linux-kernel - Re: [PATCH v2] mm: Add PM

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Message-ID: <CAHS8izPisLXTmAsvZR6w2piSXPidVvJMHfQR7oikJgkuTJyRQA@mail.gmail.com>
Date:   Thu, 4 Nov 2021 15:45:07 -0700
From:   Mina Almasry <almasrymina@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     "Paul E . McKenney" <paulmckrcu@...com>,
        Yu Zhao <yuzhao@...gle.com>, Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>,
        Ivan Teterevkov <ivan.teterevkov@...anix.com>,
        David Hildenbrand <david@...hat.com>,
        Florian Schmidt <florian.schmidt@...anix.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v2] mm: Add PM_THP to /proc/pid/pagemap

On Thu, Nov 4, 2021 at 3:08 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Thu, Nov 04, 2021 at 02:46:35PM -0700, Mina Almasry wrote:
> > Add PM_THP to allow userspace to detect whether a given virt address is
> > currently mapped by a hugepage or not.
>
> Well, no, that's not what that means.
>

Sorry, that was the intention, but I didn't implement the intention correctly.

> > @@ -1396,6 +1397,8 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
> >               flags |= PM_FILE;
> >       if (page && page_mapcount(page) == 1)
> >               flags |= PM_MMAP_EXCLUSIVE;
> > +     if (page && PageTransCompound(page))
> > +             flags |= PM_THP;
>
> All that PageTransCompound() does is call PageCompound().  It doesn't
> tell you if the underlying allocation is PMD sized, nor properly aligned.
>
> And you didn't answer my question about whether you want information about
> whether a large page is being used that's not quite as large as a PMD.
>

Sorry, I thought the implementation would make it clear but I didn't
do that correctly. Right now and for the foreseeable future what I
want to know is whether the page is mapped by a PMD. All the below
work for me:

1. Flag is set if the page is either a PMD size THP page.
2. Flag is set if the page is either a PMD size THP page or PMD size
hugetlbfs page.
3. Flag is set if the page is either a PMD size THP page or PMD size
hugetlbfs page or contig PTE size hugetlbfs page.

I prefer #2 and I think it's maybe most extensible for future use
cases that 1 flag tells whether the page is PMD hugepage and another
flag is a large cont PTE page.