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

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Message-ID: <CALOAHbAU6XwN9ti0A1_KywpPqzgKxykWTxHYcYPQOywJo7FePQ@mail.gmail.com>
Date: Mon, 26 Aug 2024 21:48:34 +0800
From: Yafang Shao <laoar.shao@...il.com>
To: Michal Hocko <mhocko@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Christoph Hellwig <hch@....de>, 
	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, Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 2/2] mm: drop PF_MEMALLOC_NORECLAIM

On Mon, Aug 26, 2024 at 4:53 PM Michal Hocko <mhocko@...nel.org> wrote:
>
> From: Michal Hocko <mhocko@...e.com>
>
> There is no existing user of the flag and the flag 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.
>
> [1] https://lore.kernel.org/all/ZcM0xtlKbAOFjv5n@tiehlicka/
>
> Signed-off-by: Michal Hocko <mhocko@...e.com>
> ---
>  include/linux/sched.h    | 1 -
>  include/linux/sched/mm.h | 7 ++-----
>  2 files changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index f8d150343d42..72dad3a6317a 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1657,7 +1657,6 @@ 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 */

To maintain consistency with the other unused bits, it would be better
to define PF__HOLE__00800000 instead.

--
Regards

Yafang