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Message-ID: <20230809224424.GB3537@monkey>
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
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