Re: [PATCH v2] mm: Add PM_THP to /proc/pid/pagemap

[Mina Almasry]

On Thu, Nov 4, 2021 at 3:45 PM Mina Almasry <almasrymina@xxxxxxxxxx> wrote:
>
> On Thu, Nov 4, 2021 at 3:08 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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.
> >

Sorry Matthew again for getting this check wrong. After taking a
deeper look, you're completely correct. My check was returning true on
all compound pages without regard to whether they are actually THP, or
whether they're mapped at the PMD level.

I've renamed the flag from PM_THP to PM_HUGE_THP_MAPPING to be more
accurate, and it looks to me like the correct check is if we're in
pagemap_pmd_range() and the underlying page is_transparent_huegpage(),
then we set the flag.

I'm about to upload v3 with this new check; please take another look.
Thank you for catching this.

> > 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.