[PATCH V2 0/3] mm/debug: Add more arch page table helper tests

[PATCH V2 0/3] mm/debug: Add more arch page table helper tests

[Anshuman Khandual]

This series adds more arch page table helper tests. The new tests here are
either related to core memory functions and advanced arch pgtable helpers.
This also creates a documentation file enlisting all expected semantics as
suggested by Mike Rapoport (https://lkml.org/lkml/2020/1/30/40).

This series has been tested on arm64 and x86 platforms. There is just one
expected failure on arm64 that will be fixed when we enable THP migration.

[   21.741634] WARNING: CPU: 0 PID: 1 at mm/debug_vm_pgtable.c:782

which corresponds to

WARN_ON(!pmd_present(pmd_mknotpresent(pmd_mkhuge(pmd))))

There are many TRANSPARENT_HUGEPAGE and ARCH_HAS_TRANSPARENT_HUGEPAGE_PUD
ifdefs scattered across the test. But consolidating all the fallback stubs
is not very straight forward because ARCH_HAS_TRANSPARENT_HUGEPAGE_PUD is
not explicitly dependent on ARCH_HAS_TRANSPARENT_HUGEPAGE.

This series has been build tested on many platforms including the ones that
subscribe the test through ARCH_HAS_DEBUG_VM_PGTABLE.

This series is based on v5.6-rc7 after applying these following patches.

1. https://patchwork.kernel.org/patch/11431277/
2. https://patchwork.kernel.org/patch/11452185/

Changes in V2:

- Dropped CONFIG_ARCH_HAS_PTE_SPECIAL per Christophe
- Dropped CONFIG_NUMA_BALANCING per Christophe
- Dropped CONFIG_HAVE_ARCH_SOFT_DIRTY per Christophe
- Dropped CONFIG_MIGRATION per Christophe
- Replaced CONFIG_S390 with __HAVE_ARCH_PMDP_INVALIDATE
- Moved page allocation & free inside swap_migration_tests() per Christophe
- Added CONFIG_TRANSPARENT_HUGEPAGE to protect pfn_pmd()
- Added CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD to protect pfn_pud()
- Added a patch for other arch advanced page table helper tests
- Added a patch creating a documentation for page table helper semantics

Changes in V1: (https://patchwork.kernel.org/patch/11408253/)

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: linux-snps-arc@lists.infradead.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linuxppc-dev@lists.ozlabs.org
Cc: linux-s390@vger.kernel.org
Cc: linux-riscv@lists.infradead.org
Cc: x86@kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-arch@vger.kernel.org
Cc: linux-kernel@vger.kernel.org

Anshuman Khandual (3):
  mm/debug: Add tests validating arch page table helpers for core features
  mm/debug: Add tests validating arch advanced page table helpers
  Documentation/mm: Add descriptions for arch page table helpers

 Documentation/vm/arch_pgtable_helpers.rst | 256 ++++++++++
 mm/debug_vm_pgtable.c                     | 594 +++++++++++++++++++++-
 2 files changed, 839 insertions(+), 11 deletions(-)
 create mode 100644 Documentation/vm/arch_pgtable_helpers.rst

-- 
2.20.1

[PATCH V2 3/3] Documentation/mm: Add descriptions for arch page table helpers

[Anshuman Khandual]

This removes existing description from the test and instead adds a specific
description file for all arch page table helpers which is in sync with the
semantics being tested via CONFIG_DEBUG_VM_PGTABLE. All future changes
either to these descriptions here or the debug test should always remain
in sync.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-doc@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
Suggested-by: Mike Rapoport <rppt@kernel.org>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
 Documentation/vm/arch_pgtable_helpers.rst | 256 ++++++++++++++++++++++
 mm/debug_vm_pgtable.c                     |  13 +-
 2 files changed, 259 insertions(+), 10 deletions(-)
 create mode 100644 Documentation/vm/arch_pgtable_helpers.rst

