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

[Mina Almasry <almasrymina@google.com>]

On Thu, Nov 4, 2021 at 3:08 PM Matthew Wilcox <willy@infradead.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.