Re: [PATCH v7 3/5] mm/hugetlb: add support for mempolicy MPOL_PREFERRED_MANY

[Hugh Dickins <hughd@google.com>]

On Tue, 10 Aug 2021, Feng Tang wrote:
> On Mon, Aug 09, 2021 at 03:19:32PM +0200, Michal Hocko wrote:
> [snip]
> > > > Do you think you can provide same helpers for other policies as well?
> > > > Maybe we can get rid of some other ifdefery as well.
> > > 
> > > Sure. I can make separate patch(es) for that.
> > > 
> > > And you mean helper like mpol_is_bind/default/local/preferred? 
> > > 
> > > I just run 'git-grep MPOL', and for places using "mode == MPOL_XXX",
> > > mostly they are in mempolicy.[ch], the only another place is in
> > > shmem.c, do we need to create all the helpers for it and the
> > > potential future users? 
> > 
> > I would just go with those instances which need to ifdef for NUMA.
> > Thanks!
> 
> Yes, following is a patch to remove one CONFIG_NUMA check, though
> an bolder idea to extend the patch by removing the CONFIG_TMPFS
> check in the same line.
> 
> Thanks,
> Feng
> 
> ---------8<---------------------------------
> 
> From 1a5858721ac8ce99c27c13d310bba2983dc73d97 Mon Sep 17 00:00:00 2001
> From: Feng Tang <feng.tang@intel.com>
> Date: Tue, 10 Aug 2021 17:00:59 +0800
> Subject: [PATCH] mm: shmem: avoid open coded check for mempolicy's mode
> 
> Add a mempolicy helper to do the check, which can also remove
> a CONFIG_NUMA option check.
> 
> Suggested-by: Michal Hocko <mhocko@suse.com>
> Signed-off-by: Feng Tang <feng.tang@intel.com>

No thanks: this is not an improvement.

The "#if defined(CONFIG_NUMA) && defined(CONFIG_TMPFS)" is there to
eliminate dead code that would not be automatically eliminated by the
optimizer: it's not there just to avoid MPOL_DEFAULT, and it's there
to cover shmem_get_sbmpol() along with shmem_show_mpol().

I know we tend to avoid #ifdefs in .c files, and that's good; and
I know you could find other code in mm/shmem.c which might also be
#ifdef'ed to eliminate other dead code in other configs; but unless
there's a new drive to purge our .c source of all #ifdefs,
please just leave this as is.

Thanks,
Hugh

> ---
>  include/linux/mempolicy.h | 14 ++++++++++++++
>  mm/shmem.c                |  8 ++++----
>  2 files changed, 18 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/mempolicy.h b/include/linux/mempolicy.h
> index 60d5e6c3340c..8fc518ad4f3c 100644
> --- a/include/linux/mempolicy.h
> +++ b/include/linux/mempolicy.h
> @@ -192,6 +192,10 @@ static inline bool mpol_is_preferred_many(struct mempolicy *pol)
>  	return  (pol->mode == MPOL_PREFERRED_MANY);
>  }
>  
> +static inline bool mpol_is_default(struct mempolicy *pol)
> +{
> +	return  (pol->mode == MPOL_DEFAULT);
> +}
>  
>  #else
>  
> @@ -287,6 +291,10 @@ static inline int mpol_parse_str(char *str, struct mempolicy **mpol)
>  }
>  #endif
>  
> +static inline void mpol_to_str(char *buffer, int maxlen, struct mempolicy *pol)
> +{
> +}
> +
>  static inline int mpol_misplaced(struct page *page, struct vm_area_struct *vma,
>  				 unsigned long address)
>  {
> @@ -309,5 +317,11 @@ static inline bool mpol_is_preferred_many(struct mempolicy *pol)
>  	return  false;
>  }
>  
> +static inline bool mpol_is_default(struct mempolicy *pol)
> +{
> +	return  false;
> +}
> +
> +
>  #endif /* CONFIG_NUMA */
>  #endif
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 96f05f6af8bb..26b195209ef7 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1437,12 +1437,12 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
>  	return 0;
>  }
>  
> -#if defined(CONFIG_NUMA) && defined(CONFIG_TMPFS)
> +#ifdef CONFIG_TMPFS
>  static void shmem_show_mpol(struct seq_file *seq, struct mempolicy *mpol)
>  {
>  	char buffer[64];
>  
> -	if (!mpol || mpol->mode == MPOL_DEFAULT)
> +	if (!mpol || mpol_is_default(mpol))
>  		return;		/* show nothing */
>  
>  	mpol_to_str(buffer, sizeof(buffer), mpol);
> @@ -1461,7 +1461,7 @@ static struct mempolicy *shmem_get_sbmpol(struct shmem_sb_info *sbinfo)
>  	}
>  	return mpol;
>  }
> -#else /* !CONFIG_NUMA || !CONFIG_TMPFS */
> +#else /* !CONFIG_TMPFS */
>  static inline void shmem_show_mpol(struct seq_file *seq, struct mempolicy *mpol)
>  {
>  }
> @@ -1469,7 +1469,7 @@ static inline struct mempolicy *shmem_get_sbmpol(struct shmem_sb_info *sbinfo)
>  {
>  	return NULL;
>  }
> -#endif /* CONFIG_NUMA && CONFIG_TMPFS */
> +#endif /* CONFIG_TMPFS */
>  #ifndef CONFIG_NUMA
>  #define vm_policy vm_private_data
>  #endif
> -- 
> 2.14.1