diff --git a/Documentation/vm/arch_pgtable_helpers.rst b/Documentation/vm/arch_pgtable_helpers.rst
new file mode 100644
index 000000000000..1fc8a1d8932a
--- /dev/null
+++ b/Documentation/vm/arch_pgtable_helpers.rst
@@ -0,0 +1,256 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _arch_page_table_helpers:
+
+===============================
+Architecture Page Table Helpers
+===============================
+
+Generic MM expects architectures (with MMU) to provide helpers to create, access
+and modify page table entries at various level for different memory functions.
+These page table helpers need to conform to a common semantics across platforms.
+Following tables describe the expected semantics which can also be tested during
+boot via CONFIG_DEBUG_VM_PGTABLE option. All future changes in here or the debug
+test need to be in sync.
+
+======================
+PTE Page Table Helpers
+======================
+
+--------------------------------------------------------------------------------
+| pte_same                  | Tests whether both PTE entries are the same      |
+--------------------------------------------------------------------------------
+| pte_bad                   | Tests a non-table mapped PTE                     |
+--------------------------------------------------------------------------------
+| pte_present               | Tests a valid mapped PTE                         |
+--------------------------------------------------------------------------------
+| pte_young                 | Tests a young PTE                                |
+--------------------------------------------------------------------------------
+| pte_dirty                 | Tests a dirty PTE                                |
+--------------------------------------------------------------------------------
+| pte_write                 | Tests a writable PTE                             |
+--------------------------------------------------------------------------------
+| pte_special               | Tests a special PTE                              |
+--------------------------------------------------------------------------------
+| pte_protnone              | Tests a PROT_NONE PTE                            |
+--------------------------------------------------------------------------------
+| pte_devmap                | Tests a ZONE_DEVICE mapped PTE                   |
+--------------------------------------------------------------------------------
+| pte_soft_dirty            | Tests a soft dirty PTE                           |
+--------------------------------------------------------------------------------
+| pte_swp_soft_dirty        | Tests a soft dirty swapped PTE                   |
+--------------------------------------------------------------------------------
+| pte_mkyoung               | Creates a young PTE                              |
+--------------------------------------------------------------------------------
+| pte_mkold                 | Creates an old PTE                               |
+--------------------------------------------------------------------------------
+| pte_mkdirty               | Creates a dirty PTE                              |
+--------------------------------------------------------------------------------
+| pte_mkclean               | Creates a clean PTE                              |
+--------------------------------------------------------------------------------
+| pte_mkwrite               | Creates a writable PTE                           |
+--------------------------------------------------------------------------------
+| pte_mkwrprotect           | Creates a write protected PTE                    |
+--------------------------------------------------------------------------------
+| pte_mkspecial             | Creates a special PTE                            |
+--------------------------------------------------------------------------------
+| pte_mkdevmap              | Creates a ZONE_DEVICE mapped PTE                 |
+--------------------------------------------------------------------------------
+| pte_mksoft_dirty          | Creates a soft dirty PTE                         |
+--------------------------------------------------------------------------------
+| pte_clear_soft_dirty      | Clears a soft dirty PTE                          |
+--------------------------------------------------------------------------------
+| pte_swp_mksoft_dirty      | Creates a soft dirty swapped PTE                 |
+--------------------------------------------------------------------------------
+| pte_swp_clear_soft_dirty  | Clears a soft dirty swapped PTE                  |
+--------------------------------------------------------------------------------
+| pte_mknotpresent          | Invalidates a mapped PTE                         |
+--------------------------------------------------------------------------------
+| ptep_get_and_clear        | Clears a PTE                                     |
+--------------------------------------------------------------------------------
+| ptep_get_and_clear_full   | Clears a PTE                                     |
+--------------------------------------------------------------------------------
+| ptep_test_and_clear_young | Clears young from a PTE                          |
+--------------------------------------------------------------------------------
+| ptep_set_wrprotect        | Converts into a write protected PTE              |
+--------------------------------------------------------------------------------
+| ptep_set_access_flags     | Converts into a more permissive PTE              |
+--------------------------------------------------------------------------------
+
+======================
+PMD Page Table Helpers
+======================
+
+--------------------------------------------------------------------------------
+| pmd_same                  | Tests whether both PMD entries are the same      |
+--------------------------------------------------------------------------------
+| pmd_bad                   | Tests a non-table mapped PMD                     |
+--------------------------------------------------------------------------------
+| pmd_leaf                  | Tests a leaf mapped PMD                          |
+--------------------------------------------------------------------------------
+| pmd_huge                  | Tests a HugeTLB mapped PMD                       |
+--------------------------------------------------------------------------------
+| pmd_trans_huge            | Tests a Transparent Huge Page (THP) at PMD       |
+--------------------------------------------------------------------------------
+| pmd_present               | Tests a valid mapped PMD                         |
+--------------------------------------------------------------------------------
+| pmd_young                 | Tests a young PMD                                |
+--------------------------------------------------------------------------------
+| pmd_dirty                 | Tests a dirty PMD                                |
+--------------------------------------------------------------------------------
+| pmd_write                 | Tests a writable PMD                             |
+--------------------------------------------------------------------------------
+| pmd_special               | Tests a special PMD                              |
+--------------------------------------------------------------------------------
+| pmd_protnone              | Tests a PROT_NONE PMD                            |
+--------------------------------------------------------------------------------
+| pmd_devmap                | Tests a ZONE_DEVICE mapped PMD                   |
+--------------------------------------------------------------------------------
+| pmd_soft_dirty            | Tests a soft dirty PMD                           |
+--------------------------------------------------------------------------------
+| pmd_swp_soft_dirty        | Tests a soft dirty swapped PMD                   |
+--------------------------------------------------------------------------------
+| pmd_mkyoung               | Creates a young PMD                              |
+--------------------------------------------------------------------------------
+| pmd_mkold                 | Creates an old PMD                               |
+--------------------------------------------------------------------------------
+| pmd_mkdirty               | Creates a dirty PMD                              |
+--------------------------------------------------------------------------------
+| pmd_mkclean               | Creates a clean PMD                              |
+--------------------------------------------------------------------------------
+| pmd_mkwrite               | Creates a writable PMD                           |
+--------------------------------------------------------------------------------
+| pmd_mkwrprotect           | Creates a write protected PMD                    |
+--------------------------------------------------------------------------------
+| pmd_mkspecial             | Creates a special PMD                            |
+--------------------------------------------------------------------------------
+| pmd_mkdevmap              | Creates a ZONE_DEVICE mapped PMD                 |
+--------------------------------------------------------------------------------
+| pmd_mksoft_dirty          | Creates a soft dirty PMD                         |
+--------------------------------------------------------------------------------
+| pmd_clear_soft_dirty      | Clears a soft dirty PMD                          |
+--------------------------------------------------------------------------------
+| pmd_swp_mksoft_dirty      | Creates a soft dirty swapped PMD                 |
+--------------------------------------------------------------------------------
+| pmd_swp_clear_soft_dirty  | Clears a soft dirty swapped PMD                  |
+--------------------------------------------------------------------------------
+| pmd_mknotpresent          | Invalidates a mapped PMD                         |
+--------------------------------------------------------------------------------
+| pmd_set_huge              | Creates a PMD huge mapping                       |
+--------------------------------------------------------------------------------
+| pmd_clear_huge            | Clears a PMD huge mapping                        |
+--------------------------------------------------------------------------------
+| pmdp_get_and_clear        | Clears a PMD                                     |
+--------------------------------------------------------------------------------
+| pmdp_get_and_clear_full   | Clears a PMD                                     |
+--------------------------------------------------------------------------------
+| pmdp_test_and_clear_young | Clears young from a PMD                          |
+--------------------------------------------------------------------------------
+| pmdp_set_wrprotect        | Converts into a write protected PMD              |
+--------------------------------------------------------------------------------
+| pmdp_set_access_flags     | Converts into a more permissive PMD              |
+--------------------------------------------------------------------------------
+
+======================
+PUD Page Table Helpers
+======================
+
+--------------------------------------------------------------------------------
+| pud_same                  | Tests whether both PUD entries are the same      |
+--------------------------------------------------------------------------------
+| pud_bad                   | Tests a non-table mapped PUD                     |
+--------------------------------------------------------------------------------
+| pud_leaf                  | Tests a leaf mapped PUD                          |
+--------------------------------------------------------------------------------
+| pud_huge                  | Tests a HugeTLB mapped PUD                       |
+--------------------------------------------------------------------------------
+| pud_trans_huge            | Tests a Transparent Huge Page (THP) at PUD       |
+--------------------------------------------------------------------------------
+| pud_present               | Tests a valid mapped PUD                         |
+--------------------------------------------------------------------------------
+| pud_young                 | Tests a young PUD                                |
+--------------------------------------------------------------------------------
+| pud_dirty                 | Tests a dirty PUD                                |
+--------------------------------------------------------------------------------
+| pud_write                 | Tests a writable PUD                             |
+--------------------------------------------------------------------------------
+| pud_devmap                | Tests a ZONE_DEVICE mapped PUD                   |
+--------------------------------------------------------------------------------
+| pud_mkyoung               | Creates a young PUD                              |
+--------------------------------------------------------------------------------
+| pud_mkold                 | Creates an old PUD                               |
+--------------------------------------------------------------------------------
+| pud_mkdirty               | Creates a dirty PUD                              |
+--------------------------------------------------------------------------------
+| pud_mkclean               | Creates a clean PUD                              |
+--------------------------------------------------------------------------------
+| pud_mkwrite               | Creates a writable PMD                           |
+--------------------------------------------------------------------------------
+| pud_mkwrprotect           | Creates a write protected PMD                    |
+--------------------------------------------------------------------------------
+| pud_mkdevmap              | Creates a ZONE_DEVICE mapped PMD                 |
+--------------------------------------------------------------------------------
+| pud_mknotpresent          | Invalidates a mapped PUD                         |
+--------------------------------------------------------------------------------
+| pud_set_huge              | Creates a PUD huge mapping                       |
+--------------------------------------------------------------------------------
+| pud_clear_huge            | Clears a PUD huge mapping                        |
+--------------------------------------------------------------------------------
+| pudp_get_and_clear        | Clears a PUD                                     |
+--------------------------------------------------------------------------------
+| pudp_get_and_clear_full   | Clears a PUD                                     |
+--------------------------------------------------------------------------------
+| pudp_test_and_clear_young | Clears young from a PUD                          |
+--------------------------------------------------------------------------------
+| pudp_set_wrprotect        | Converts into a write protected PUD              |
+--------------------------------------------------------------------------------
+| pudp_set_access_flags     | Converts into a more permissive PUD              |
+--------------------------------------------------------------------------------
+
+==========================
+HugeTLB Page Table Helpers
+==========================
+
+--------------------------------------------------------------------------------
+| pte_huge                  | Tests a HugeTLB                                  |
+--------------------------------------------------------------------------------
+| pte_mkhuge                | Creates a HugeTLB                                |
+--------------------------------------------------------------------------------
+| huge_pte_dirty            | Tests a dirty HugeTLB                            |
+--------------------------------------------------------------------------------
+| huge_pte_write            | Tests a writable HugeTLB                         |
+--------------------------------------------------------------------------------
+| huge_pte_mkdirty          | Creates a dirty HugeTLB                          |
+--------------------------------------------------------------------------------
+| huge_pte_mkwrite          | Creates a writable HugeTLB                       |
+--------------------------------------------------------------------------------
+| huge_pte_mkwrprotect      | Creates a write protected HugeTLB                |
+--------------------------------------------------------------------------------
+| huge_ptep_get_and_clear   | Clears a HugeTLB                                 |
+--------------------------------------------------------------------------------
+| huge_ptep_set_wrprotect   | Converts into a write protected HugeTLB          |
+--------------------------------------------------------------------------------
+| huge_ptep_set_access_flags  | Converts into a more permissive HugeTLB        |
+--------------------------------------------------------------------------------
+
+========================
+SWAP Page Table Helpers
+========================
+
+--------------------------------------------------------------------------------
+| __pte_to_swp_entry        | Creates a swapped entry (arch) from a mapepd PTE |
+--------------------------------------------------------------------------------
+| __swp_to_pte_entry        | Creates a mapped PTE from a swapped entry (arch) |
+--------------------------------------------------------------------------------
+| __pmd_to_swp_entry        | Creates a swapped entry (arch) from a mapepd PMD |
+--------------------------------------------------------------------------------
+| __swp_to_pmd_entry        | Creates a mapped PMD from a swapped entry (arch) |
+--------------------------------------------------------------------------------
+| is_migration_entry        | Tests a migration (read or write) swapped entry  |
+--------------------------------------------------------------------------------
+| is_write_migration_entry  | Tests a write migration swapped entry            |
+--------------------------------------------------------------------------------
+| make_migration_entry_read | Converts into read migration swapped entry       |
+--------------------------------------------------------------------------------
+| make_migration_entry      | Creates a migration swapped entry (read or write)|
+--------------------------------------------------------------------------------
diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
index 87b4b495333b..7c210f99a812 100644
--- a/mm/debug_vm_pgtable.c
+++ b/mm/debug_vm_pgtable.c
@@ -32,16 +32,9 @@
 #include <asm/tlbflush.h>
 
