Re: [PATCH 07/12] huge_memory: Allow mappings of PMD sized pages

Dan Williams
2024-09-27T02:49:47Z

Alistair Popple wrote:
> Currently DAX folio/page reference counts are managed differently to
> normal pages. To allow these to be managed the same as normal pages
> introduce dax_insert_pfn_pmd. This will map the entire PMD-sized folio
> and take references as it would for a normally mapped page.
>
> This is distinct from the current mechanism, vmf_insert_pfn_pmd, which
> simply inserts a special devmap PMD entry into the page table without
> holding a reference to the page for the mapping.

It would be useful to mention the rationale for the locking changes and
your understanding of the new "pgtable deposit" handling, because those
things make this not a trivial conversion.

>
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> ---
> include/linux/huge_mm.h | 1 +-
> mm/huge_memory.c | 57 ++++++++++++++++++++++++++++++++++--------
> 2 files changed, 48 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index d3a1872..eaf3f78 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -40,6 +40,7 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>
> vm_fault_t vmf_insert_pfn_pmd(struct vm_fault *vmf, pfn_t pfn, bool write);
> vm_fault_t vmf_insert_pfn_pud(struct vm_fault *vmf, pfn_t pfn, bool write);
> +vm_fault_t dax_insert_pfn_pmd(struct vm_fault *vmf, pfn_t pfn, bool write);
> vm_fault_t dax_insert_pfn_pud(struct vm_fault *vmf, pfn_t pfn, bool write);
>
> enum transparent_hugepage_flag {
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index e8985a4..790041e 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1237,14 +1237,12 @@ static void insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
> {
> struct mm_struct *mm = vma->vm_mm;
> pmd_t entry;
> - spinlock_t *ptl;
>
> - ptl = pmd_lock(mm, pmd);
> if (!pmd_none(*pmd)) {
> if (write) {
> if (pmd_pfn(*pmd) != pfn_t_to_pfn(pfn)) {
> WARN_ON_ONCE(!is_huge_zero_pmd(*pmd));
> - goto out_unlock;
> + return;
> }
> entry = pmd_mkyoung(*pmd);
> entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
> @@ -1252,7 +1250,7 @@ static void insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
> update_mmu_cache_pmd(vma, addr, pmd);
> }
>
> - goto out_unlock;
> + return;
> }
>
> entry = pmd_mkhuge(pfn_t_pmd(pfn, prot));
> @@ -1271,11 +1269,6 @@ static void insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
>
> set_pmd_at(mm, addr, pmd, entry);
> update_mmu_cache_pmd(vma, addr, pmd);
> -
> -out_unlock:
> - spin_unlock(ptl);
> - if (pgtable)
> - pte_free(mm, pgtable);
> }
>
> /**
> @@ -1294,6 +1287,7 @@ vm_fault_t vmf_insert_pfn_pmd(struct vm_fault *vmf, pfn_t pfn, bool write)
> struct vm_area_struct *vma = vmf->vma;
> pgprot_t pgprot = vma->vm_page_prot;
> pgtable_t pgtable = NULL;
> + spinlock_t *ptl;
>
> /*
> * If we had pmd_special, we could avoid all these restrictions,
> @@ -1316,12 +1310,55 @@ vm_fault_t vmf_insert_pfn_pmd(struct vm_fault *vmf, pfn_t pfn, bool write)
> }
>
> track_pfn_insert(vma, &pgprot, pfn);
> -
> + ptl = pmd_lock(vma->vm_mm, vmf->pmd);
> insert_pfn_pmd(vma, addr, vmf->pmd, pfn, pgprot, write, pgtable);
> + spin_unlock(ptl);
> + if (pgtable)
> + pte_free(vma->vm_mm, pgtable);
> +
> return VM_FAULT_NOPAGE;
> }
> EXPORT_SYMBOL_GPL(vmf_insert_pfn_pmd);
>
> +vm_fault_t dax_insert_pfn_pmd(struct vm_fault *vmf, pfn_t pfn, bool write)
> +{
> + struct vm_area_struct *vma = vmf->vma;
> + unsigned long addr = vmf->address & PMD_MASK;
> + struct mm_struct *mm = vma->vm_mm;
> + spinlock_t *ptl;
> + pgtable_t pgtable = NULL;
> + struct folio *folio;
> + struct page *page;
> +
> + if (addr < vma->vm_start || addr >= vma->vm_end)
> + return VM_FAULT_SIGBUS;
> +
> + if (arch_needs_pgtable_deposit()) {
> + pgtable = pte_alloc_one(vma->vm_mm);
> + if (!pgtable)
> + return VM_FAULT_OOM;
> + }
> +
> + track_pfn_insert(vma, &vma->vm_page_prot, pfn);
> +
> + ptl = pmd_lock(mm, vmf->pmd);
> + if (pmd_none(*vmf->pmd)) {
> + page = pfn_t_to_page(pfn);
> + folio = page_folio(page);
> + folio_get(folio);
> + folio_add_file_rmap_pmd(folio, page, vma);
> + add_mm_counter(mm, mm_counter_file(folio), HPAGE_PMD_NR);
> + }
> + insert_pfn_pmd(vma, addr, vmf->pmd, pfn, vma->vm_page_prot,
> + write, pgtable);
> + spin_unlock(ptl);
> + if (pgtable)
> + pte_free(mm, pgtable);

