Message-ID: <26e23ce3-6108-4428-902c-95863f25d3ff@nvidia.com>
Date: Wed, 8 Oct 2025 19:33:58 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Jakub Acs <acsjakub@...zon.de>, aliceryhl@...gle.com, djwong@...nel.org
Cc: akpm@...ux-foundation.org, axelrasmussen@...gle.com,
 chengming.zhou@...ux.dev, david@...hat.com, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, peterx@...hat.com,
 rust-for-linux@...r.kernel.org, xu.xin16@....com.cn
Subject: Re: [PATCH] mm: use enum for vm_flags

On 10/8/25 5:54 AM, Jakub Acs wrote:
> redefine VM_* flag constants with BIT()
> 
> Make VM_* flag constant definitions consistent - unify all to use BIT()
> macro and define them within an enum.
> 
> The bindgen tool is better able to handle BIT(_) declarations when used
> in an enum.
> 
> Also add enum definitions for tracepoints.
> 
> We have previously changed VM_MERGEABLE in a separate bugfix. This is a
> follow-up to make all the VM_* flag constant definitions consistent, as
> suggested by David in [1].
> 
> [1]: https://lore.kernel.org/all/85f852f9-8577-4230-adc7-c52e7f479454@redhat.com/
> 
> Signed-off-by: Jakub Acs <acsjakub@...zon.de>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Xu Xin <xu.xin16@....com.cn>
> Cc: Chengming Zhou <chengming.zhou@...ux.dev>
> Cc: Peter Xu <peterx@...hat.com>
> Cc: Axel Rasmussen <axelrasmussen@...gle.com>
> Cc: linux-mm@...ck.org
> Cc: linux-kernel@...r.kernel.org
> ---
> 
> Hi Alice,
> 
> thanks for the patch, I squashed it in (should I add your signed-off-by
> too?) and added the TRACE_DEFINE_ENUM calls pointed out by Derrick.
> 
> I have the following points to still address, though: 
> 
> - can the fact that we're not controlling the type of the values if
>   using enum be a problem? (likely the indirect control we have through
>   the highest value is good enough, but I'm not sure)
> 
> - where do TRACE_DEFINE_ENUM calls belong?
>   I see them placed e.g. in include/trace/misc/nfs.h for nfs or
>   arch/x86/kvm/mmu/mmutrace.h, but I don't see a corresponding file for
>   mm.h - does this warrant creating a separate file for these
>   definitions?
> 
> - with the need for TRACE_DEFINE_ENUM calls, do we still deem this
>   to be a good trade-off? - isn't fixing all of these in
>   rust/bindings/bindings_helper.h better?
> 
> @Derrick, can you point me to how to test for the issue you pointed out?
> 
> Thanks,
> Jakub
> 
> 
>  include/linux/mm.h              | 142 ++++++++++++++++++++++----------
>  rust/bindings/bindings_helper.h |   1 -
>  2 files changed, 98 insertions(+), 45 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 70a2a76007d4..8b9e7a9e7042 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -36,6 +36,7 @@
>  #include <linux/rcuwait.h>
>  #include <linux/bitmap.h>
>  #include <linux/bitops.h>
> +#include <linux/tracepoint.h>
>  
>  struct mempolicy;
>  struct anon_vma;
> @@ -273,57 +274,58 @@ extern unsigned int kobjsize(const void *objp);
>   * vm_flags in vm_area_struct, see mm_types.h.
>   * When changing, update also include/trace/events/mmflags.h
>   */
> -#define VM_NONE		0x00000000
> +enum {
> +	VM_NONE		= 0,
>  
> -#define VM_READ		0x00000001	/* currently active flags */
> -#define VM_WRITE	0x00000002
> -#define VM_EXEC		0x00000004
> -#define VM_SHARED	0x00000008
> +	VM_READ		= BIT(0),		/* currently active flags */
> +	VM_WRITE	= BIT(1),
> +	VM_EXEC		= BIT(2),
> +	VM_SHARED	= BIT(3),
>  
>  /* mprotect() hardcodes VM_MAYREAD >> 4 == VM_READ, and so for r/w/x bits. */
> -#define VM_MAYREAD	0x00000010	/* limits for mprotect() etc */
> -#define VM_MAYWRITE	0x00000020
> -#define VM_MAYEXEC	0x00000040
> -#define VM_MAYSHARE	0x00000080
> +	VM_MAYREAD	= BIT(4),		/* limits for mprotect() etc */
> +	VM_MAYWRITE	= BIT(5),
> +	VM_MAYEXEC	= BIT(6),
> +	VM_MAYSHARE	= BIT(7),
>  
> -#define VM_GROWSDOWN	0x00000100	/* general info on the segment */
> +	VM_GROWSDOWN	= BIT(8),		/* general info on the segment */