 /*
- * Basic operations
- *
- * mkold(entry)			= An old and not a young entry
- * mkyoung(entry)		= A young and not an old entry
- * mkdirty(entry)		= A dirty and not a clean entry
- * mkclean(entry)		= A clean and not a dirty entry
- * mkwrite(entry)		= A write and not a write protected entry
- * wrprotect(entry)		= A write protected and not a write entry
- * pxx_bad(entry)		= A mapped and non-table entry
- * pxx_same(entry1, entry2)	= Both entries hold the exact same value
+ * Please refer Documentation/vm/arch_pgtable_helpers.rst for the semantics
+ * expectations that are being validated here. All future changes in here
+ * or the documentation need to be in sync.
  */
 #define VMFLAGS	(VM_READ|VM_WRITE|VM_EXEC)
 
-- 
2.20.1

Re: [PATCH V2 0/3] mm/debug: Add more arch page table helper tests

[Anshuman Khandual]

On 03/24/2020 10:52 AM, Anshuman Khandual wrote:
> This series adds more arch page table helper tests. The new tests here are
> either related to core memory functions and advanced arch pgtable helpers.
> This also creates a documentation file enlisting all expected semantics as
> suggested by Mike Rapoport (https://lkml.org/lkml/2020/1/30/40).
> 
> This series has been tested on arm64 and x86 platforms.

If folks can test these patches out on remaining ARCH_HAS_DEBUG_VM_PGTABLE
enabled platforms i.e s390, arc, powerpc (32 and 64), that will be really
appreciated. Thank you.

- Anshuman

Re: [PATCH V2 0/3] mm/debug: Add more arch page table helper tests

[Christophe Leroy]

Le 26/03/2020 à 03:23, Anshuman Khandual a écrit :
> 
> 
> On 03/24/2020 10:52 AM, Anshuman Khandual wrote:
>> This series adds more arch page table helper tests. The new tests here are
>> either related to core memory functions and advanced arch pgtable helpers.
>> This also creates a documentation file enlisting all expected semantics as
>> suggested by Mike Rapoport (https://lkml.org/lkml/2020/1/30/40).
>>
>> This series has been tested on arm64 and x86 platforms.
> 
> If folks can test these patches out on remaining ARCH_HAS_DEBUG_VM_PGTABLE
> enabled platforms i.e s390, arc, powerpc (32 and 64), that will be really
> appreciated. Thank you.
> 

On powerpc 8xx (PPC32), I get:

