed includes three types of pages
> +	 * freed outside of vmscan:
> +	 * (1) Slab pages.
> +	 * (2) Clean file pages from pruned inodes.
> +	 * (3) XFS freed buffer pages.
> +	 *
> +	 * For all of these cases, we have no way of finding out whether these
> +	 * pages were related to the memcg under reclaim. For example, a freed
> +	 * slab page could have had only a single object charged to the memcg

Minor nits:
s/could have had/could have

> +	 * under reclaim. Also, populated inodes are not on shrinker LRUs
> +	 * anymore except on highmem systems.
> +	 *
> +	 * Instead of over-reporting the reclaimed pages in a memcg reclaim,
> +	 * only count such pages in global reclaim. This prevents unnecessary

May be clearer to say:
This prevents under-reclaimaing the target memcg, and unnecessary

> +	 * retries during memcg charging and false positive from proactive
> +	 * reclaim (memory.reclaim).
> +	 *

Tim