linux-kernel - Re: [PATCHi v2] mm: put_and_wait_on_page

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Date:   Tue, 27 Nov 2018 13:08:50 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     Mike Rapoport <rppt@...ux.ibm.com>
cc:     Matthew Wilcox <willy@...radead.org>,
        Hugh Dickins <hughd@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Baoquan He <bhe@...hat.com>, Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        David Herrmann <dh.herrmann@...il.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Kan Liang <kan.liang@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Christoph Lameter <cl@...ux.com>,
        Nick Piggin <npiggin@...il.com>, pifang@...hat.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHi v2] mm: put_and_wait_on_page_locked() while page is
 migrated

On Tue, 27 Nov 2018, Mike Rapoport wrote:
> On Mon, Nov 26, 2018 at 11:27:07AM -0800, Hugh Dickins wrote:
> > 
> > +/*
> > + * A choice of three behaviors for wait_on_page_bit_common():
> > + */
> > +enum behavior {
> > +	EXCLUSIVE,	/* Hold ref to page and take the bit when woken, like
> > +			 * __lock_page() waiting on then setting PG_locked.
> > +			 */
> > +	SHARED,		/* Hold ref to page and check the bit when woken, like
> > +			 * wait_on_page_writeback() waiting on PG_writeback.
> > +			 */
> > +	DROP,		/* Drop ref to page before wait, no check when woken,
> > +			 * like put_and_wait_on_page_locked() on PG_locked.
> > +			 */
> > +};
> 
> Can we please make it:
> 
> /**
>  * enum behavior - a choice of three behaviors for wait_on_page_bit_common()
>  */
> enum behavior {
> 	/**
> 	 * @EXCLUSIVE: Hold ref to page and take the bit when woken,
> 	 * like __lock_page() waiting on then setting %PG_locked.
> 	 */
> 	EXCLUSIVE,
> 	/**
> 	 * @SHARED: Hold ref to page and check the bit when woken,
> 	 * like wait_on_page_writeback() waiting on %PG_writeback.
> 	 */
> 	SHARED,
> 	/**
> 	 * @DROP: Drop ref to page before wait, no check when woken,
> 	 * like put_and_wait_on_page_locked() on %PG_locked.
> 	 */
> 	DROP,
> };

I'm with Matthew, I'd prefer not: the first looks a more readable,
less cluttered comment to me than the second: this is just an arg
to an internal helper in mm/filemap.c, itself not kernel-doc'ed.

But the comment is not there for me: if consensus is that the
second is preferable, then sure, we can change it over.

Hugh