linux-kernel - Re: [PATCH -V3] mm: Add PageLayzyFree() helper functions for MADV

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Message-ID: <20200309121300.GL8447@dhcp22.suse.cz>
Date:   Mon, 9 Mar 2020 13:13:00 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     "Huang, Ying" <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
        Zi Yan <ziy@...dia.com>, Peter Zijlstra <peterz@...radead.org>,
        Minchan Kim <minchan@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH -V3] mm: Add PageLayzyFree() helper functions for
 MADV_FREE

On Mon 09-03-20 09:55:38, David Hildenbrand wrote:
> On 09.03.20 03:17, Huang, Ying wrote:
[...]
> > @@ -1235,7 +1234,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> >  		 * Try to allocate it some swap space here.
> >  		 * Lazyfree page could be freed directly
> >  		 */
> > -		if (PageAnon(page) && PageSwapBacked(page)) {
> > +		if (PageAnon(page) && !__PageLazyFree(page)) {
> >  			if (!PageSwapCache(page)) {
> >  				if (!(sc->gfp_mask & __GFP_IO))
> >  					goto keep_locked;
> > @@ -1411,7 +1410,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> >  			}
> >  		}
> >  
> > -		if (PageAnon(page) && !PageSwapBacked(page)) {
> > +		if (PageLazyFree(page)) {
> >  			/* follow __remove_mapping for reference */
> >  			if (!page_ref_freeze(page, 1))
> >  				goto keep_locked;
> > 
> 
> I still prefer something like
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index fd6d4670ccc3..7538501230bd 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -63,6 +63,10 @@
>   * page_waitqueue(page) is a wait queue of all tasks waiting for the page
>   * to become unlocked.
>   *
> + * PG_swapbacked used with anonymous pages (PageAnon()) indicates that a
> + * page is backed by swap. Anonymous pages without PG_swapbacked are
> + * pages that can be lazily freed (e.g., MADV_FREE) on demand.
> + *
>   * PG_uptodate tells whether the page's contents is valid.  When a read
>   * completes, the page becomes uptodate, unless a disk I/O error happened.
>   *
> 
> and really don't like the use of !__PageLazyFree() instead of PageSwapBacked().

I have to say that I do not have a strong opinion about helper
functions. In general I tend to be against adding them unless there is a
very good reason for them. This particular patch is in a gray zone a bit.

There are few places which are easier to follow but others sound like,
we have a hammer let's use it. E.g. shrink_page_list path above. There
is a clear comment explaining PageAnon && PageSwapBacked check being
LazyFree related but do I have to know that this is LazyFree path? I
believe that seeing PageSwapBacked has a more meaning to me because it
tells me that anonymous pages without a backing store doesn't really
need swap entry.  This happens to be Lazy free related today but with a
heavy overloading of our flags this might differ in the future. You have
effectively made a more generic description more specific without a very
good reason.

On the other hand having PG_swapbacked description in page-flags.h above
gives a very useful information which was previously hidden at the
definition so this is a clear improvement.

That being said I think that the patch is not helpful enough. I would
much rather see a simply documentation update.

-- 
Michal Hocko
SUSE Labs