linux-kernel - Re: [PATCH 2/2] mm: drop PF_MEMALLOC

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Message-ID: <f9c1dbfd-322a-4021-928b-1d9dcaccbaec@suse.cz>
Date: Thu, 29 Aug 2024 23:45:31 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Michal Hocko <mhocko@...e.com>, Matthew Wilcox <willy@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Christoph Hellwig
 <hch@....de>, Yafang Shao <laoar.shao@...il.com>,
 Kent Overstreet <kent.overstreet@...ux.dev>, jack@...e.cz,
 Christian Brauner <brauner@...nel.org>,
 Alexander Viro <viro@...iv.linux.org.uk>, Paul Moore <paul@...l-moore.com>,
 James Morris <jmorris@...ei.org>, "Serge E. Hallyn" <serge@...lyn.com>,
 linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
 linux-bcachefs@...r.kernel.org, linux-security-module@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: drop PF_MEMALLOC_NORECLAIM

On 8/26/24 21:18, Michal Hocko wrote:
> On Mon 26-08-24 18:49:23, Matthew Wilcox wrote:
>> On Mon, Aug 26, 2024 at 06:51:55PM +0200, Michal Hocko wrote:
> [...]
>> > If a plan revert is preferably, I will go with it.
>> 
>> There aren't any other users of PF_MEMALLOC_NOWARN and it definitely
>> seems like something you want at a callsite rather than blanket for every
>> allocation below this point.  We don't seem to have many PF_ flags left,
>> so let's not keep it around if there's no immediate plans for it.
> 
> Good point. What about this?
> --- 
> From 923cd429d4b1a3520c93bcf46611ae74a3158865 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@...e.com>
> Date: Mon, 26 Aug 2024 21:15:02 +0200
> Subject: [PATCH] Revert "mm: introduce PF_MEMALLOC_NORECLAIM,
>  PF_MEMALLOC_NOWARN"
> 
> This reverts commit eab0af905bfc3e9c05da2ca163d76a1513159aa4.
> 
> There is no existing user of those flags. PF_MEMALLOC_NOWARN is
> dangerous because a nested allocation context can use GFP_NOFAIL which
> could cause unexpected failure. Such a code would be hard to maintain
> because it could be deeper in the call chain.
> 
> PF_MEMALLOC_NORECLAIM has been added even when it was pointed out [1]
> that such a allocation contex is inherently unsafe if the context
> doesn't fully control all allocations called from this context.
> 
> While PF_MEMALLOC_NOWARN is not dangerous the way PF_MEMALLOC_NORECLAIM
> is it doesn't have any user and as Matthew has pointed out we are
> running out of those flags so better reclaim it without any real users.
> 
> [1] https://lore.kernel.org/all/ZcM0xtlKbAOFjv5n@tiehlicka/
> 
> Signed-off-by: Michal Hocko <mhocko@...e.com>

Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  include/linux/sched.h    |  4 ++--
>  include/linux/sched/mm.h | 17 ++++-------------
>  2 files changed, 6 insertions(+), 15 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index f8d150343d42..731ff1078c9e 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1657,8 +1657,8 @@ 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_MEMALLOC_NORECLAIM	0x00800000	/* All allocation requests will clear __GFP_DIRECT_RECLAIM */
> -#define PF_MEMALLOC_NOWARN	0x01000000	/* All allocation requests will inherit __GFP_NOWARN */
> +#define PF__HOLE__00800000	0x00800000
> +#define PF__HOLE__01000000	0x01000000
>  #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 91546493c43d..07c4fde32827 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -258,25 +258,16 @@ 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_NORECLAIM |
> -			       PF_MEMALLOC_NOWARN |
> -			       PF_MEMALLOC_PIN))) {
> +	if (unlikely(pflags & (PF_MEMALLOC_NOIO | PF_MEMALLOC_NOFS | PF_MEMALLOC_PIN))) {
>  		/*
> -		 * Stronger flags before weaker flags:
> -		 * NORECLAIM implies NOIO, which in turn implies NOFS
> +		 * NOIO implies both NOIO and NOFS and it is a weaker context
> +		 * so always make sure it makes precedence
>  		 */
> -		if (pflags & PF_MEMALLOC_NORECLAIM)
> -			flags &= ~__GFP_DIRECT_RECLAIM;
> -		else if (pflags & PF_MEMALLOC_NOIO)
> +		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;
>  	}