[   53.338368] debug_vm_pgtable: debug_vm_pgtable: Validating 
architecture page table helpers
[   53.347403] ------------[ cut here ]------------
[   53.351832] WARNING: CPU: 0 PID: 1 at mm/debug_vm_pgtable.c:647 
debug_vm_pgtable+0x280/0x3f4
[   53.360140] CPU: 0 PID: 1 Comm: swapper Not tainted 
5.6.0-rc7-s3k-dev-01090-g92710e99881f #3544
[   53.368718] NIP:  c0777c04 LR: c0777bb8 CTR: 00000000
[   53.373720] REGS: c9023df0 TRAP: 0700   Not tainted 
(5.6.0-rc7-s3k-dev-01090-g92710e99881f)
[   53.382042] MSR:  00029032 <EE,ME,IR,DR,RI>  CR: 22000222  XER: 20000000
[   53.388667]
[   53.388667] GPR00: c0777bb8 c9023ea8 c6120000 00000001 1e410000 
00000000 00000000 007641c9
[   53.388667] GPR08: 00000000 00000001 00000000 ffffffff 82000222 
00000000 c00039b8 00000000
[   53.388667] GPR16: 00000000 00000000 00000000 fffffff0 065fc000 
1e410000 c6600000 000001e4
[   53.388667] GPR24: 000001d9 c062d14c c65fc000 c642d448 000006c9 
00000000 c65f8000 c65fc040
[   53.423400] NIP [c0777c04] debug_vm_pgtable+0x280/0x3f4
[   53.428559] LR [c0777bb8] debug_vm_pgtable+0x234/0x3f4
[   53.433593] Call Trace:
[   53.436048] [c9023ea8] [c0777bb8] debug_vm_pgtable+0x234/0x3f4 
(unreliable)
[   53.442936] [c9023f28] [c00039e0] kernel_init+0x28/0x124
[   53.448184] [c9023f38] [c000f174] ret_from_kernel_thread+0x14/0x1c
[   53.454245] Instruction dump:
[   53.457180] 41a20008 4bea3ed9 62890021 7d36b92e 7d36b82e 71290fd0 
3149ffff 7d2a4910
[   53.464838] 0f090000 5789077e 3149ffff 7d2a4910 <0f090000> 38c00000 
38a00000 38800000
[   53.472671] ---[ end trace fd5dd92744dc0065 ]---
[   53.519778] Freeing unused kernel memory: 608K

Re: [PATCH V2 0/3] mm/debug: Add more arch page table helper tests

[Anshuman Khandual]

On 03/26/2020 08:53 PM, Christophe Leroy wrote:
> 
> 
> Le 26/03/2020 à 03:23, Anshuman Khandual a écrit :
>>
>>
>> On 03/24/2020 10:52 AM, Anshuman Khandual wrote:
>>> This series adds more arch page table helper tests. The new tests here are
>>> either related to core memory functions and advanced arch pgtable helpers.
>>> This also creates a documentation file enlisting all expected semantics as
>>> suggested by Mike Rapoport (https://lkml.org/lkml/2020/1/30/40).
>>>
>>> This series has been tested on arm64 and x86 platforms.
>>
>> If folks can test these patches out on remaining ARCH_HAS_DEBUG_VM_PGTABLE
>> enabled platforms i.e s390, arc, powerpc (32 and 64), that will be really
>> appreciated. Thank you.
>>
> 
> On powerpc 8xx (PPC32), I get:
> 
> [   53.338368] debug_vm_pgtable: debug_vm_pgtable: Validating architecture page table helpers
> [   53.347403] ------------[ cut here ]------------
> [   53.351832] WARNING: CPU: 0 PID: 1 at mm/debug_vm_pgtable.c:647 debug_vm_pgtable+0x280/0x3f4

mm/debug_vm_pgtable.c:647 ?

With the following commits in place

53a8338ce (HEAD) Documentation/mm: Add descriptions for arch page table helper
5d4913fc1 mm/debug: Add tests validating arch advanced page table helpers
bcaf120a7 mm/debug: Add tests validating arch page table helpers for core features
d6ed5a4a5 x86/memory: Drop pud_mknotpresent()
0739d1f8d mm/debug: Add tests validating architecture page table helpers
16fbf79b0 (tag: v5.6-rc7) Linux 5.6-rc7

mm/debug_vm_pgtable.c:647 is here.

#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
static void __init pmd_swap_tests(unsigned long pfn, pgprot_t prot)
{
        swp_entry_t swp;
        pmd_t pmd;  -----------------------------> Line #647

        pmd = pfn_pmd(pfn, prot);
        swp = __pmd_to_swp_entry(pmd);
        WARN_ON(!pmd_same(pmd, __swp_entry_to_pmd(swp)));
}
#else
static void __init pmd_swap_tests(unsigned long pfn, pgprot_t prot) { }
#end

Did I miss something ?

> [   53.360140] CPU: 0 PID: 1 Comm: swapper Not tainted 5.6.0-rc7-s3k-dev-01090-g92710e99881f #3544
> [   53.368718] NIP:  c0777c04 LR: c0777bb8 CTR: 00000000
> [   53.373720] REGS: c9023df0 TRAP: 0700   Not tainted (5.6.0-rc7-s3k-dev-01090-g92710e99881f)
> [   53.382042] MSR:  00029032 <EE,ME,IR,DR,RI>  CR: 22000222  XER: 20000000
> [   53.388667]
> [   53.388667] GPR00: c0777bb8 c9023ea8 c6120000 00000001 1e410000 00000000 00000000 007641c9
> [   53.388667] GPR08: 00000000 00000001 00000000 ffffffff 82000222 00000000 c00039b8 00000000
> [   53.388667] GPR16: 00000000 00000000 00000000 fffffff0 065fc000 1e410000 c6600000 000001e4
> [   53.388667] GPR24: 000001d9 c062d14c c65fc000 c642d448 000006c9 00000000 c65f8000 c65fc040
> [   53.423400] NIP [c0777c04] debug_vm_pgtable+0x280/0x3f4
> [   53.428559] LR [c0777bb8] debug_vm_pgtable+0x234/0x3f4
> [   53.433593] Call Trace:
> [   53.436048] [c9023ea8] [c0777bb8] debug_vm_pgtable+0x234/0x3f4 (unreliable)
> [   53.442936] [c9023f28] [c00039e0] kernel_init+0x28/0x124
> [   53.448184] [c9023f38] [c000f174] ret_from_kernel_thread+0x14/0x1c
> [   53.454245] Instruction dump:
> [   53.457180] 41a20008 4bea3ed9 62890021 7d36b92e 7d36b82e 71290fd0 3149ffff 7d2a4910
> [   53.464838] 0f090000 5789077e 3149ffff 7d2a4910 <0f090000> 38c00000 38a00000 38800000
> [   53.472671] ---[ end trace fd5dd92744dc0065 ]---
Could you please point me to the exact test which is failing ?

> [   53.519778] Freeing unused kernel memory: 608K
> 
> 
So I assume that the system should have come till runtime just fine apart from
the above warning message because.

