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Message-ID: <177cb5d1-4fde-4fa0-adbc-8e295fba403b@lucifer.local>
Date: Thu, 12 Jun 2025 17:49:35 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, nvdimm@...ts.linux.dev,
linux-cxl@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
Alistair Popple <apopple@...dia.com>,
"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>, Dan Williams <dan.j.williams@...el.com>,
Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH v2 3/3] mm/huge_memory: don't mark refcounted folios
special in vmf_insert_folio_pud()
On Wed, Jun 11, 2025 at 02:06:54PM +0200, David Hildenbrand wrote:
> Marking PUDs that map a "normal" refcounted folios as special is
> against our rules documented for vm_normal_page().
Might be worth referring to specifically which rule. I'm guessing it's the
general one of special == don't touch (from vm_normal_page() comment):
/*
* vm_normal_page -- This function gets the "struct page" associated with a pte.
*
* "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
* case, NULL is returned here. "Normal" mappings do have a struct page.
*
* ...
*
*/
But don't we already violate this E.g.:
if (vma->vm_ops && vma->vm_ops->find_special_page)
return vma->vm_ops->find_special_page(vma, addr);
I mean this in itself perhaps means we should update this comment to say 'except
when file-backed and there is a find_special_page() hook'.
>
> Fortunately, there are not that many pud_special() check that can be
> mislead and are right now rather harmless: e.g., none so far
> bases decisions whether to grab a folio reference on that decision.
>
> Well, and GUP-fast will fallback to GUP-slow. All in all, so far no big
> implications as it seems.
>
> Getting this right will get more important as we introduce
> folio_normal_page_pud() and start using it in more place where we
> currently special-case based on other VMA flags.
>
> Fix it just like we fixed vmf_insert_folio_pmd().
>
> Add folio_mk_pud() to mimic what we do with folio_mk_pmd().
>
> Fixes: dbe54153296d ("mm/huge_memory: add vmf_insert_folio_pud()")
> Signed-off-by: David Hildenbrand <david@...hat.com>
LGTM, so:
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Couple nits/comments below.
> ---
> include/linux/mm.h | 19 ++++++++++++++++-
> mm/huge_memory.c | 51 +++++++++++++++++++++++++---------------------
> 2 files changed, 46 insertions(+), 24 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index fa538feaa8d95..912b6d40a12d6 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1816,7 +1816,24 @@ static inline pmd_t folio_mk_pmd(struct folio *folio, pgprot_t pgprot)
> {
> return pmd_mkhuge(pfn_pmd(folio_pfn(folio), pgprot));
> }
> -#endif
> +
> +#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
> +/**
> + * folio_mk_pud - Create a PUD for this folio
> + * @folio: The folio to create a PUD for
> + * @pgprot: The page protection bits to use
> + *
> + * Create a page table entry for the first page of this folio.
> + * This is suitable for passing to set_pud_at().
> + *
> + * Return: A page table entry suitable for mapping this folio.
> + */
> +static inline pud_t folio_mk_pud(struct folio *folio, pgprot_t pgprot)
Nice to have some consistency around pud, it seems so often we do a pmd version
of relevant functions then with pud we go 'meh whatever' :)
> +{
> + return pud_mkhuge(pfn_pud(folio_pfn(folio), pgprot));
> +}
> +#endif /* CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD */
> +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> #endif /* CONFIG_MMU */
>
> static inline bool folio_has_pincount(const struct folio *folio)
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 7e3e9028873e5..4734de1dc0ae4 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1535,15 +1535,18 @@ static pud_t maybe_pud_mkwrite(pud_t pud, struct vm_area_struct *vma)
> return pud;
> }
>
> -static void insert_pfn_pud(struct vm_area_struct *vma, unsigned long addr,
> - pud_t *pud, pfn_t pfn, pgprot_t prot, bool write)
> +static void insert_pud(struct vm_area_struct *vma, unsigned long addr,
> + pud_t *pud, struct folio_or_pfn fop, pgprot_t prot, bool write)
> {
> struct mm_struct *mm = vma->vm_mm;
> pud_t entry;
>
> if (!pud_none(*pud)) {
> + const unsigned long pfn = fop.is_folio ? folio_pfn(fop.folio) :
> + pfn_t_to_pfn(fop.pfn);
> +
> if (write) {
> - if (WARN_ON_ONCE(pud_pfn(*pud) != pfn_t_to_pfn(pfn)))
> + if (WARN_ON_ONCE(pud_pfn(*pud) != pfn))
> return;
> entry = pud_mkyoung(*pud);
> entry = maybe_pud_mkwrite(pud_mkdirty(entry), vma);
> @@ -1553,11 +1556,19 @@ static void insert_pfn_pud(struct vm_area_struct *vma, unsigned long addr,
> return;
> }
>
> - entry = pud_mkhuge(pfn_t_pud(pfn, prot));
> - if (pfn_t_devmap(pfn))
> - entry = pud_mkdevmap(entry);
> - else
> - entry = pud_mkspecial(entry);
> + if (fop.is_folio) {
> + entry = folio_mk_pud(fop.folio, vma->vm_page_prot);
> +
> + folio_get(fop.folio);
> + folio_add_file_rmap_pud(fop.folio, &fop.folio->page, vma);
> + add_mm_counter(mm, mm_counter_file(fop.folio), HPAGE_PUD_NR);
Nit, but might be nice to abstract for PMD/PUD.
