Re: [PATCH 1/2] fs/proc/task_mmu: add guard region bit to pagemap

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Fri, Feb 21, 2025 at 09:10:42AM -0800, Kalesh Singh wrote:
> On Fri, Feb 21, 2025 at 4:05 AM Lorenzo Stoakes
> <lorenzo.stoakes@oracle.com> wrote:
> >
> > Currently there is no means by which users can determine whether a given
> > page in memory is in fact a guard region, that is having had the
> > MADV_GUARD_INSTALL madvise() flag applied to it.
> >
> > This is intentional, as to provide this information in VMA metadata would
> > contradict the intent of the feature (providing a means to change fault
> > behaviour at a page table level rather than a VMA level), and would require
> > VMA metadata operations to scan page tables, which is unacceptable.
> >
> > In many cases, users have no need to reflect and determine what regions
> > have been designated guard regions, as it is the user who has established
> > them in the first place.
> >
> > But in some instances, such as monitoring software, or software that relies
> > upon being able to ascertain the nature of mappings within a remote process
> > for instance, it becomes useful to be able to determine which pages have
> > the guard region marker applied.
> >
> > This patch makes use of an unused pagemap bit (58) to provide this
> > information.
> >
> > This patch updates the documentation at the same time as making the change
> > such that the implementation of the feature and the documentation of it are
> > tied together.
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> > ---
> >  Documentation/admin-guide/mm/pagemap.rst | 3 ++-
> >  fs/proc/task_mmu.c                       | 6 +++++-
> >  2 files changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
> > index caba0f52dd36..a297e824f990 100644
> > --- a/Documentation/admin-guide/mm/pagemap.rst
> > +++ b/Documentation/admin-guide/mm/pagemap.rst
> > @@ -21,7 +21,8 @@ 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
> >        Documentation/admin-guide/mm/userfaultfd.rst)
> > -    * Bits 58-60 zero
> > +    * Bit  58    pte is a guard region (since 6.15) (see madvise (2) man page)
>
> Should this be 6.14 ?

We're aiming for the 6.15 merge window so this is correct :>) I don't think
this could be considered a hotfix haha!

>
> Other than that: Reviewed-by: Kalesh Singh <kaleshsingh@google.com>

Thanks! And thanks for review on the other patch also! Appreciated.

>
> Thanks,
> Kalesh
>
> > +    * 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 f02cd362309a..c17615e21a5d 100644
> > --- a/fs/proc/task_mmu.c
> > +++ b/fs/proc/task_mmu.c
> > @@ -1632,6 +1632,7 @@ 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_GUARD_REGION                BIT_ULL(58)
> >  #define PM_FILE                        BIT_ULL(61)
> >  #define PM_SWAP                        BIT_ULL(62)
> >  #define PM_PRESENT             BIT_ULL(63)
> > @@ -1732,6 +1733,8 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
> >                         page = pfn_swap_entry_to_page(entry);
> >                 if (pte_marker_entry_uffd_wp(entry))
> >                         flags |= PM_UFFD_WP;
> > +               if (is_guard_swp_entry(entry))
> > +                       flags |=  PM_GUARD_REGION;
> >         }
> >
> >         if (page) {
> > @@ -1931,7 +1934,8 @@ static const struct mm_walk_ops pagemap_ops = {
> >   * Bit  55    pte is soft-dirty (see Documentation/admin-guide/mm/soft-dirty.rst)
> >   * Bit  56    page exclusively mapped
> >   * Bit  57    pte is uffd-wp write-protected
> > - * Bits 58-60 zero
> > + * Bit  58    pte is a guard region
> > + * Bits 59-60 zero
> >   * Bit  61    page is file-page or shared-anon
> >   * Bit  62    page swapped
> >   * Bit  63    page present
> > --
> > 2.48.1
> >