linux-kernel - Re: [PATCH 1/4] mm: migrate: use a folio in add_page_for

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Date:   Wed, 9 Aug 2023 15:44:24 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Kefeng Wang <wangkefeng.wang@...wei.com>
Cc:     Zi Yan <ziy@...dia.com>, Naoya Horiguchi <naoya.horiguchi@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Huang Ying <ying.huang@...el.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 1/4] mm: migrate: use a folio in add_page_for_migration()

On 08/09/23 13:53, Mike Kravetz wrote:
> On 08/09/23 20:37, Kefeng Wang wrote:
> > > 
> > > Cc Mike to help us clarify the expected behavior of hugetlb.
> > > 
> > > Hi Mike, what is the expected behavior, if a user tries to use move_pages()
> > > to migrate a non head page of a hugetlb page?
> > 
> > Could you give some advise, thanks
> > 
> 
> Sorry, I was away for a while.
> 
> It seems unfortunate that move_pages says the passed user addresses
> should be aligned to page boundaries.  However, IIUC this is not checked
> or enforced.  Otherwise, passing a hugetlb page should return the same
> error.
> 
> One thought would be that hugetlb mappings should behave the same
> non-hugetlb mappings.  If passed the address of a hugetlb tail page, align
> the address to a hugetlb boundary and migrate the page.  This changes the
> existing behavior.  However, it would be hard to imagine anyone depending
> on this.
> 
> After taking a closer look at the add_page_for_migration(), it seems to
> just ignore passed tail pages and do nothing for such passed addresses.
> Correct?  Or, am I missing something?  Perhaps that is behavior we want/
> need to preserve?

My mistake, status -EACCES is returned when passing a tail page of a
hugetlb page.

Back to the question of 'What is the expected behavior if a tail page is
passed?'.  I do not think we have defined an expected behavior.  If
anything is 'expected' I would say it is -EACCES as returned today.

BTW - hugetlb pages not migrated due to passing a tail page does not
seem to contribute to a 'Positive return value' indicating the number of
non-migrated pages.
-- 
Mike Kravetz