> + } else {
> + entry = pud_mkhuge(pfn_t_pud(fop.pfn, prot));
Same incredibly pedantic whitespace comment from previous patch :)
> + if (pfn_t_devmap(fop.pfn))
> + entry = pud_mkdevmap(entry);
> + else
> + entry = pud_mkspecial(entry);
> + }
> if (write) {
> entry = pud_mkyoung(pud_mkdirty(entry));
> entry = maybe_pud_mkwrite(entry, vma);
> @@ -1581,6 +1592,9 @@ vm_fault_t vmf_insert_pfn_pud(struct vm_fault *vmf, pfn_t pfn, bool write)
> unsigned long addr = vmf->address & PUD_MASK;
> struct vm_area_struct *vma = vmf->vma;
> pgprot_t pgprot = vma->vm_page_prot;
> + struct folio_or_pfn fop = {
> + .pfn = pfn,
> + };
> spinlock_t *ptl;
>
> /*
> @@ -1600,7 +1614,7 @@ vm_fault_t vmf_insert_pfn_pud(struct vm_fault *vmf, pfn_t pfn, bool write)
> pfnmap_setup_cachemode_pfn(pfn_t_to_pfn(pfn), &pgprot);
>
> ptl = pud_lock(vma->vm_mm, vmf->pud);
> - insert_pfn_pud(vma, addr, vmf->pud, pfn, pgprot, write);
> + insert_pud(vma, addr, vmf->pud, fop, pgprot, write);
> spin_unlock(ptl);
>
> return VM_FAULT_NOPAGE;
> @@ -1622,6 +1636,10 @@ vm_fault_t vmf_insert_folio_pud(struct vm_fault *vmf, struct folio *folio,
> unsigned long addr = vmf->address & PUD_MASK;
> pud_t *pud = vmf->pud;
> struct mm_struct *mm = vma->vm_mm;
> + struct folio_or_pfn fop = {
> + .folio = folio,
> + .is_folio = true,
> + };
> spinlock_t *ptl;
>
> if (addr < vma->vm_start || addr >= vma->vm_end)
> @@ -1631,20 +1649,7 @@ vm_fault_t vmf_insert_folio_pud(struct vm_fault *vmf, struct folio *folio,
> return VM_FAULT_SIGBUS;
>
> ptl = pud_lock(mm, pud);
> -
> - /*
> - * If there is already an entry present we assume the folio is
> - * already mapped, hence no need to take another reference. We
> - * still call insert_pfn_pud() though in case the mapping needs
> - * upgrading to writeable.
> - */
> - if (pud_none(*vmf->pud)) {
> - folio_get(folio);
> - folio_add_file_rmap_pud(folio, &folio->page, vma);
> - add_mm_counter(mm, mm_counter_file(folio), HPAGE_PUD_NR);
> - }
> - insert_pfn_pud(vma, addr, vmf->pud, pfn_to_pfn_t(folio_pfn(folio)),
> - vma->vm_page_prot, write);
> + insert_pud(vma, addr, vmf->pud, fop, vma->vm_page_prot, write);
> spin_unlock(ptl);
>
> return VM_FAULT_NOPAGE;
> --
> 2.49.0
>
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