Re: [PATCH V2 0/3] mm/debug: Add more arch page table helper tests

[Christophe Leroy]

On 03/27/2020 06:46 AM, Anshuman Khandual wrote:
> 
> On 03/26/2020 08:53 PM, Christophe Leroy wrote:
>>
>>
>> Le 26/03/2020 à 03:23, Anshuman Khandual a écrit :
>>>
>>>
>>> On 03/24/2020 10:52 AM, Anshuman Khandual wrote:
>>>> This series adds more arch page table helper tests. The new tests here are
>>>> either related to core memory functions and advanced arch pgtable helpers.
>>>> This also creates a documentation file enlisting all expected semantics as
>>>> suggested by Mike Rapoport (https://lkml.org/lkml/2020/1/30/40).
>>>>
>>>> This series has been tested on arm64 and x86 platforms.
>>>
>>> If folks can test these patches out on remaining ARCH_HAS_DEBUG_VM_PGTABLE
>>> enabled platforms i.e s390, arc, powerpc (32 and 64), that will be really
>>> appreciated. Thank you.
>>>
>>
>> On powerpc 8xx (PPC32), I get:
>>
>> [   53.338368] debug_vm_pgtable: debug_vm_pgtable: Validating architecture page table helpers
>> [   53.347403] ------------[ cut here ]------------
>> [   53.351832] WARNING: CPU: 0 PID: 1 at mm/debug_vm_pgtable.c:647 debug_vm_pgtable+0x280/0x3f4
> 
> mm/debug_vm_pgtable.c:647 ?
> 
> With the following commits in place
> 
> 53a8338ce (HEAD) Documentation/mm: Add descriptions for arch page table helper
> 5d4913fc1 mm/debug: Add tests validating arch advanced page table helpers
> bcaf120a7 mm/debug: Add tests validating arch page table helpers for core features
> d6ed5a4a5 x86/memory: Drop pud_mknotpresent()
> 0739d1f8d mm/debug: Add tests validating architecture page table helpers
> 16fbf79b0 (tag: v5.6-rc7) Linux 5.6-rc7

I have:

facaa5eb5909 (HEAD -> helpers0) mm/debug: Add tests validating arch 
advanced page table helpers
6389fed515fc mm/debug: Add tests validating arch page table helpers for 
core features
dc14ecc8b94e mm/debug: add tests validating architecture page table helpers
c6624071c338 (origin/merge, merge) Automatic merge of branches 'master', 
'next' and 'fixes' into merge
58e05c5508e6 Automatic merge of branches 'master', 'next' and 'fixes' 
into merge
1b649e0bcae7 (origin/master, origin/HEAD) Merge 
git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net

origin is https://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux.git

I can't see your last patch in powerpc mailing list 
(https://patchwork.ozlabs.org/project/linuxppc-dev/list/?series=166237)

> 
> mm/debug_vm_pgtable.c:647 is here.

Line 647 is:

	WARN_ON(!pte_same(pte, __swp_entry_to_pte(swp)));


> 
> #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> static void __init pmd_swap_tests(unsigned long pfn, pgprot_t prot)
> {
>          swp_entry_t swp;
>          pmd_t pmd;  -----------------------------> Line #647
> 
>          pmd = pfn_pmd(pfn, prot);
>          swp = __pmd_to_swp_entry(pmd);
>          WARN_ON(!pmd_same(pmd, __swp_entry_to_pmd(swp)));
> }
> #else
> static void __init pmd_swap_tests(unsigned long pfn, pgprot_t prot) { }
> #end
> 
> Did I miss something ?
> 

[...]

> Could you please point me to the exact test which is failing ?
> 
>> [   53.519778] Freeing unused kernel memory: 608K
>>
>>
> So I assume that the system should have come till runtime just fine apart from
> the above warning message because.
> 

Yes it boots fine otherwise.

Christophe

Re: [PATCH V2 0/3] mm/debug: Add more arch page table helper tests

[Anshuman Khandual]

On 03/27/2020 12:30 PM, Christophe Leroy wrote:
> 
> 
> On 03/27/2020 06:46 AM, Anshuman Khandual wrote:
>>
>> On 03/26/2020 08:53 PM, Christophe Leroy wrote:
>>>
>>>
>>> Le 26/03/2020 à 03:23, Anshuman Khandual a écrit :
>>>>
>>>>
>>>> On 03/24/2020 10:52 AM, Anshuman Khandual wrote:
>>>>> This series adds more arch page table helper tests. The new tests here are
>>>>> either related to core memory functions and advanced arch pgtable helpers.
>>>>> This also creates a documentation file enlisting all expected semantics as
>>>>> suggested by Mike Rapoport (https://lkml.org/lkml/2020/1/30/40).
>>>>>
>>>>> This series has been tested on arm64 and x86 platforms.
>>>>
>>>> If folks can test these patches out on remaining ARCH_HAS_DEBUG_VM_PGTABLE
>>>> enabled platforms i.e s390, arc, powerpc (32 and 64), that will be really
>>>> appreciated. Thank you.
>>>>
>>>
>>> On powerpc 8xx (PPC32), I get:
>>>
>>> [   53.338368] debug_vm_pgtable: debug_vm_pgtable: Validating architecture page table helpers
>>> [   53.347403] ------------[ cut here ]------------
>>> [   53.351832] WARNING: CPU: 0 PID: 1 at mm/debug_vm_pgtable.c:647 debug_vm_pgtable+0x280/0x3f4
>>
>> mm/debug_vm_pgtable.c:647 ?
>>
>> With the following commits in place
>>
>> 53a8338ce (HEAD) Documentation/mm: Add descriptions for arch page table helper
>> 5d4913fc1 mm/debug: Add tests validating arch advanced page table helpers
>> bcaf120a7 mm/debug: Add tests validating arch page table helpers for core features
>> d6ed5a4a5 x86/memory: Drop pud_mknotpresent()
>> 0739d1f8d mm/debug: Add tests validating architecture page table helpers
>> 16fbf79b0 (tag: v5.6-rc7) Linux 5.6-rc7
> 
> I have:
> 
> facaa5eb5909 (HEAD -> helpers0) mm/debug: Add tests validating arch advanced page table helpers
> 6389fed515fc mm/debug: Add tests validating arch page table helpers for core features
> dc14ecc8b94e mm/debug: add tests validating architecture page table helpers
> c6624071c338 (origin/merge, merge) Automatic merge of branches 'master', 'next' and 'fixes' into merge
> 58e05c5508e6 Automatic merge of branches 'master', 'next' and 'fixes' into merge
> 1b649e0bcae7 (origin/master, origin/HEAD) Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net
> 
> origin is https://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux.git
> 
> I can't see your last patch in powerpc mailing list (https://patchwork.ozlabs.org/project/linuxppc-dev/list/?series=166237)

My bad, did not update the last patch with required lists (will fix).

> 
>>
>> mm/debug_vm_pgtable.c:647 is here.
> 
> Line 647 is:
> 
>     WARN_ON(!pte_same(pte, __swp_entry_to_pte(swp)));

Both set of definitions suggest that the last three bits (if present)
on the PTE will be discarded during PTE->SWP->PTE conversion which
might be leading to this mismatch and subsequent failure.

arch/powerpc/include/asm/nohash/32/pgtable.h
arch/powerpc/include/asm/book3s/32/pgtable.h

#define __pte_to_swp_entry(pte)         ((swp_entry_t) { pte_val(pte) >> 3 })
#define __swp_entry_to_pte(x)           ((pte_t) { (x).val << 3 })

Also there are some more architectures (microblaze, sh, etc) where these
conversions are not always preserving. On powerpc64, it sets back _PAGE_PTE
irrespective of whether the bit was originally set or not.

Probably it is wrong to expect that PTE->SWP->PTE conversion will be always
preserving. So wondering if it is worth changing this test to accommodate
all such architectures or just drop it instead.

