Re: [PATCH v2 1/6] mm: introduce vma->vm_flags modifier functions

Re: [PATCH v2 1/6] mm: introduce vma->vm_flags modifier functions

[Peter Zijlstra]

On Wed, Jan 25, 2023 at 12:38:46AM -0800, Suren Baghdasaryan wrote:

> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 2d6d790d9bed..6c7c70bf50dd 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -491,7 +491,13 @@ struct vm_area_struct {
>  	 * See vmf_insert_mixed_prot() for discussion.
>  	 */
>  	pgprot_t vm_page_prot;
> -	unsigned long vm_flags;		/* Flags, see mm.h. */
> +
> +	/*
> +	 * Flags, see mm.h.
> +	 * WARNING! Do not modify directly.
> +	 * Use {init|reset|set|clear|mod}_vm_flags() functions instead.
> +	 */
> +	unsigned long vm_flags;

We have __private and ACCESS_PRIVATE() to help with enforcing this.
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Re: [PATCH v2 1/6] mm: introduce vma->vm_flags modifier functions

[Matthew Wilcox]

On Wed, Jan 25, 2023 at 08:49:50AM -0800, Suren Baghdasaryan wrote:
> On Wed, Jan 25, 2023 at 1:10 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > > +     /*
> > > +      * Flags, see mm.h.
> > > +      * WARNING! Do not modify directly.
> > > +      * Use {init|reset|set|clear|mod}_vm_flags() functions instead.
> > > +      */
> > > +     unsigned long vm_flags;
> >
> > We have __private and ACCESS_PRIVATE() to help with enforcing this.
> 
> Thanks for pointing this out, Peter! I guess for that I'll need to
> convert all read accesses and provide get_vm_flags() too? That will
> cause some additional churt (a quick search shows 801 hits over 248
> files) but maybe it's worth it? I think Michal suggested that too in
> another patch. Should I do that while we are at it?

Here's a trick I saw somewhere in the VFS:

	union {
		const vm_flags_t vm_flags;
		vm_flags_t __private __vm_flags;
	};

Now it can be read by anybody but written only by those using
ACCESS_PRIVATE.
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Re: [PATCH v2 1/6] mm: introduce vma->vm_flags modifier functions

[Matthew Wilcox]

On Wed, Jan 25, 2023 at 12:38:46AM -0800, Suren Baghdasaryan wrote:
> +/* Use when VMA is not part of the VMA tree and needs no locking */
> +static inline void init_vm_flags(struct vm_area_struct *vma,
> +				 unsigned long flags)
> +{
> +	vma->vm_flags = flags;

vm_flags are supposed to have type vm_flags_t.  That's not been
fully realised yet, but perhaps we could avoid making it worse?

>  	pgprot_t vm_page_prot;
> -	unsigned long vm_flags;		/* Flags, see mm.h. */
> +
> +	/*
> +	 * Flags, see mm.h.
> +	 * WARNING! Do not modify directly.
> +	 * Use {init|reset|set|clear|mod}_vm_flags() functions instead.
> +	 */
> +	unsigned long vm_flags;

Including changing this line to vm_flags_t
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Re: [PATCH v2 1/6] mm: introduce vma->vm_flags modifier functions

[Matthew Wilcox]

On Thu, Jan 26, 2023 at 04:50:59PM +0200, Mike Rapoport wrote:
> On Thu, Jan 26, 2023 at 11:17:09AM +0200, Mike Rapoport wrote:
> > On Wed, Jan 25, 2023 at 12:38:46AM -0800, Suren Baghdasaryan wrote:
> > > +/* Use when VMA is not part of the VMA tree and needs no locking */
> > > +static inline void init_vm_flags(struct vm_area_struct *vma,
> > > +				 unsigned long flags)
> > 
> > I'd suggest to make it vm_flags_init() etc.
> 
> Thinking more about it, it will be even clearer to name these vma_flags_xyz()

Perhaps vma_VERB_flags()?

vma_init_flags()
vma_reset_flags()
vma_set_flags()
vma_clear_flags()
vma_mod_flags()

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