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

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Message-ID: <ZsyzKmbmNs06Zrt9@tiehlicka>
Date: Mon, 26 Aug 2024 18:54:02 +0200
From: Michal Hocko <mhocko@...e.com>
To: Yafang Shao <laoar.shao@...il.com>
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
Subject: Re: [PATCH 2/2] mm: drop PF_MEMALLOC_NORECLAIM

On Mon 26-08-24 21:48:34, Yafang Shao wrote:
> 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.

OK

-- 
Michal Hocko
SUSE Labs