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Subject: Re: [PATCH 3/4] mm/vmscan: Attempt to migrate page in lieu of discard
To:     Yang Shi <shy828301@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Keith Busch <keith.busch@intel.com>
References: <20191016221148.F9CCD155@viggo.jf.intel.com>
 <20191016221152.BF2171A3@viggo.jf.intel.com>
 <CAHbLzkp_2UD50Vt8f_atxKcz4x8J3GB3YzTqMOd6Src_y2Yg2g@mail.gmail.com>
From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <6ab6ca82-71d4-bdea-ae95-e0bebb5e71df@intel.com>
Date:   Fri, 18 Oct 2019 11:15:07 -0700
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On 10/17/19 10:30 AM, Yang Shi wrote:
>> +               if (!PageHuge(page)) {
>> +                       int rc = migrate_demote_mapping(page);
>> +
>> +                       /*
>> +                        * -ENOMEM on a THP may indicate either migration is
>> +                        * unsupported or there was not enough contiguous
>> +                        * space. Split the THP into base pages and retry the
>> +                        * head immediately. The tail pages will be considered
>> +                        * individually within the current loop's page list.
>> +                        */
>> +                       if (rc == -ENOMEM && PageTransHuge(page) &&
>> +                           !split_huge_page_to_list(page, page_list))
>> +                               rc = migrate_demote_mapping(page);
> I recalled when Keith posted the patch at the first time, I raised
> question about why not just migrating THP in a whole? The
> migrate_pages() could handle this. If it fails, it just fallbacks to
> base page.

There's a pair of migrate_demote_mapping()s in there.  I expected that
the first will migrate the whole THP and the second plus the split is
only used if fails the whole migration.

Am I reading it wrong?