Message-ID: <ca28318f-290e-4d03-b2fe-a6ff00b9a9e6@lucifer.local>
Date: Fri, 18 Jul 2025 13:35:34 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        xen-devel@...ts.xenproject.org, linux-fsdevel@...r.kernel.org,
        nvdimm@...ts.linux.dev, Andrew Morton <akpm@...ux-foundation.org>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Zi Yan <ziy@...dia.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
        Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
        Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
        Hugh Dickins <hughd@...gle.com>, Oscar Salvador <osalvador@...e.de>,
        Lance Yang <lance.yang@...ux.dev>
Subject: Re: [PATCH v2 7/9] mm/memory: factor out common code from
 vm_normal_page_*()

On Thu, Jul 17, 2025 at 10:12:37PM +0200, David Hildenbrand wrote:
> > >
> > > -/*
> > > - * vm_normal_page -- This function gets the "struct page" associated with a pte.
> > > +/**
> > > + * vm_normal_page_pfn() - Get the "struct page" associated with a PFN in a
> > > + *			  non-special page table entry.
> >
> > This is a bit nebulous/confusing, I mean you'll get PTE entries with PTE special
> > bit that'll have a PFN but just no struct page/folio to look at, or should not
> > be touched.
> >
> > So the _pfn() bit doesn't really properly describe what it does.
> >
> > I wonder if it'd be better to just separate out the special handler, have
> > that return a boolean indicating special of either form, and then separate
> > other shared code separately from that?
>
> Let me think about that; I played with various approaches and this was the
> best I was come up with before running in circles.

Thanks

>
> >
> > > + * @vma: The VMA mapping the @pfn.
> > > + * @addr: The address where the @pfn is mapped.
> > > + * @pfn: The PFN.
> > > + * @entry: The page table entry value for error reporting purposes.
> > >    *
> > >    * "Special" mappings do not wish to be associated with a "struct page" (either
> > >    * it doesn't exist, or it exists but they don't want to touch it). In this
> > > @@ -603,10 +608,10 @@ static void print_bad_page_map(struct vm_area_struct *vma,
> > >    * (such as GUP) can still identify these mappings and work with the
> > >    * underlying "struct page".
> > >    *
> > > - * There are 2 broad cases. Firstly, an architecture may define a pte_special()
> > > - * pte bit, in which case this function is trivial. Secondly, an architecture
> > > - * may not have a spare pte bit, which requires a more complicated scheme,
> > > - * described below.
> > > + * There are 2 broad cases. Firstly, an architecture may define a "special"
> > > + * page table entry bit (e.g., pte_special()), in which case this function is
> > > + * trivial. Secondly, an architecture may not have a spare page table
> > > + * entry bit, which requires a more complicated scheme, described below.
> >
> > Strikes me this bit of the comment should be with vm_normal_page(). As this
> > implies the 2 broad cases are handled here and this isn't the case.
>
> Well, pragmatism. Splitting up the doc doesn't make sense. Having it at
> vm_normal_page() doesn't make sense.
>
> I'm sure the educated reader will be able to make sense of it :P
>
> But I'm happy to hear suggestions on how to do it differently :)

Right yeah.

I feel like having separate 'special' handling for each case as separate
functions, each with their own specific explanation would work.

But I don't want to hold up the series _too_ much on this, generally I just
find the _pfn thing confusing.

I mean the implementation is a total pain anyway...

I feel like we could even have separate special handling functions like

#ifdef CONFIG_ARCH_HAS_PTE_SPECIAL

/*
 * < description of pte special special page >
 *
 * If returns true, then pagep set to NULL or, if a page can be found, that
 * page.
 *
 */
static struct bool is_special_page(struct vm_area_struct *vma, unsigned long addr,
		pte_t pte, struct page **pagep)
{
	unsigned long pfn = pte_pfn(pte);

	if (likely(!pte_special(pte))) {
		if (pfn <= highest_memmap_pfn)
			return false;

		goto bad;
	}

	if (vma->vm_ops && vma->vm_ops->find_special_page) {
		*pagep = vma->vm_ops->find_special_page(vma, addr);
		return true;
	} else if (vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP)) {
		goto special;
	}

	if (is_zero_pfn(pfn))
		goto special;

