linux-kernel - Re: [RFC PATCH v3 06/10] mm/mshare: Add vm flag for shared PTEs

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Message-ID: <CADrL8HV51t44EBKFwXoT-A48miq2TT7w1yjSUFo6uc5WDN=z9A@mail.gmail.com>
Date: Tue, 3 Sep 2024 16:40:43 -0700
From: James Houghton <jthoughton@...gle.com>
To: Anthony Yznaga <anthony.yznaga@...cle.com>
Cc: akpm@...ux-foundation.org, willy@...radead.org, markhemm@...glemail.com, 
	viro@...iv.linux.org.uk, david@...hat.com, khalid@...nel.org, 
	andreyknvl@...il.com, dave.hansen@...el.com, luto@...nel.org, 
	brauner@...nel.org, arnd@...db.de, ebiederm@...ssion.com, 
	catalin.marinas@....com, linux-arch@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, mhiramat@...nel.org, 
	rostedt@...dmis.org, vasily.averin@...ux.dev, xhao@...ux.alibaba.com, 
	pcc@...gle.com, neilb@...e.de, maz@...nel.org
Subject: Re: [RFC PATCH v3 06/10] mm/mshare: Add vm flag for shared PTEs

On Tue, Sep 3, 2024 at 4:23 PM Anthony Yznaga <anthony.yznaga@...cle.com> wrote:
>
> From: Khalid Aziz <khalid@...nel.org>
>
> Add a bit to vm_flags to indicate a vma shares PTEs with others. Add
> a function to determine if a vma shares PTEs by checking this flag.
> This is to be used to find the shared page table entries on page fault
> for vmas sharing PTEs.
>
> Signed-off-by: Khalid Aziz <khalid@...nel.org>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> Signed-off-by: Anthony Yznaga <anthony.yznaga@...cle.com>
> ---
>  include/linux/mm.h             | 7 +++++++
>  include/trace/events/mmflags.h | 3 +++
>  mm/internal.h                  | 5 +++++
>  3 files changed, 15 insertions(+)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 6549d0979b28..3aa0b3322284 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -413,6 +413,13 @@ extern unsigned int kobjsize(const void *objp);
>  #define VM_DROPPABLE           VM_NONE
>  #endif
>
> +#ifdef CONFIG_64BIT
> +#define VM_SHARED_PT_BIT       41
> +#define VM_SHARED_PT           BIT(VM_SHARED_PT_BIT)
> +#else
> +#define VM_SHARED_PT           VM_NONE
> +#endif
> +
>  #ifdef CONFIG_64BIT
>  /* VM is sealed, in vm_flags */
>  #define VM_SEALED      _BITUL(63)
> diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
> index b63d211bd141..e1ae1e60d086 100644
> --- a/include/trace/events/mmflags.h
> +++ b/include/trace/events/mmflags.h
> @@ -167,8 +167,10 @@ IF_HAVE_PG_ARCH_X(arch_3)
>
>  #ifdef CONFIG_64BIT
>  # define IF_HAVE_VM_DROPPABLE(flag, name) {flag, name},
> +# define IF_HAVE_VM_SHARED_PT(flag, name) {flag, name},
>  #else
>  # define IF_HAVE_VM_DROPPABLE(flag, name)
> +# define IF_HAVE_VM_SHARED_PT(flag, name)
>  #endif
>
>  #define __def_vmaflag_names                                            \
> @@ -204,6 +206,7 @@ IF_HAVE_VM_SOFTDIRTY(VM_SOFTDIRTY,  "softdirty"     )               \
>         {VM_HUGEPAGE,                   "hugepage"      },              \
>         {VM_NOHUGEPAGE,                 "nohugepage"    },              \
>  IF_HAVE_VM_DROPPABLE(VM_DROPPABLE,     "droppable"     )               \
> +IF_HAVE_VM_SHARED_PT(VM_SHARED_PT,     "sharedpt"      )               \
>         {VM_MERGEABLE,                  "mergeable"     }               \
>
>  #define show_vma_flags(flags)                                          \
> diff --git a/mm/internal.h b/mm/internal.h
> index b4d86436565b..8005d5956b6e 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -1578,4 +1578,9 @@ void unlink_file_vma_batch_init(struct unlink_vma_file_batch *);
>  void unlink_file_vma_batch_add(struct unlink_vma_file_batch *, struct vm_area_struct *);
>  void unlink_file_vma_batch_final(struct unlink_vma_file_batch *);
>

Hi Anthony,

I'm really excited to see this series on the mailing list again! :) I
won't have time to review this series in too much detail, but I hope
something like it gets merged eventually.

> +static inline bool vma_is_shared(const struct vm_area_struct *vma)
> +{
> +       return VM_SHARED_PT && (vma->vm_flags & VM_SHARED_PT);
> +}

Tiny comment - I find vma_is_shared() to be a bit of a confusing name,
especially given how vma_is_shared_maywrite() is defined. (Sorry if
this has already been discussed before.)

How about vma_is_shared_pt()?