linux-kernel - Re: [PATCH V4 4/6] mm: reclaim MADV

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Message-ID: <20170223171901.GA20444@shli-mbp.local>
Date:   Thu, 23 Feb 2017 09:19:01 -0800
From:   Shaohua Li <shli@...com>
To:     Johannes Weiner <hannes@...xchg.org>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <Kernel-team@...com>, <mhocko@...e.com>, <minchan@...nel.org>,
        <hughd@...gle.com>, <riel@...hat.com>,
        <mgorman@...hsingularity.net>, <akpm@...ux-foundation.org>
Subject: Re: [PATCH V4 4/6] mm: reclaim MADV_FREE pages

On Thu, Feb 23, 2017 at 11:13:42AM -0500, Johannes Weiner wrote:
> On Wed, Feb 22, 2017 at 10:50:42AM -0800, Shaohua Li wrote:
> > @@ -1424,6 +1424,12 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
> >  				dec_mm_counter(mm, MM_ANONPAGES);
> >  				rp->lazyfreed++;
> >  				goto discard;
> > +			} else if (!PageSwapBacked(page)) {
> > +				/* dirty MADV_FREE page */
> > +				set_pte_at(mm, address, pvmw.pte, pteval);
> > +				ret = SWAP_DIRTY;
> > +				page_vma_mapped_walk_done(&pvmw);
> > +				break;
> >  			}
> >  
> >  			if (swap_duplicate(entry) < 0) {
> > @@ -1525,8 +1531,8 @@ int try_to_unmap(struct page *page, enum ttu_flags flags)
> >  
> >  	if (ret != SWAP_MLOCK && !page_mapcount(page)) {
> >  		ret = SWAP_SUCCESS;
> > -		if (rp.lazyfreed && !PageDirty(page))
> > -			ret = SWAP_LZFREE;
> > +		if (rp.lazyfreed && PageDirty(page))
> > +			ret = SWAP_DIRTY;
> 
> Can this actually happen? If the page is dirty, ret should already be
> SWAP_DIRTY, right? How would a dirty page get fully unmapped?
> 
> It seems to me rp.lazyfreed can be removed entirely now that we don't
> have to identify the lazyfree case anymore. The failure case is much
> easier to identify - all it takes is a single pte to be dirty.

ok, I get mixed up. Yes, this couldn't happen any more since we changed the
behavior of try_to_unmap_one. Will delete this in next post.

Thanks,
Shaohua