>  #ifdef CONFIG_MMU
> -#define VM_UFFD_MISSING	0x00000200	/* missing pages tracking */
> +	VM_UFFD_MISSING	= BIT(9),		/* missing pages tracking */
>  #else /* CONFIG_MMU */
> -#define VM_MAYOVERLAY	0x00000200	/* nommu: R/O MAP_PRIVATE mapping that might overlay a file mapping */
> +	VM_MAYOVERLAY	= BIT(9),		/* nommu: R/O MAP_PRIVATE mapping that might overlay a file mapping */
>  #define VM_UFFD_MISSING	0
>  #endif /* CONFIG_MMU */
> -#define VM_PFNMAP	0x00000400	/* Page-ranges managed without "struct page", just pure PFN */
> -#define VM_UFFD_WP	0x00001000	/* wrprotect pages tracking */
> -
> -#define VM_LOCKED	0x00002000
> -#define VM_IO           0x00004000	/* Memory mapped I/O or similar */
> -
> -					/* Used by sys_madvise() */
> -#define VM_SEQ_READ	0x00008000	/* App will access data sequentially */
> -#define VM_RAND_READ	0x00010000	/* App will not benefit from clustered reads */
> -
> -#define VM_DONTCOPY	0x00020000      /* Do not copy this vma on fork */
> -#define VM_DONTEXPAND	0x00040000	/* Cannot expand with mremap() */
> -#define VM_LOCKONFAULT	0x00080000	/* Lock the pages covered when they are faulted in */
> -#define VM_ACCOUNT	0x00100000	/* Is a VM accounted object */
> -#define VM_NORESERVE	0x00200000	/* should the VM suppress accounting */
> -#define VM_HUGETLB	0x00400000	/* Huge TLB Page VM */
> -#define VM_SYNC		0x00800000	/* Synchronous page faults */
> -#define VM_ARCH_1	0x01000000	/* Architecture-specific flag */
> -#define VM_WIPEONFORK	0x02000000	/* Wipe VMA contents in child. */
> -#define VM_DONTDUMP	0x04000000	/* Do not include in the core dump */
> +	VM_PFNMAP	= BIT(10),		/* Page-ranges managed without "struct page", just pure PFN */
> +	VM_UFFD_WP	= BIT(12),		/* wrprotect pages tracking */
> +
> +	VM_LOCKED	= BIT(13),
> +	VM_IO           = BIT(14),		/* Memory mapped I/O or similar */
> +
> +						/* Used by sys_madvise() */
> +	VM_SEQ_READ	= BIT(15),		/* App will access data sequentially */
> +	VM_RAND_READ	= BIT(16),		/* App will not benefit from clustered reads */
> +
> +	VM_DONTCOPY	= BIT(17),		/* Do not copy this vma on fork */
> +	VM_DONTEXPAND	= BIT(18),		/* Cannot expand with mremap() */
> +	VM_LOCKONFAULT	= BIT(19),		/* Lock the pages covered when they are faulted in */
> +	VM_ACCOUNT	= BIT(20),		/* Is a VM accounted object */
> +	VM_NORESERVE	= BIT(21),		/* should the VM suppress accounting */
> +	VM_HUGETLB	= BIT(22),		/* Huge TLB Page VM */
> +	VM_SYNC		= BIT(23),		/* Synchronous page faults */
> +	VM_ARCH_1	= BIT(24),		/* Architecture-specific flag */
> +	VM_WIPEONFORK	= BIT(25),		/* Wipe VMA contents in child. */
> +	VM_DONTDUMP	= BIT(26),		/* Do not include in the core dump */
>  
>  #ifdef CONFIG_MEM_SOFT_DIRTY
> -# define VM_SOFTDIRTY	0x08000000	/* Not soft dirty clean area */
> +	VM_SOFTDIRTY	= BIT(27),		/* Not soft dirty clean area */
>  #else
>  # define VM_SOFTDIRTY	0
>  #endif
>  
> -#define VM_MIXEDMAP	0x10000000	/* Can contain "struct page" and pure PFN pages */
> -#define VM_HUGEPAGE	0x20000000	/* MADV_HUGEPAGE marked this vma */
> -#define VM_NOHUGEPAGE	0x40000000	/* MADV_NOHUGEPAGE marked this vma */
> -#define VM_MERGEABLE	BIT(31)		/* KSM may merge identical pages */
> +	VM_MIXEDMAP	= BIT(28),		/* Can contain "struct page" and pure PFN pages */
> +	VM_HUGEPAGE	= BIT(29),		/* MADV_HUGEPAGE marked this vma */
> +	VM_NOHUGEPAGE	= BIT(30),		/* MADV_NOHUGEPAGE marked this vma */
> +	VM_MERGEABLE	= BIT(31),		/* KSM may merge identical pages */
>  
>  #ifdef CONFIG_ARCH_USES_HIGH_VMA_FLAGS
>  #define VM_HIGH_ARCH_BIT_0	32	/* bit only usable on 64-bit architectures */
> @@ -333,14 +335,66 @@ extern unsigned int kobjsize(const void *objp);
>  #define VM_HIGH_ARCH_BIT_4	36	/* bit only usable on 64-bit architectures */
>  #define VM_HIGH_ARCH_BIT_5	37	/* bit only usable on 64-bit architectures */
>  #define VM_HIGH_ARCH_BIT_6	38	/* bit only usable on 64-bit architectures */

