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Message-ID: <77ed5b25-1a82-44b5-be67-698b9674547f@lucifer.local>
Date: Tue, 12 Aug 2025 20:43:20 +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, linuxppc-dev@...ts.ozlabs.org,
Andrew Morton <akpm@...ux-foundation.org>,
Madhavan Srinivasan <maddy@...ux.ibm.com>,
Michael Ellerman <mpe@...erman.id.au>,
Nicholas Piggin <npiggin@...il.com>,
Christophe Leroy <christophe.leroy@...roup.eu>,
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>,
David Vrabel <david.vrabel@...rix.com>,
Wei Yang <richard.weiyang@...il.com>
Subject: Re: [PATCH v3 11/11] mm: rename vm_ops->find_special_page() to
vm_ops->find_normal_page()
On Mon, Aug 11, 2025 at 01:26:31PM +0200, David Hildenbrand wrote:
> ... and hide it behind a kconfig option. There is really no need for
> any !xen code to perform this check.
>
> The naming is a bit off: we want to find the "normal" page when a PTE
> was marked "special". So it's really not "finding a special" page.
>
> Improve the documentation, and add a comment in the code where XEN ends
> up performing the pte_mkspecial() through a hypercall. More details can
> be found in commit 923b2919e2c3 ("xen/gntdev: mark userspace PTEs as
> special on x86 PV guests").
>
> Cc: David Vrabel <david.vrabel@...rix.com>
> Reviewed-by: Oscar Salvador <osalvador@...e.de>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Reviewed-by: Wei Yang <richard.weiyang@...il.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>
Oh I already reviewed it. But anyway, may as well say - THANKS fof this
it's great again :)
> ---
> drivers/xen/Kconfig | 1 +
> drivers/xen/gntdev.c | 5 +++--
> include/linux/mm.h | 18 +++++++++++++-----
> mm/Kconfig | 2 ++
> mm/memory.c | 12 ++++++++++--
> tools/testing/vma/vma_internal.h | 18 +++++++++++++-----
> 6 files changed, 42 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
> index 24f485827e039..f9a35ed266ecf 100644
> --- a/drivers/xen/Kconfig
> +++ b/drivers/xen/Kconfig
> @@ -138,6 +138,7 @@ config XEN_GNTDEV
> depends on XEN
> default m
> select MMU_NOTIFIER
> + select FIND_NORMAL_PAGE
> help
> Allows userspace processes to use grants.
>
> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> index 1f21607656182..26f13b37c78e6 100644
> --- a/drivers/xen/gntdev.c
> +++ b/drivers/xen/gntdev.c
> @@ -321,6 +321,7 @@ static int find_grant_ptes(pte_t *pte, unsigned long addr, void *data)
> BUG_ON(pgnr >= map->count);
> pte_maddr = arbitrary_virt_to_machine(pte).maddr;
>
> + /* Note: this will perform a pte_mkspecial() through the hypercall. */
> gnttab_set_map_op(&map->map_ops[pgnr], pte_maddr, flags,
> map->grants[pgnr].ref,
> map->grants[pgnr].domid);
> @@ -528,7 +529,7 @@ static void gntdev_vma_close(struct vm_area_struct *vma)
> gntdev_put_map(priv, map);
> }
>
> -static struct page *gntdev_vma_find_special_page(struct vm_area_struct *vma,
> +static struct page *gntdev_vma_find_normal_page(struct vm_area_struct *vma,
> unsigned long addr)
> {
> struct gntdev_grant_map *map = vma->vm_private_data;
> @@ -539,7 +540,7 @@ static struct page *gntdev_vma_find_special_page(struct vm_area_struct *vma,
> static const struct vm_operations_struct gntdev_vmops = {
> .open = gntdev_vma_open,
> .close = gntdev_vma_close,
> - .find_special_page = gntdev_vma_find_special_page,
> + .find_normal_page = gntdev_vma_find_normal_page,
> };
>
> /* ------------------------------------------------------------------ */
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 8ca7d2fa71343..3868ca1a25f9c 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -657,13 +657,21 @@ struct vm_operations_struct {
> struct mempolicy *(*get_policy)(struct vm_area_struct *vma,
> unsigned long addr, pgoff_t *ilx);
> #endif
> +#ifdef CONFIG_FIND_NORMAL_PAGE
> /*
> - * Called by vm_normal_page() for special PTEs to find the
> - * page for @addr. This is useful if the default behavior
> - * (using pte_page()) would not find the correct page.
