linux-kernel - Re: [PATCH V3 3/7] mm: reclaim MADV

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Date:   Fri, 17 Feb 2017 11:11:05 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Shaohua Li <shli@...com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Kernel-team@...com, mhocko@...e.com,
        hughd@...gle.com, riel@...hat.com, mgorman@...hsingularity.net,
        akpm@...ux-foundation.org
Subject: Re: [PATCH V3 3/7] mm: reclaim MADV_FREE pages

Hi Minchan,

On Fri, Feb 17, 2017 at 02:45:55PM +0900, Minchan Kim wrote:
> On Thu, Feb 16, 2017 at 04:27:18PM -0800, Shaohua Li wrote:
> > On Thu, Feb 16, 2017 at 01:40:18PM -0500, Johannes Weiner wrote:
> > > On Tue, Feb 14, 2017 at 11:36:09AM -0800, Shaohua Li wrote:
> > > > @@ -1419,11 +1419,18 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
> > > >  			VM_BUG_ON_PAGE(!PageSwapCache(page) && PageSwapBacked(page),
> > > >  				page);
> > > >  
> > > > -			if (!PageDirty(page) && (flags & TTU_LZFREE)) {
> > > > -				/* It's a freeable page by MADV_FREE */
> > > > -				dec_mm_counter(mm, MM_ANONPAGES);
> > > > -				rp->lazyfreed++;
> > > > -				goto discard;
> > > > +			if (flags & TTU_LZFREE) {
> > > > +				if (!PageDirty(page)) {
> > > > +					/* It's a freeable page by MADV_FREE */
> > > > +					dec_mm_counter(mm, MM_ANONPAGES);
> > > > +					rp->lazyfreed++;
> > > > +					goto discard;
> > > > +				} else {
> > > > +					set_pte_at(mm, address, pvmw.pte, pteval);
> > > > +					ret = SWAP_FAIL;
> > > > +					page_vma_mapped_walk_done(&pvmw);
> > > > +					break;
> > > > +				}
> > > 
> > > I don't understand why we need the TTU_LZFREE bit in general. More on
> > > that below at the callsite.
> > 
> > Sounds useless flag, don't see any reason we shouldn't free the MADV_FREE page
> > in places other than reclaim. Looks TTU_UNMAP is useless too..
> 
> Agree on TTU_UNMAP but for example, THP split doesn't mean free lazyfree pages,
> I think.

Anon THP splitting uses the migration branch, so we should be fine.