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Date:   Wed, 10 Nov 2021 15:03:06 +0800
From:   Peter Xu <peterx@redhat.com>
To:     Mina Almasry <almasrymina@google.com>
Cc:     David Hildenbrand <david@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        "Paul E . McKenney" <paulmckrcu@fb.com>,
        Yu Zhao <yuzhao@google.com>, Jonathan Corbet <corbet@lwn.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ivan Teterevkov <ivan.teterevkov@nutanix.com>,
        Florian Schmidt <florian.schmidt@nutanix.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH v4] mm: Add PM_HUGE_THP_MAPPING to /proc/pid/pagemap
Message-ID: <YYtuqsnOSxA44AUX@t490s>
References: <20211107235754.1395488-1-almasrymina@google.com>
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Hi, Mina,

Sorry to comment late.

On Sun, Nov 07, 2021 at 03:57:54PM -0800, Mina Almasry wrote:
> diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
> index fdc19fbc10839..8a0f0064ff336 100644
> --- a/Documentation/admin-guide/mm/pagemap.rst
> +++ b/Documentation/admin-guide/mm/pagemap.rst
> @@ -23,7 +23,8 @@ There are four components to pagemap:
>      * Bit  56    page exclusively mapped (since 4.2)
>      * Bit  57    pte is uffd-wp write-protected (since 5.13) (see
>        :ref:`Documentation/admin-guide/mm/userfaultfd.rst <userfaultfd>`)
> -    * Bits 57-60 zero
> +    * Bit  58    page is a huge (PMD size) THP mapping
> +    * Bits 59-60 zero
>      * Bit  61    page is file-page or shared-anon (since 3.5)
>      * Bit  62    page swapped
>      * Bit  63    page present
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index ad667dbc96f5c..6f1403f83b310 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1302,6 +1302,7 @@ struct pagemapread {
>  #define PM_SOFT_DIRTY		BIT_ULL(55)
>  #define PM_MMAP_EXCLUSIVE	BIT_ULL(56)
>  #define PM_UFFD_WP		BIT_ULL(57)
> +#define PM_HUGE_THP_MAPPING	BIT_ULL(58)

The ending "_MAPPING" seems redundant to me, how about just call it "PM_THP" or
"PM_HUGE" (as THP also means HUGE already)?

IMHO the core problem is about permission controls, and it seems to me we're
actually trying to workaround it by duplicating some information we have.. so
it's kind of a pity.  Totally not against this patch, but imho it'll be nicer
if it's the permission part that to be enhanced, rather than a new but slightly
duplicated interface.

Thanks,

-- 
Peter Xu