linux-kernel - Re: [PATCH 3/5] mm: Attempt to migrate page in lieu of discard

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Date:   Fri, 22 Mar 2019 10:34:41 -0600
From:   Keith Busch <kbusch@...nel.org>
To:     Yang Shi <shy828301@...il.com>
Cc:     Keith Busch <keith.busch@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>, linux-nvdimm@...ts.01.org,
        Dave Hansen <dave.hansen@...el.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH 3/5] mm: Attempt to migrate page in lieu of discard

On Thu, Mar 21, 2019 at 04:58:16PM -0700, Yang Shi wrote:
> On Thu, Mar 21, 2019 at 1:03 PM Keith Busch <keith.busch@...el.com> wrote:
> > +               if (!PageCompound(page)) {
> > +                       if (migrate_demote_mapping(page)) {
> > +                                unlock_page(page);
> > +                                if (likely(put_page_testzero(page)))
> > +                                        goto free_it;
> > +
> > +                                /*
> > +                                * Speculative reference will free this page,
> > +                                * so leave it off the LRU.
> > +                                */
> > +                                nr_reclaimed++;
> > +                                continue;
> > +                        }
> > +               }
> 
> It looks the reclaim path would fall through if the migration is
> failed. But, it looks, with patch #4, you may end up trying reclaim an
> anon page on swapless system if migration is failed?

Right, and add_to_swap() will fail and the page jumps to activate_locked
label, placing it back where it was before.

> And, actually I have the same question with Yan Zi. Why not just put
> the demote candidate into a separate list, then migrate all the
> candidates in bulk with migrate_pages()?

The page is already locked at the point we know we want to migrate it.