Re: [PATCH V3 3/7] mm: reclaim MADV_FREE pages

[Minchan Kim <minchan@kernel.org>]

On Fri, Feb 17, 2017 at 02:41:08PM +0900, Minchan Kim wrote:
> Hi Johannes,
> 
> 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.
> 
> The reason I introduced it was ttu is used for migration/THP split path
> as well as reclaim. It's clear to discard them in reclaim path because
> it means surely memory pressure now but not sure with other path.
> 
> If you guys think it's always win to discard them in try_to_unmap
> unconditionally, I think it would be better to be separate patch.

I was totally wrong.

Anon page with THP split/migration/HWPoison will not reach to discard path
in try_to_unmap_one so Johannes is right. We don't need TTU_LZFREE.

Sorry for the noise.

Thanks.

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