	/* If we reach here something's gone wrong. */

bad:
	print_bad_pte(vma, addr, pte, NULL);
special:
	*pagep = NULL;
	return true;
}
#else
/*
 * < description for not-pte special special page >
 */
static struct bool is_special_page(struct vm_area_struct *vma, unsigned long addr,
		pte_t pte, struct page **pagep)
{
	unsigned long pfn = pte_pfn(pte);

	if (is_zero_pfn(pfn))
		goto special;

	if (vma->vm_flags & VM_MIXEDMAP) {
		if (!pfn_valid(pfn) || is_zero_pfn(pfn))
			goto special;
	} else if (vma->vm_flags & VM_PFNMAP) {
		unsigned long off;

		off = (addr - vma->vm_start) >> PAGE_SHIFT;
		if (pfn == vma->vm_pgoff + off)
			goto special;
		/* Hell's bells we allow CoW !arch_has_pte_special of PFN pages! help! */
		if (!is_cow_mapping(vma->vm_flags))
			goto special;
	}

	if (pfn > highest_memmap_pfn) {
		print_bad_pte(vma, addr, pte, NULL);
		goto special;
	}

	return false;
special:
	*pagep = NULL;
	return true;
}

#endif

And then obviously invoke as makes sense... This is rough and untested,
just to give a sense :>)

>
> >
> > >    *
> > >    * A raw VM_PFNMAP mapping (ie. one that is not COWed) is always considered a
> > >    * special mapping (even if there are underlying and valid "struct pages").
> > > @@ -639,15 +644,72 @@ static void print_bad_page_map(struct vm_area_struct *vma,
> > >    * don't have to follow the strict linearity rule of PFNMAP mappings in
> > >    * order to support COWable mappings.
> > >    *
> > > + * This function is not expected to be called for obviously special mappings:
> > > + * when the page table entry has the "special" bit set.
> >
> > Hmm this is is a bit weird though, saying "obviously" special, because you're
> > handling "special" mappings here, but only for architectures that don't specify
> > the PTE special bit.
> >
> > So it makes it quite nebulous what constitutes 'obviously' here, really you mean
> > pte_special().
>
> Yes, I can clarify that.

Thanks!

>
> >
> > > + *
> > > + * Return: Returns the "struct page" if this is a "normal" mapping. Returns
> > > + *	   NULL if this is a "special" mapping.
> > > + */
> > > +static inline struct page *vm_normal_page_pfn(struct vm_area_struct *vma,
> > > +		unsigned long addr, unsigned long pfn, unsigned long long entry)
> > > +{
> > > +	/*
> > > +	 * With CONFIG_ARCH_HAS_PTE_SPECIAL, any special page table mappings
> > > +	 * (incl. shared zero folios) are marked accordingly and are handled
> > > +	 * by the caller.
> > > +	 */
> > > +	if (!IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL)) {
> > > +		if (unlikely(vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP))) {
> > > +			if (vma->vm_flags & VM_MIXEDMAP) {
> > > +				/* If it has a "struct page", it's "normal". */
> > > +				if (!pfn_valid(pfn))
> > > +					return NULL;
> > > +			} else {
> > > +				unsigned long off = (addr - vma->vm_start) >> PAGE_SHIFT;
> > > +
> > > +				/* Only CoW'ed anon folios are "normal". */
> > > +				if (pfn == vma->vm_pgoff + off)
> > > +					return NULL;
> > > +				if (!is_cow_mapping(vma->vm_flags))
> > > +					return NULL;
> > > +			}
> > > +		}
> > > +
> > > +		if (is_zero_pfn(pfn) || is_huge_zero_pfn(pfn))
> >
> > This handles zero/zero huge page handling for non-pte_special() case
> > only. I wonder if we even need to bother having these marked special
> > generally since you can just check the PFN every time anyway.
>
> Well, that makes (a) pte_special() a bit weird -- not set for some special
> pages and (b) requires additional runtime checks for the case we all really
> care about -- pte_special().
>
> So I don't think we should change that.

OK, best to be consistent in setting for special pages.

>
> [...]
>
> > >
> > > +/**
> > > + * vm_normal_folio() - Get the "struct folio" associated with a PTE
> > > + * @vma: The VMA mapping the @pte.
> > > + * @addr: The address where the @pte is mapped.
> > > + * @pte: The PTE.
> > > + *
> > > + * Get the "struct folio" associated with a PTE. See vm_normal_page_pfn()
> > > + * for details.
> > > + *
> > > + * Return: Returns the "struct folio" if this is a "normal" mapping. Returns
> > > + *	   NULL if this is a "special" mapping.
> > > + */
> >
> > Nice to add a comment, but again feels weird to have the whole explanation in
> > vm_normal_page_pfn() but then to invoke vm_normal_page()..
>
> You want people to do pointer chasing to find what they are looking for? :)

Yes.

Only joking :P

Hopefully the ideas mentioned above clarify things... a bit maybe... :>)

>
> --
> Cheers,
>
> David / dhildenb
>

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