Subject: Re: [PATCH 03/13] mm: Introduce __GFP_MEMALLOC to allow access to emergency reserves

From: Mel Gorman <[email-masked]>
Date: Tue, 26 Apr 2011 11:36:46 +0100

On Tue, Apr 26, 2011 at 07:49:47PM +1000, NeilBrown wrote:
> On Tue, 26 Apr 2011 08:36:44 +0100 Mel Gorman <[email-masked]> wrote:
> 
> > __GFP_MEMALLOC will allow the allocation to disregard the watermarks,
> > much like PF_MEMALLOC. It allows one to pass along the memalloc state in
> > object related allocation flags as opposed to task related flags, such
> > as sk->sk_allocation. This removes the need for ALLOC_PFMEMALLOC as
> > callers using __GFP_MEMALLOC can get the ALLOC_NO_WATERMARK flag which
> > is now enough to identify allocations related to page reclaim.
> > 
> > Signed-off-by: Peter Zijlstra <[email-masked]>
> > Signed-off-by: Mel Gorman <[email-masked]>
> > ---
> >  include/linux/gfp.h      |    4 +++-
> >  include/linux/mm_types.h |    2 +-
> >  mm/page_alloc.c          |   14 ++++++--------
> >  mm/slab.c                |    2 +-
> >  4 files changed, 11 insertions(+), 11 deletions(-)
> > 
> > diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> > index bfb8f93..4e011e7 100644
> > --- a/include/linux/gfp.h
> > +++ b/include/linux/gfp.h
> > @@ -23,6 +23,7 @@ struct vm_area_struct;
> >  #define ___GFP_REPEAT		0x400u
> >  #define ___GFP_NOFAIL		0x800u
> >  #define ___GFP_NORETRY		0x1000u
> > +#define ___GFP_MEMALLOC		0x2000u
> >  #define ___GFP_COMP		0x4000u
> >  #define ___GFP_ZERO		0x8000u
> >  #define ___GFP_NOMEMALLOC	0x10000u
> > @@ -75,6 +76,7 @@ struct vm_area_struct;
> >  #define __GFP_REPEAT	((__force gfp_t)___GFP_REPEAT)	/* See above */
> >  #define __GFP_NOFAIL	((__force gfp_t)___GFP_NOFAIL)	/* See above */
> >  #define __GFP_NORETRY	((__force gfp_t)___GFP_NORETRY) /* See above */
> > +#define __GFP_MEMALLOC	((__force gfp_t)___GFP_MEMALLOC)/* Allow access to emergency reserves */
> >  #define __GFP_COMP	((__force gfp_t)___GFP_COMP)	/* Add compound page metadata */
> >  #define __GFP_ZERO	((__force gfp_t)___GFP_ZERO)	/* Return zeroed page on success */
> >  #define __GFP_NOMEMALLOC ((__force gfp_t)___GFP_NOMEMALLOC) /* Don't use emergency reserves */
> 
> Having both "MEMALLOC" and  "NOMEMALLOC" seems ... unfortunate.
> 
> It appears that NOMEMALLOC over-rides MEMALLOC.  It might be good to document
> this 
> 

I can document it. Right now, a better name than NOMEMALLOC does not
spring to mind.

> > +#define __GFP_MEMALLOC	((__force gfp_t)___GFP_MEMALLOC)/* Allow access to emergency reserves
>                                                                    unless __GFP_NOMEMALLOC is set*/
> 
> >  #define __GFP_NOMEMALLOC ((__force gfp_t)___GFP_NOMEMALLOC) /* Don't use emergency reserves
>                                                                   Overrides __GFP_MEMALLOC */
> 
> I suspect that it is never valid to set both.  So NOMEMALLOC is really
> NO_PF_MEMALLOC, but making that change is probably just noise.
> 
> Maybe a
>    WARN_ON((gfp_mask & __GFP_MEMALLOC) && (gfp_mask & __GFP_NOMEMALLOC));
> might be wise?
> 

Both MEMALLOC and NOMEMALLOC are related to PFMEMALLOC reserves so
it's reasonable for them to have similar names. This warning will
also trigger because it's a combination of flags that does happen.

Consider for example

any interrupt
  -> __netdev_alloc_skb		(mask == GFP_ATOMIC)
    -> __alloc_skb		(mask == GFP_ATOMIC)
       if (sk_memalloc_socks() && (flags & SKB_ALLOC_RX))
               gfp_mask |= __GFP_MEMALLOC;
				(mask == GFP_ATOMIC|__GFP_NOMEMALLOC)
      -> __kmalloc_reserve
		First attempt tries to avoid reserves so adds __GFP_MEMALLOC
				(mask == GFP_ATOMIC|__GFP_NOMEMALLOC|__GFP_MEMALLOC)

You're right in that __GFP_NOMEMALLOC overrides __GFP_MEMALLOC so that
could do with a note.

-- 
Mel Gorman
SUSE Labs

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