linux-kernel - Re: [PATCH v2 01/13] mm: support madvise(MADV

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Date:	Thu, 5 Nov 2015 08:39:10 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Michael Kerrisk <mtk.manpages@...il.com>,
	linux-api@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Jason Evans <je@...com>, Daniel Micay <danielmicay@...il.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Shaohua Li <shli@...nel.org>, Michal Hocko <mhocko@...e.cz>,
	yalin.wang2010@...il.com
Subject: Re: [PATCH v2 01/13] mm: support madvise(MADV_FREE)

Hi Sergey,

On Wed, Nov 04, 2015 at 11:16:24AM +0900, Sergey Senozhatsky wrote:
> Hi Minchan,
> 
> On (11/04/15 10:25), Minchan Kim wrote:
> [..]
> >+static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> >+                               unsigned long end, struct mm_walk *walk)
> >+
> ...
> > +	if (pmd_trans_unstable(pmd))
> > +		return 0;
> 
> I think it makes sense to update pmd_trans_unstable() and
> pmd_none_or_trans_huge_or_clear_bad() comments in asm-generic/pgtable.h
> Because they explicitly mention MADV_DONTNEED only. Just a thought.

Hmm, When I read comments(but actually I don't understand it 100%), it
says pmd disappearing from MADV_DONTNEED with mmap_sem read-side
lock. But MADV_FREE doesn't remove the pmd. So, I don't understand
what I should add comment. Please suggest if I am missing something.

> 
> 
> > @@ -379,6 +502,14 @@ madvise_vma(struct vm_area_struct *vma, struct vm_area_struct **prev,
> >  		return madvise_remove(vma, prev, start, end);
> >  	case MADV_WILLNEED:
> >  		return madvise_willneed(vma, prev, start, end);
> > +	case MADV_FREE:
> > +		/*
> > +		 * XXX: In this implementation, MADV_FREE works like
> 		  ^^^^
> 		XXX

What does it mean?

> 
> > +		 * MADV_DONTNEED on swapless system or full swap.
> > +		 */
> > +		if (get_nr_swap_pages() > 0)
> > +			return madvise_free(vma, prev, start, end);
> > +		/* passthrough */
> 
> 	-ss
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