Is there really any value in having these defines, at this point?

See below...

> -#define VM_HIGH_ARCH_0	BIT(VM_HIGH_ARCH_BIT_0)
> -#define VM_HIGH_ARCH_1	BIT(VM_HIGH_ARCH_BIT_1)
> -#define VM_HIGH_ARCH_2	BIT(VM_HIGH_ARCH_BIT_2)
> -#define VM_HIGH_ARCH_3	BIT(VM_HIGH_ARCH_BIT_3)
> -#define VM_HIGH_ARCH_4	BIT(VM_HIGH_ARCH_BIT_4)
> -#define VM_HIGH_ARCH_5	BIT(VM_HIGH_ARCH_BIT_5)
> -#define VM_HIGH_ARCH_6	BIT(VM_HIGH_ARCH_BIT_6)
> +	VM_HIGH_ARCH_0	= BIT(VM_HIGH_ARCH_BIT_0),

Or just omit those bit 0 to bit 7 defines, and write:

	VM_HIGH_ARCH_0	= BIT(32),
	...etc

> +	VM_HIGH_ARCH_1	= BIT(VM_HIGH_ARCH_BIT_1),
> +	VM_HIGH_ARCH_2	= BIT(VM_HIGH_ARCH_BIT_2),
> +	VM_HIGH_ARCH_3	= BIT(VM_HIGH_ARCH_BIT_3),
> +	VM_HIGH_ARCH_4	= BIT(VM_HIGH_ARCH_BIT_4),
> +	VM_HIGH_ARCH_5	= BIT(VM_HIGH_ARCH_BIT_5),
> +	VM_HIGH_ARCH_6	= BIT(VM_HIGH_ARCH_BIT_6),