Are not the deposit rules that the extra page table stick around for the
lifetime of the inserted pte? So would that not require this incremental
change?

---
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index ea65c2db2bb1..5ef1e5d21a96 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1232,7 +1232,7 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)

static void insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
pmd_t *pmd, unsigned long pfn, pgprot_t prot,
- bool write, pgtable_t pgtable)
+ bool write, pgtable_t *pgtable)
{
struct mm_struct *mm = vma->vm_mm;
pmd_t entry;
@@ -1258,10 +1258,10 @@ static void insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
entry = maybe_pmd_mkwrite(entry, vma);
}

- if (pgtable) {
- pgtable_trans_huge_deposit(mm, pmd, pgtable);
+ if (*pgtable) {
+ pgtable_trans_huge_deposit(mm, pmd, *pgtable);
mm_inc_nr_ptes(mm);
- pgtable = NULL;
+ *pgtable = NULL;
}

set_pmd_at(mm, addr, pmd, entry);
@@ -1306,7 +1306,7 @@ vm_fault_t vmf_insert_pfn_pmd(struct vm_fault *vmf, unsigned long pfn, bool writ

track_pfn_insert(vma, &pgprot, pfn);
ptl = pmd_lock(vma->vm_mm, vmf->pmd);
- insert_pfn_pmd(vma, addr, vmf->pmd, pfn, pgprot, write, pgtable);
+ insert_pfn_pmd(vma, addr, vmf->pmd, pfn, pgprot, write, &pgtable);
spin_unlock(ptl);
if (pgtable)
pte_free(vma->vm_mm, pgtable);
@@ -1344,8 +1344,8 @@ vm_fault_t dax_insert_pfn_pmd(struct vm_fault *vmf, unsigned long pfn, bool writ
folio_add_file_rmap_pmd(folio, page, vma);
add_mm_counter(mm, mm_counter_file(folio), HPAGE_PMD_NR);
}
- insert_pfn_pmd(vma, addr, vmf->pmd, pfn, vma->vm_page_prot,
- write, pgtable);
+ insert_pfn_pmd(vma, addr, vmf->pmd, pfn, vma->vm_page_prot, write,
+ &pgtable);
spin_unlock(ptl);
if (pgtable)
pte_free(mm, pgtable);
---

Along these lines it would be lovely if someone from the PowerPC side
could test these changes, or if someone has a canned qemu command line
to test radix vs hash with pmem+dax that they can share?

> +
> + return VM_FAULT_NOPAGE;
> +}
> +EXPORT_SYMBOL_GPL(dax_insert_pfn_pmd);

Like I mentioned before, lets make the exported function
vmf_insert_folio() and move the pte, pmd, pud internal private / static
details of the implementation. The "dax_" specific aspect of this was
removed at the conversion of a dax_pfn to a folio.