Re: [PATCH v13 1/4] mm: Add become_kswapd and restore_kswapd

[Dave Chinner <david@fromorbit.com>]

On Thu, Dec 17, 2020 at 09:11:54AM +0800, Yafang Shao wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> 
> Since XFS needs to pretend to be kswapd in some of its worker threads,
> create methods to save & restore kswapd state.  Don't bother restoring
> kswapd state in kswapd -- the only time we reach this code is when we're
> exiting and the task_struct is about to be destroyed anyway.
> 
> Cc: Dave Chinner <david@fromorbit.com>
> Acked-by: Michal Hocko <mhocko@suse.com>
> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> ---
>  fs/xfs/libxfs/xfs_btree.c | 14 ++++++++------
>  include/linux/sched/mm.h  | 23 +++++++++++++++++++++++
>  mm/vmscan.c               | 16 +---------------
>  3 files changed, 32 insertions(+), 21 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_btree.c b/fs/xfs/libxfs/xfs_btree.c
> index 51dbff9b0908..0f35b7a38e76 100644
> --- a/fs/xfs/libxfs/xfs_btree.c
> +++ b/fs/xfs/libxfs/xfs_btree.c
> @@ -2813,8 +2813,9 @@ xfs_btree_split_worker(
>  {
>  	struct xfs_btree_split_args	*args = container_of(work,
>  						struct xfs_btree_split_args, work);
> +	bool			is_kswapd = args->kswapd;
>  	unsigned long		pflags;
> -	unsigned long		new_pflags = PF_MEMALLOC_NOFS;
> +	int			memalloc_nofs;
>  
>  	/*
>  	 * we are in a transaction context here, but may also be doing work
> @@ -2822,16 +2823,17 @@ xfs_btree_split_worker(
>  	 * temporarily to ensure that we don't block waiting for memory reclaim
>  	 * in any way.
>  	 */
> -	if (args->kswapd)
> -		new_pflags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;
> -
> -	current_set_flags_nested(&pflags, new_pflags);
> +	if (is_kswapd)
> +		pflags = become_kswapd();
> +	memalloc_nofs = memalloc_nofs_save();
>  
>  	args->result = __xfs_btree_split(args->cur, args->level, args->ptrp,
>  					 args->key, args->curp, args->stat);
>  	complete(args->done);
>  
> -	current_restore_flags_nested(&pflags, new_pflags);
> +	memalloc_nofs_restore(memalloc_nofs);
> +	if (is_kswapd)
> +		restore_kswapd(pflags);
>  }
>  
>  /*
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index d5ece7a9a403..2faf03e79a1e 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -278,6 +278,29 @@ static inline void memalloc_nocma_restore(unsigned int flags)
>  }
>  #endif
>  
> +/*
> + * Tell the memory management code that this thread is working on behalf
> + * of background memory reclaim (like kswapd).  That means that it will
> + * get access to memory reserves should it need to allocate memory in
> + * order to make forward progress.  With this great power comes great
> + * responsibility to not exhaust those reserves.
> + */
> +#define KSWAPD_PF_FLAGS		(PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD)
> +
> +static inline unsigned long become_kswapd(void)
> +{
> +	unsigned long flags = current->flags & KSWAPD_PF_FLAGS;
> +
> +	current->flags |= KSWAPD_PF_FLAGS;
> +
> +	return flags;
> +}

You can get rid of the empty lines out of this function.

> +static inline void restore_kswapd(unsigned long flags)
> +{
> +	current->flags &= ~(flags ^ KSWAPD_PF_FLAGS);
> +}

Urk, that requires thinking about to determine whether it is
correct. And it is 3 runtime logic operations (^, ~ and &) too. The
way all the memalloc_*_restore() functions restore the previous
flags is obviously correct and only requires 2 runtime logic
operations because the compiler calculates the ~ operation on the
constant. So why do it differently here? i.e.:

	current->flags = (current->flags & ~KSWAPD_PF_FLAGS) | flags;

> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3870,19 +3870,7 @@ static int kswapd(void *p)
>  	if (!cpumask_empty(cpumask))
>  		set_cpus_allowed_ptr(tsk, cpumask);
>  
> -	/*
> -	 * Tell the memory management that we're a "memory allocator",
> -	 * and that if we need more memory we should get access to it
> -	 * regardless (see "__alloc_pages()"). "kswapd" should
> -	 * never get caught in the normal page freeing logic.
> -	 *
> -	 * (Kswapd normally doesn't need memory anyway, but sometimes
> -	 * you need a small amount of memory in order to be able to
> -	 * page out something else, and this flag essentially protects
> -	 * us from recursively trying to free more memory as we're
> -	 * trying to free the first piece of memory in the first place).
> -	 */
> -	tsk->flags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;
> +	become_kswapd();
>  	set_freezable();
>  
>  	WRITE_ONCE(pgdat->kswapd_order, 0);
> @@ -3932,8 +3920,6 @@ static int kswapd(void *p)
>  			goto kswapd_try_sleep;
>  	}
>  
> -	tsk->flags &= ~(PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD);
> -

Missing a restore_kswapd()?

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com