Re: [PATCH 00/19] mm: Support huge pfnmaps

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2024/8/16 3:20, Peter Xu wrote:
> On Wed, Aug 14, 2024 at 09:37:15AM -0300, Jason Gunthorpe wrote:
>>> Currently, only x86_64 (1G+2M) and arm64 (2M) are supported.
>>
>> There is definitely interest here in extending ARM to support the 1G
>> size too, what is missing?
> 
> Currently PUD pfnmap relies on THP_PUD config option:
> 
> config ARCH_SUPPORTS_PUD_PFNMAP
> 	def_bool y
> 	depends on ARCH_SUPPORTS_HUGE_PFNMAP && HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
> 
> Arm64 unfortunately doesn't yet support dax 1G, so not applicable yet.
> 
> Ideally, pfnmap is too simple comparing to real THPs and it shouldn't
> require to depend on THP at all, but we'll need things like below to land
> first:
> 
> https://lore.kernel.org/r/20240717220219.3743374-1-peterx@redhat.com
> 
> I sent that first a while ago, but I didn't collect enough inputs, and I
> decided to unblock this series from that, so x86_64 shouldn't be affected,
> and arm64 will at least start to have 2M.
> 
>>
>>> The other trick is how to allow gup-fast working for such huge mappings
>>> even if there's no direct sign of knowing whether it's a normal page or
>>> MMIO mapping.  This series chose to keep the pte_special solution, so that
>>> it reuses similar idea on setting a special bit to pfnmap PMDs/PUDs so that
>>> gup-fast will be able to identify them and fail properly.
>>
>> Make sense
>>
>>> More architectures / More page sizes
>>> ------------------------------------
>>>
>>> Currently only x86_64 (2M+1G) and arm64 (2M) are supported.
>>>
>>> For example, if arm64 can start to support THP_PUD one day, the huge pfnmap
>>> on 1G will be automatically enabled.

A draft patch to enable THP_PUD on arm64, only passed with 
DEBUG_VM_PGTABLE, we may test pud pfnmaps on arm64.

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index a2f8ff354ca6..ff0d27c72020 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -184,6 +184,7 @@ config ARM64
  	select HAVE_ARCH_THREAD_STRUCT_WHITELIST
  	select HAVE_ARCH_TRACEHOOK
  	select HAVE_ARCH_TRANSPARENT_HUGEPAGE
+	select HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD if PGTABLE_LEVELS > 2
  	select HAVE_ARCH_VMAP_STACK
  	select HAVE_ARM_SMCCC
  	select HAVE_ASM_MODVERSIONS
diff --git a/arch/arm64/include/asm/pgtable.h 
b/arch/arm64/include/asm/pgtable.h
index 7a4f5604be3f..e013fe458476 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -763,6 +763,25 @@ static inline unsigned long pmd_page_vaddr(pmd_t pmd)
  #define pud_valid(pud)		pte_valid(pud_pte(pud))
  #define pud_user(pud)		pte_user(pud_pte(pud))
  #define pud_user_exec(pud)	pte_user_exec(pud_pte(pud))
+#define pud_dirty(pud)		pte_dirty(pud_pte(pud))
+#define pud_devmap(pud)		pte_devmap(pud_pte(pud))
+#define pud_wrprotect(pud)	pte_pud(pte_wrprotect(pud_pte(pud)))
+#define pud_mkold(pud)		pte_pud(pte_mkold(pud_pte(pud)))
+#define pud_mkwrite(pud)	pte_pud(pte_mkwrite_novma(pud_pte(pud)))
+#define pud_mkclean(pud)	pte_pud(pte_mkclean(pud_pte(pud)))
+#define pud_mkdirty(pud)	pte_pud(pte_mkdirty(pud_pte(pud)))
+
+#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
+static inline int pud_trans_huge(pud_t pud)
+{
+	return pud_val(pud) && pud_present(pud) && !(pud_val(pud) & 
PUD_TABLE_BIT);
+}
+
+static inline pud_t pud_mkdevmap(pud_t pud)
+{
+	return pte_pud(set_pte_bit(pud_pte(pud), __pgprot(PTE_DEVMAP)));
+}
+#endif

  static inline bool pgtable_l4_enabled(void);

@@ -1137,10 +1156,20 @@ static inline int pmdp_set_access_flags(struct 
vm_area_struct *vma,
  							pmd_pte(entry), dirty);
  }

+static inline int pudp_set_access_flags(struct vm_area_struct *vma,
+					unsigned long address, pud_t *pudp,
+					pud_t entry, int dirty)
+{
+	return __ptep_set_access_flags(vma, address, (pte_t *)pudp,
+							pud_pte(entry), dirty);
+}
+
+#ifndef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
  static inline int pud_devmap(pud_t pud)
  {
  	return 0;
  }
+#endif

  static inline int pgd_devmap(pgd_t pgd)
  {
@@ -1213,6 +1242,13 @@ static inline int 
pmdp_test_and_clear_young(struct vm_area_struct *vma,
  {
  	return __ptep_test_and_clear_young(vma, address, (pte_t *)pmdp);
  }
+
+static inline int pudp_test_and_clear_young(struct vm_area_struct *vma,
+					    unsigned long address,
+					    pud_t *pudp)
+{
+	return __ptep_test_and_clear_young(vma, address, (pte_t *)pudp);
+}
  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */

  static inline pte_t __ptep_get_and_clear(struct mm_struct *mm,
@@ -1433,6 +1469,7 @@ static inline void update_mmu_cache_range(struct 
vm_fault *vmf,
  #define update_mmu_cache(vma, addr, ptep) \
  	update_mmu_cache_range(NULL, vma, addr, ptep, 1)
  #define update_mmu_cache_pmd(vma, address, pmd) do { } while (0)
+#define update_mmu_cache_pud(vma, address, pud) do { } while (0)

  #ifdef CONFIG_ARM64_PA_BITS_52
  #define phys_to_ttbr(addr)	(((addr) | ((addr) >> 46)) & 
TTBR_BADDR_MASK_52)
-- 
2.27.0


>>
>> Oh that sounds like a bigger step..
> 
> Just to mention, no real THP 1G needed here for pfnmaps.  The real gap here
> is only about the pud helpers that only exists so far with CONFIG_THP_PUD
> in huge_memory.c.
> 
>>   
>>> VFIO is so far the only consumer for the huge pfnmaps after this series
>>> applied.  Besides above remap_pfn_range() generic optimization, device
>>> driver can also try to optimize its mmap() on a better VA alignment for
>>> either PMD/PUD sizes.  This may, iiuc, normally require userspace changes,
>>> as the driver doesn't normally decide the VA to map a bar.  But I don't
>>> think I know all the drivers to know the full picture.
>>
>> How does alignment work? In most caes I'm aware of the userspace does
>> not use MAP_FIXED so the expectation would be for the kernel to
>> automatically select a high alignment. I suppose your cases are
>> working because qemu uses MAP_FIXED and naturally aligns the BAR
>> addresses?
>>
>>> - x86_64 + AMD GPU
>>>    - Needs Alex's modified QEMU to guarantee proper VA alignment to make
>>>      sure all pages to be mapped with PUDs
>>
>> Oh :(
> 
> So I suppose this answers above. :) Yes, alignment needed.
>