>  #endif /* CONFIG_ARCH_USES_HIGH_VMA_FLAGS */
> +};
> +
> +TRACE_DEFINE_ENUM(VM_NONE);
> +TRACE_DEFINE_ENUM(VM_READ);
> +TRACE_DEFINE_ENUM(VM_WRITE);
> +TRACE_DEFINE_ENUM(VM_EXEC);
> +TRACE_DEFINE_ENUM(VM_SHARED);
> +TRACE_DEFINE_ENUM(VM_MAYREAD);
> +TRACE_DEFINE_ENUM(VM_MAYWRITE);
> +TRACE_DEFINE_ENUM(VM_MAYEXEC);
> +TRACE_DEFINE_ENUM(VM_MAYSHARE);
> +TRACE_DEFINE_ENUM(VM_GROWSDOWN);
> +TRACE_DEFINE_ENUM(VM_UFFD_MISSING);
> +
> +#ifndef CONFIG_MMU
> +TRACE_DEFINE_ENUM(VM_MAYOVERLAY);

The ifdef handling here for these two items, VM_UFFD_MISSING
and VM_MAYOVERLAY, does not seem to match the way they are handled
above in the enum.

> +#endif /* CONFIG_MMU */
> +
> +TRACE_DEFINE_ENUM(VM_PFNMAP);
> +TRACE_DEFINE_ENUM(VM_UFFD_WP);
> +TRACE_DEFINE_ENUM(VM_LOCKED);
> +TRACE_DEFINE_ENUM(VM_IO);
> +TRACE_DEFINE_ENUM(VM_SEQ_READ);
> +TRACE_DEFINE_ENUM(VM_RAND_READ);
> +TRACE_DEFINE_ENUM(VM_DONTCOPY);
> +TRACE_DEFINE_ENUM(VM_DONTEXPAND);
> +TRACE_DEFINE_ENUM(VM_LOCKONFAULT);
> +TRACE_DEFINE_ENUM(VM_ACCOUNT);
> +TRACE_DEFINE_ENUM(VM_NORESERVE);
> +TRACE_DEFINE_ENUM(VM_HUGETLB);
> +TRACE_DEFINE_ENUM(VM_SYNC);
> +TRACE_DEFINE_ENUM(VM_ARCH_1);
> +TRACE_DEFINE_ENUM(VM_WIPEONFORK);
> +TRACE_DEFINE_ENUM(VM_DONTDUMP);
> +
> +TRACE_DEFINE_ENUM(VM_SOFTDIRTY);
> +
> +TRACE_DEFINE_ENUM(VM_MIXEDMAP);
> +TRACE_DEFINE_ENUM(VM_HUGEPAGE);
> +TRACE_DEFINE_ENUM(VM_NOHUGEPAGE);
> +TRACE_DEFINE_ENUM(VM_MERGEABLE);
> +
> +#ifdef CONFIG_ARCH_USES_HIGH_VMA_FLAGS
> +TRACE_DEFINE_ENUM(VM_HIGH_ARCH_0);
> +TRACE_DEFINE_ENUM(VM_HIGH_ARCH_1);
> +TRACE_DEFINE_ENUM(VM_HIGH_ARCH_2);
> +TRACE_DEFINE_ENUM(VM_HIGH_ARCH_3);
> +TRACE_DEFINE_ENUM(VM_HIGH_ARCH_4);
> +TRACE_DEFINE_ENUM(VM_HIGH_ARCH_5);
> +TRACE_DEFINE_ENUM(VM_HIGH_ARCH_6);
> +#endif /* CONFIG_ARCH_USES_HIGH_VMA_FLAGS */
> +
>  
>  #ifdef CONFIG_ARCH_HAS_PKEYS
>  # define VM_PKEY_SHIFT VM_HIGH_ARCH_BIT_0
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index 2e43c66635a2..04b75d4d01c3 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -108,7 +108,6 @@ const xa_mark_t RUST_CONST_HELPER_XA_PRESENT = XA_PRESENT;
>  
>  const gfp_t RUST_CONST_HELPER_XA_FLAGS_ALLOC = XA_FLAGS_ALLOC;
>  const gfp_t RUST_CONST_HELPER_XA_FLAGS_ALLOC1 = XA_FLAGS_ALLOC1;
> -const vm_flags_t RUST_CONST_HELPER_VM_MERGEABLE = VM_MERGEABLE;
>  
>  #if IS_ENABLED(CONFIG_ANDROID_BINDER_IPC_RUST)
>  #include "../../drivers/android/binder/rust_binder.h"

thanks,
-- 
John Hubbard

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