> + * Called by vm_normal_page() for special PTEs in @vma at @addr. This
> + * allows for returning a "normal" page from vm_normal_page() even
> + * though the PTE indicates that the "struct page" either does not exist
> + * or should not be touched: "special".
> + *
> + * Do not add new users: this really only works when a "normal" page
> + * was mapped, but then the PTE got changed to something weird (+
> + * marked special) that would not make pte_pfn() identify the originally
> + * inserted page.
> */
> - struct page *(*find_special_page)(struct vm_area_struct *vma,
> - unsigned long addr);
> + struct page *(*find_normal_page)(struct vm_area_struct *vma,
> + unsigned long addr);
> +#endif /* CONFIG_FIND_NORMAL_PAGE */
> };
>
> #ifdef CONFIG_NUMA_BALANCING
> diff --git a/mm/Kconfig b/mm/Kconfig
> index e443fe8cd6cf2..59a04d0b2e272 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -1381,6 +1381,8 @@ config PT_RECLAIM
>
> Note: now only empty user PTE page table pages will be reclaimed.
>
> +config FIND_NORMAL_PAGE
> + def_bool n
>
> source "mm/damon/Kconfig"
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 6f806bf3cc994..002c28795d8b7 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -639,6 +639,12 @@ static void print_bad_page_map(struct vm_area_struct *vma,
> * trivial. Secondly, an architecture may not have a spare page table
> * entry bit, which requires a more complicated scheme, described below.
> *
> + * With CONFIG_FIND_NORMAL_PAGE, we might have the "special" bit set on
> + * page table entries that actually map "normal" pages: however, that page
> + * cannot be looked up through the PFN stored in the page table entry, but
> + * instead will be looked up through vm_ops->find_normal_page(). So far, this
> + * only applies to PTEs.
> + *
> * 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").
> * COWed pages of a VM_PFNMAP are always normal.
> @@ -679,8 +685,10 @@ static inline struct page *__vm_normal_page(struct vm_area_struct *vma,
> {
> if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL)) {
> if (unlikely(special)) {
> - if (vma->vm_ops && vma->vm_ops->find_special_page)
> - return vma->vm_ops->find_special_page(vma, addr);
> +#ifdef CONFIG_FIND_NORMAL_PAGE
> + if (vma->vm_ops && vma->vm_ops->find_normal_page)
> + return vma->vm_ops->find_normal_page(vma, addr);
> +#endif /* CONFIG_FIND_NORMAL_PAGE */
> if (vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP))
> return NULL;
> if (is_zero_pfn(pfn) || is_huge_zero_pfn(pfn))
> diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h
> index 3639aa8dd2b06..cb1c2a8afe265 100644
> --- a/tools/testing/vma/vma_internal.h
> +++ b/tools/testing/vma/vma_internal.h
> @@ -467,13 +467,21 @@ struct vm_operations_struct {
> struct mempolicy *(*get_policy)(struct vm_area_struct *vma,
> unsigned long addr, pgoff_t *ilx);
> #endif
> +#ifdef CONFIG_FIND_NORMAL_PAGE
> /*
> - * Called by vm_normal_page() for special PTEs to find the
> - * page for @addr. This is useful if the default behavior
> - * (using pte_page()) would not find the correct page.
> + * Called by vm_normal_page() for special PTEs in @vma at @addr. This
> + * allows for returning a "normal" page from vm_normal_page() even
> + * though the PTE indicates that the "struct page" either does not exist
> + * or should not be touched: "special".
> + *
> + * Do not add new users: this really only works when a "normal" page
> + * was mapped, but then the PTE got changed to something weird (+
> + * marked special) that would not make pte_pfn() identify the originally
> + * inserted page.
> */
> - struct page *(*find_special_page)(struct vm_area_struct *vma,
> - unsigned long addr);
> + struct page *(*find_normal_page)(struct vm_area_struct *vma,
> + unsigned long addr);
> +#endif /* CONFIG_FIND_NORMAL_PAGE */
> };
>
> struct vm_unmapped_area_info {
> --
> 2.50.1
>
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