Re: [PATCH 03/24] mm: use __SetPageSwapBacked and don't ClearPageSwapBacked

[Hugh Dickins]

On Wed, 25 Feb 2015, Mel Gorman wrote:
> On Fri, Feb 20, 2015 at 07:56:15PM -0800, Hugh Dickins wrote:
> > Commit 07a427884348 ("mm: shmem: avoid atomic operation during
> > shmem_getpage_gfp") rightly replaced one instance of SetPageSwapBacked
> > by __SetPageSwapBacked, pointing out that the newly allocated page is
> > not yet visible to other users (except speculative get_page_unless_zero-
> > ers, who may not update page flags before their further checks).
> > 
> > That was part of a series in which Mel was focused on tmpfs profiles:
> > but almost all SetPageSwapBacked uses can be so optimized, with the
> > same justification.  And remove the ClearPageSwapBacked from
> > read_swap_cache_async()'s and zswap_get_swap_cache_page()'s error
> > paths: it's not an error to free a page with PG_swapbacked set.
> > 
> > (There's probably scope for further __SetPageFlags in other places,
> > but SwapBacked is the one I'm interested in at the moment.)
> > 
> > Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> > ---
> >  mm/migrate.c    |    6 +++---
> >  mm/rmap.c       |    2 +-
> >  mm/shmem.c      |    4 ++--
> >  mm/swap_state.c |    3 +--
> >  mm/zswap.c      |    3 +--
> >  5 files changed, 8 insertions(+), 10 deletions(-)
> > 
> > <SNIP>
> > --- thpfs.orig/mm/shmem.c	2015-02-08 18:54:22.000000000 -0800
> > +++ thpfs/mm/shmem.c	2015-02-20 19:33:35.676074594 -0800
> > @@ -987,8 +987,8 @@ static int shmem_replace_page(struct pag
> >  	flush_dcache_page(newpage);
> >  
> >  	__set_page_locked(newpage);
> > +	__SetPageSwapBacked(newpage);
> >  	SetPageUptodate(newpage);
> > -	SetPageSwapBacked(newpage);
> >  	set_page_private(newpage, swap_index);
> >  	SetPageSwapCache(newpage);
> >  
> 
> It's clear why you did this but ...
> 
> > @@ -1177,8 +1177,8 @@ repeat:
> >  			goto decused;
> >  		}
> >  
> > -		__SetPageSwapBacked(page);
> >  		__set_page_locked(page);
> > +		__SetPageSwapBacked(page);
> >  		if (sgp == SGP_WRITE)
> >  			__SetPageReferenced(page);
> >  
> 
> It's less clear why this was necessary.

I don't think the reordering was necessary in either case
(though perhaps the first hunk makes a subsequent patch smaller).
I just get irritated by seeing the same lines of code permuted
in different ways for no reason, and thought I'd tidy them up
to establish one familiar sequence, that's all.

> I don't think it causes any problems though so
> 
> Reviewed-by: Mel Gorman <mgorman@xxxxxxx>

Thanks!

Hugh
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