> 
> 
>>
>> #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
>> static void __init pmd_swap_tests(unsigned long pfn, pgprot_t prot)
>> {
>>          swp_entry_t swp;
>>          pmd_t pmd;  -----------------------------> Line #647
>>
>>          pmd = pfn_pmd(pfn, prot);
>>          swp = __pmd_to_swp_entry(pmd);
>>          WARN_ON(!pmd_same(pmd, __swp_entry_to_pmd(swp)));
>> }
>> #else
>> static void __init pmd_swap_tests(unsigned long pfn, pgprot_t prot) { }
>> #end
>>
>> Did I miss something ?
>>
> 
> [...]
> 
>> Could you please point me to the exact test which is failing ?
>>
>>> [   53.519778] Freeing unused kernel memory: 608K
>>>
>>>
>> So I assume that the system should have come till runtime just fine apart from
>> the above warning message because.
>>
> 
> Yes it boots fine otherwise.

Cool, that is good to know.

> 
> Christophe
> 

Re: [PATCH V2 0/3] mm/debug: Add more arch page table helper tests

[Gerald Schaefer]

On Tue, 24 Mar 2020 10:52:52 +0530
Anshuman Khandual <anshuman.khandual@arm.com> wrote:

> This series adds more arch page table helper tests. The new tests here are
> either related to core memory functions and advanced arch pgtable helpers.
> This also creates a documentation file enlisting all expected semantics as
> suggested by Mike Rapoport (https://lkml.org/lkml/2020/1/30/40).
> 
> This series has been tested on arm64 and x86 platforms. There is just one
> expected failure on arm64 that will be fixed when we enable THP migration.
> 
> [   21.741634] WARNING: CPU: 0 PID: 1 at mm/debug_vm_pgtable.c:782
> 
> which corresponds to
> 
> WARN_ON(!pmd_present(pmd_mknotpresent(pmd_mkhuge(pmd))))
> 
> There are many TRANSPARENT_HUGEPAGE and ARCH_HAS_TRANSPARENT_HUGEPAGE_PUD
> ifdefs scattered across the test. But consolidating all the fallback stubs
> is not very straight forward because ARCH_HAS_TRANSPARENT_HUGEPAGE_PUD is
> not explicitly dependent on ARCH_HAS_TRANSPARENT_HUGEPAGE.
> 
> This series has been build tested on many platforms including the ones that
> subscribe the test through ARCH_HAS_DEBUG_VM_PGTABLE.
> 

Hi Anshuman,

thanks for the update. There are a couple of issues on s390, some might
also affect other archs.

1) The pxd_huge_tests are using pxd_set/clear_huge, which defaults to
returning 0 if !CONFIG_HAVE_ARCH_HUGE_VMAP. As result, the checks for
!pxd_test/clear_huge in the pxd_huge_tests will always trigger the
warning. This should affect all archs w/o CONFIG_HAVE_ARCH_HUGE_VMAP.
Could be fixed like this:

@@ -923,8 +923,10 @@ void __init debug_vm_pgtable(void)
        pmd_leaf_tests(pmd_aligned, prot);
        pud_leaf_tests(pud_aligned, prot);
 
-       pmd_huge_tests(pmdp, pmd_aligned, prot);
-       pud_huge_tests(pudp, pud_aligned, prot);
+       if (IS_ENABLED(CONFIG_HAVE_ARCH_HUGE_VMAP)) {
+               pmd_huge_tests(pmdp, pmd_aligned, prot);
+               pud_huge_tests(pudp, pud_aligned, prot);
+       }
 
        pte_savedwrite_tests(pte_aligned, prot);
        pmd_savedwrite_tests(pmd_aligned, prot);

BTW, please add some comments to the various #ifdef/#else stuff, especially
when the different parts are far away and/or nested.

2) The hugetlb_advanced_test will fail because it directly de-references
huge *ptep pointers instead of using huge_ptep_get() for this. We have
very different pagetable entry layout for pte and (large) pmd on s390,
and unfortunately the whole hugetlbfs code is using pte_t instead of pmd_t
like THP. For this reason, huge_ptep_get() was introduced, which will
return a "converted" pte, because directly reading from a *ptep (pointing
to a large pmd) will not return a proper pte. Only ARM has also an
implementation of huge_ptep_get(), so they could be affected, depending
on what exactly they need it for.

Could be fixed like this (the first de-reference is a bit special,
because at that point *ptep does not really point to a large (pmd) entry
yet, it is initially an invalid pte entry, which breaks our huge_ptep_get()
conversion logic. I also added PMD_MASK alignment for RANDOM_ORVALUE,
because we do have some special bits there in our large pmds. It seems
to also work w/o that alignment, but it feels a bit wrong):

@@ -731,26 +731,26 @@ static void __init hugetlb_advanced_test
                                          unsigned long vaddr, pgprot_t prot)
 {
        struct page *page = pfn_to_page(pfn);
-       pte_t pte = READ_ONCE(*ptep);
+       pte_t pte;

-       pte = __pte(pte_val(pte) | RANDOM_ORVALUE);
+       pte = pte_mkhuge(mk_pte_phys(RANDOM_ORVALUE & PMD_MASK, prot));
        set_huge_pte_at(mm, vaddr, ptep, pte);
        barrier();
        WARN_ON(!pte_same(pte, huge_ptep_get(ptep)));
        huge_pte_clear(mm, vaddr, ptep, PMD_SIZE);
-       pte = READ_ONCE(*ptep);
+       pte = huge_ptep_get(ptep);
        WARN_ON(!huge_pte_none(pte));
 
        pte = mk_huge_pte(page, prot);
        set_huge_pte_at(mm, vaddr, ptep, pte);
        huge_ptep_set_wrprotect(mm, vaddr, ptep);
-       pte = READ_ONCE(*ptep);
+       pte = huge_ptep_get(ptep);
        WARN_ON(huge_pte_write(pte));
 
        pte = mk_huge_pte(page, prot);
        set_huge_pte_at(mm, vaddr, ptep, pte);
        huge_ptep_get_and_clear(mm, vaddr, ptep);
-       pte = READ_ONCE(*ptep);
+       pte = huge_ptep_get(ptep);
        WARN_ON(!huge_pte_none(pte));
 
        pte = mk_huge_pte(page, prot);
@@ -759,7 +759,7 @@ static void __init hugetlb_advanced_test
        pte = huge_pte_mkwrite(pte);
        pte = huge_pte_mkdirty(pte);
        huge_ptep_set_access_flags(vma, vaddr, ptep, pte, 1);
-       pte = READ_ONCE(*ptep);
+       pte = huge_ptep_get(ptep);
        WARN_ON(!(huge_pte_write(pte) && huge_pte_dirty(pte)));
 }
 #else

3) The pmd_protnone_tests() has an issue, because it passes a pmd to
pmd_protnone() which has not been marked as large. We check for large
pmd in the s390 implementation of pmd_protnone(), and will fail if a
pmd is not large. We had similar issues before, in other helpers, where
I changed the logic on s390 to not require the pmd large check, but I'm
not so sure in this case. Is there a valid use case for doing
pmd_protnone() on "normal" pmds? Or could this be changed like this:

