Re: [PATCH 2/3] mm: introduce PF_MEMALLOC_NORECLAIM, PF_MEMALLOC_NOWARN

[Vlastimil Babka <vbabka@suse.cz>]

On 2/6/24 22:50, Kent Overstreet wrote:
> Introduce PF_MEMALLOC_* equivalents of some GFP_ flags:
> 
> PF_MEMALLOC_NORECLAIM	-> GFP_NOWAIT

In an ideal world, this would be nice, but we are in a world with implicit
"too small to fail" guarantees that has so far been impossible to get away
from [1] for small order GFP_KERNEL allocations, and this scoping would be
only safe if no allocations underneath relied on this behavior. But how to
ensure that's the case?

[1] https://lwn.net/Articles/723317/

> PF_MEMALLOC_NOWARN	-> __GFP_NOWARN
> 
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Darrick J. Wong <djwong@kernel.org>
> Cc: linux-mm@kvack.org
> Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
> ---
>  include/linux/sched.h    |  4 ++--
>  include/linux/sched/mm.h | 17 +++++++++++++----
>  2 files changed, 15 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 292c31697248..ca08d92b20ac 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1755,8 +1755,8 @@ extern struct pid *cad_pid;
>  						 * I am cleaning dirty pages from some other bdi. */
>  #define PF_KTHREAD		0x00200000	/* I am a kernel thread */
>  #define PF_RANDOMIZE		0x00400000	/* Randomize virtual address space */
> -#define PF__HOLE__00800000	0x00800000
> -#define PF__HOLE__01000000	0x01000000
> +#define PF_MEMALLOC_NORECLAIM	0x00800000	/* All allocation requests will inherit __GFP_NOWARN */
> +#define PF_MEMALLOC_NOWARN	0x01000000	/* All allocation requests will inherit __GFP_NOWARN */
>  #define PF__HOLE__02000000	0x02000000
>  #define PF_NO_SETAFFINITY	0x04000000	/* Userland is not allowed to meddle with cpus_mask */
>  #define PF_MCE_EARLY		0x08000000      /* Early kill for mce process policy */
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index f00d7ecc2adf..c29059a76052 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -236,16 +236,25 @@ static inline gfp_t current_gfp_context(gfp_t flags)
>  {
>  	unsigned int pflags = READ_ONCE(current->flags);
>  
> -	if (unlikely(pflags & (PF_MEMALLOC_NOIO | PF_MEMALLOC_NOFS | PF_MEMALLOC_PIN))) {
> +	if (unlikely(pflags & (PF_MEMALLOC_NOIO |
> +			       PF_MEMALLOC_NOFS |
> +			       PF_MEMALLOC_NORECLAIM |
> +			       PF_MEMALLOC_NOWARN |
> +			       PF_MEMALLOC_PIN))) {
>  		/*
> -		 * NOIO implies both NOIO and NOFS and it is a weaker context
> -		 * so always make sure it makes precedence
> +		 * Stronger flags before weaker flags:
> +		 * NORECLAIM implies NOIO, which in turn implies NOFS
>  		 */
> -		if (pflags & PF_MEMALLOC_NOIO)
> +		if (pflags & PF_MEMALLOC_NORECLAIM)
> +			flags &= ~__GFP_DIRECT_RECLAIM;
> +		else if (pflags & PF_MEMALLOC_NOIO)
>  			flags &= ~(__GFP_IO | __GFP_FS);
>  		else if (pflags & PF_MEMALLOC_NOFS)
>  			flags &= ~__GFP_FS;
>  
> +		if (pflags & PF_MEMALLOC_NOWARN)
> +			flags |= __GFP_NOWARN;
> +
>  		if (pflags & PF_MEMALLOC_PIN)
>  			flags &= ~__GFP_MOVABLE;
>  	}