@@ -537,7 +537,7 @@ static void __init pte_protnone_tests(un
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 static void __init pmd_protnone_tests(unsigned long pfn, pgprot_t prot)
 {
-       pmd_t pmd = pfn_pmd(pfn, prot);
+       pmd_t pmd = mk_huge_pmd(pfn_to_page(pfn), prot);

        if (!IS_ENABLED(CONFIG_NUMA_BALANCING))
                return;

Regards,
Gerald

Re: [PATCH V2 0/3] mm/debug: Add more arch page table helper tests

[Anshuman Khandual]

On 03/31/2020 06:00 PM, Gerald Schaefer wrote:
> On Tue, 24 Mar 2020 10:52:52 +0530
> Anshuman Khandual <anshuman.khandual@arm.com> wrote:
> 
>> This series adds more arch page table helper tests. The new tests here are
>> either related to core memory functions and advanced arch pgtable helpers.
>> This also creates a documentation file enlisting all expected semantics as
>> suggested by Mike Rapoport (https://lkml.org/lkml/2020/1/30/40).
>>
>> This series has been tested on arm64 and x86 platforms. There is just one
>> expected failure on arm64 that will be fixed when we enable THP migration.
>>
>> [   21.741634] WARNING: CPU: 0 PID: 1 at mm/debug_vm_pgtable.c:782
>>
>> which corresponds to
>>
>> WARN_ON(!pmd_present(pmd_mknotpresent(pmd_mkhuge(pmd))))
>>
>> There are many TRANSPARENT_HUGEPAGE and ARCH_HAS_TRANSPARENT_HUGEPAGE_PUD
>> ifdefs scattered across the test. But consolidating all the fallback stubs
>> is not very straight forward because ARCH_HAS_TRANSPARENT_HUGEPAGE_PUD is
>> not explicitly dependent on ARCH_HAS_TRANSPARENT_HUGEPAGE.
>>
>> This series has been build tested on many platforms including the ones that
>> subscribe the test through ARCH_HAS_DEBUG_VM_PGTABLE.
>>
> 
> Hi Anshuman,
> 
> thanks for the update. There are a couple of issues on s390, some might
> also affect other archs.

Sure, thanks for taking a look and giving it a spin on s390.

> 
> 1) The pxd_huge_tests are using pxd_set/clear_huge, which defaults to
> returning 0 if !CONFIG_HAVE_ARCH_HUGE_VMAP. As result, the checks for
> !pxd_test/clear_huge in the pxd_huge_tests will always trigger the
> warning. This should affect all archs w/o CONFIG_HAVE_ARCH_HUGE_VMAP.
> Could be fixed like this:
> 
> @@ -923,8 +923,10 @@ void __init debug_vm_pgtable(void)
>         pmd_leaf_tests(pmd_aligned, prot);
>         pud_leaf_tests(pud_aligned, prot);
>  
> -       pmd_huge_tests(pmdp, pmd_aligned, prot);
> -       pud_huge_tests(pudp, pud_aligned, prot);
> +       if (IS_ENABLED(CONFIG_HAVE_ARCH_HUGE_VMAP)) {
> +               pmd_huge_tests(pmdp, pmd_aligned, prot);
> +               pud_huge_tests(pudp, pud_aligned, prot);
> +       }

That is correct. It was an omission on my part and will fix it.

>  
>         pte_savedwrite_tests(pte_aligned, prot);
>         pmd_savedwrite_tests(pmd_aligned, prot);
> 
> BTW, please add some comments to the various #ifdef/#else stuff, especially
> when the different parts are far away and/or nested.

Sure, will do.

> 
> 2) The hugetlb_advanced_test will fail because it directly de-references
> huge *ptep pointers instead of using huge_ptep_get() for this. We have
> very different pagetable entry layout for pte and (large) pmd on s390,
> and unfortunately the whole hugetlbfs code is using pte_t instead of pmd_t
> like THP. For this reason, huge_ptep_get() was introduced, which will
> return a "converted" pte, because directly reading from a *ptep (pointing
> to a large pmd) will not return a proper pte. Only ARM has also an
> implementation of huge_ptep_get(), so they could be affected, depending
> on what exactly they need it for.

Currently, we dont support ARM (32). But as huge_ptep_get() already got a
fallback, its better to use that than a direct READ_ONCE().

> 
> Could be fixed like this (the first de-reference is a bit special,
> because at that point *ptep does not really point to a large (pmd) entry
> yet, it is initially an invalid pte entry, which breaks our huge_ptep_get()

There seems to be an inconsistency on s390 platform. Even though it defines
a huge_ptep_get() override, it does not subscribe __HAVE_ARCH_HUGE_PTEP_GET
which should have forced it fallback on generic huge_ptep_get() but it does
not :) Then I realized that __HAVE_ARCH_HUGE_PTEP_GET only makes sense when
an arch uses <asm-generic/hugetlb.h>. s390 does not use that and hence gets
away with it's own huge_ptep_get() without __HAVE_ARCH_HUGE_PTEP_GET. Sounds
confusing ? But I might not have the entire context here.

> conversion logic. I also added PMD_MASK alignment for RANDOM_ORVALUE,
> because we do have some special bits there in our large pmds. It seems
> to also work w/o that alignment, but it feels a bit wrong):

Sure, we can accommodate that.

> 
> @@ -731,26 +731,26 @@ static void __init hugetlb_advanced_test
>                                           unsigned long vaddr, pgprot_t prot)
>  {
>         struct page *page = pfn_to_page(pfn);
> -       pte_t pte = READ_ONCE(*ptep);
> +       pte_t pte;
> 
> -       pte = __pte(pte_val(pte) | RANDOM_ORVALUE);
> +       pte = pte_mkhuge(mk_pte_phys(RANDOM_ORVALUE & PMD_MASK, prot));

So that keeps the existing value in 'ptep' pointer at bay and instead
construct a PTE from scratch. I would rather have READ_ONCE(*ptep) at
least provide the seed that can be ORed with RANDOM_ORVALUE before
being masked with PMD_MASK. Do you see any problem ?

Some thing like this instead.

pte_t pte = READ_ONCE(*ptep);
pte = pte_mkhuge(__pte((pte_val(pte) | RANDOM_ORVALUE) & PMD_MASK));

We cannot use mk_pte_phys() as it is defined only on some platforms
without any generic fallback for others.

>         set_huge_pte_at(mm, vaddr, ptep, pte);
>         barrier();
>         WARN_ON(!pte_same(pte, huge_ptep_get(ptep)));
>         huge_pte_clear(mm, vaddr, ptep, PMD_SIZE);
> -       pte = READ_ONCE(*ptep);
> +       pte = huge_ptep_get(ptep);
>         WARN_ON(!huge_pte_none(pte));
>  
>         pte = mk_huge_pte(page, prot);
>         set_huge_pte_at(mm, vaddr, ptep, pte);
>         huge_ptep_set_wrprotect(mm, vaddr, ptep);
> -       pte = READ_ONCE(*ptep);
> +       pte = huge_ptep_get(ptep);
>         WARN_ON(huge_pte_write(pte));
>  
>         pte = mk_huge_pte(page, prot);
>         set_huge_pte_at(mm, vaddr, ptep, pte);
>         huge_ptep_get_and_clear(mm, vaddr, ptep);
> -       pte = READ_ONCE(*ptep);
> +       pte = huge_ptep_get(ptep);
>         WARN_ON(!huge_pte_none(pte));
>  
>         pte = mk_huge_pte(page, prot);
> @@ -759,7 +759,7 @@ static void __init hugetlb_advanced_test
>         pte = huge_pte_mkwrite(pte);
>         pte = huge_pte_mkdirty(pte);
>         huge_ptep_set_access_flags(vma, vaddr, ptep, pte, 1);
> -       pte = READ_ONCE(*ptep);
> +       pte = huge_ptep_get(ptep);
>         WARN_ON(!(huge_pte_write(pte) && huge_pte_dirty(pte)));
>  }
>  #else
> 
> 3) The pmd_protnone_tests() has an issue, because it passes a pmd to
> pmd_protnone() which has not been marked as large. We check for large
> pmd in the s390 implementation of pmd_protnone(), and will fail if a
> pmd is not large. We had similar issues before, in other helpers, where
> I changed the logic on s390 to not require the pmd large check, but I'm
> not so sure in this case. Is there a valid use case for doing
> pmd_protnone() on "normal" pmds? Or could this be changed like this:

That is a valid question. IIUC, all existing callers for pmd_protnone()
ensure that it is indeed a huge PMD. But even assuming otherwise should
not the huge PMD requirement get checked in the caller itself rather than
in the arch helper which is just supposed to check the existence of the
dedicated PTE bit(s) for this purpose. Purely from a helper perspective
pmd_protnone() should not really care about being large even though it
might never get used without one.

Also all platforms (except s390) derive the pmd_protnone() from their
respective pte_protnone(). I wonder why should s390 be any different
unless it is absolutely necessary.

> 
> @@ -537,7 +537,7 @@ static void __init pte_protnone_tests(un
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  static void __init pmd_protnone_tests(unsigned long pfn, pgprot_t prot)
>  {
> -       pmd_t pmd = pfn_pmd(pfn, prot);
> +       pmd_t pmd = mk_huge_pmd(pfn_to_page(pfn), prot);
> 
>         if (!IS_ENABLED(CONFIG_NUMA_BALANCING))
>                 return;
> 
> Regards,
> Gerald
> 
> 

Re: [PATCH V2 0/3] mm/debug: Add more arch page table helper tests

[Gerald Schaefer]

On Sun, 5 Apr 2020 17:58:14 +0530
Anshuman Khandual <anshuman.khandual@arm.com> wrote:

[...]
> > 
> > Could be fixed like this (the first de-reference is a bit special,
> > because at that point *ptep does not really point to a large (pmd) entry
> > yet, it is initially an invalid pte entry, which breaks our huge_ptep_get()  
> 
> There seems to be an inconsistency on s390 platform. Even though it defines
> a huge_ptep_get() override, it does not subscribe __HAVE_ARCH_HUGE_PTEP_GET
> which should have forced it fallback on generic huge_ptep_get() but it does
> not :) Then I realized that __HAVE_ARCH_HUGE_PTEP_GET only makes sense when
> an arch uses <asm-generic/hugetlb.h>. s390 does not use that and hence gets
> away with it's own huge_ptep_get() without __HAVE_ARCH_HUGE_PTEP_GET. Sounds
> confusing ? But I might not have the entire context here.

Yes, that sounds very confusing. Also a bit ironic, since huge_ptep_get()
was initially introduced because of s390, and now we don't select
__HAVE_ARCH_HUGE_PTEP_GET...

As you realized, I guess this is because we do not use generic hugetlb.h.
And when __HAVE_ARCH_HUGE_PTEP_GET was introduced with commit 544db7597ad
("hugetlb: introduce generic version of huge_ptep_get"), that was probably
the reason why we did not get our share of __HAVE_ARCH_HUGE_PTEP_GET.

Nothing really wrong with that, but yes, very confusing. Maybe we could
also select it for s390, even though it wouldn't have any functional
impact (so far), just for less confusion. Maybe also thinking about
using the generic hugetlb.h, not sure if the original reasons for not
doing so would still apply. Now I only need to find the time...

> 
> > conversion logic. I also added PMD_MASK alignment for RANDOM_ORVALUE,
> > because we do have some special bits there in our large pmds. It seems
> > to also work w/o that alignment, but it feels a bit wrong):  
> 
> Sure, we can accommodate that.
> 
> > 
> > @@ -731,26 +731,26 @@ static void __init hugetlb_advanced_test
> >                                           unsigned long vaddr, pgprot_t prot)
> >  {
> >         struct page *page = pfn_to_page(pfn);
> > -       pte_t pte = READ_ONCE(*ptep);
> > +       pte_t pte;
> > 
> > -       pte = __pte(pte_val(pte) | RANDOM_ORVALUE);
> > +       pte = pte_mkhuge(mk_pte_phys(RANDOM_ORVALUE & PMD_MASK, prot));  
> 
> So that keeps the existing value in 'ptep' pointer at bay and instead
> construct a PTE from scratch. I would rather have READ_ONCE(*ptep) at
> least provide the seed that can be ORed with RANDOM_ORVALUE before
> being masked with PMD_MASK. Do you see any problem ?

Yes, unfortunately. The problem is that the resulting pte is not marked
as present. The conversion pte -> (huge) pmd, which is done in
set_huge_pte_at() for s390, will establish an empty pmd for non-present
ptes, all the RANDOM_ORVALUE stuff is lost. And a subsequent
huge_ptep_get() will not result in the same original pte value. If you
want to preserve and check the RANDOM_ORVALUE, it has to be a present
pte, hence the mk_pte(_phys).

> 
> Some thing like this instead.
> 
> pte_t pte = READ_ONCE(*ptep);
> pte = pte_mkhuge(__pte((pte_val(pte) | RANDOM_ORVALUE) & PMD_MASK));
> 
> We cannot use mk_pte_phys() as it is defined only on some platforms
> without any generic fallback for others.

Oh, didn't know that, sorry. What about using mk_pte() instead, at least
it would result in a present pte:

pte = pte_mkhuge(mk_pte(phys_to_page(RANDOM_ORVALUE & PMD_MASK), prot));

And if you also want to do some with the existing value, which seems
to be an empty pte, then maybe just check if writing and reading that
value with set_huge_pte_at() / huge_ptep_get() returns the same,
i.e. initially w/o RANDOM_ORVALUE.

So, in combination, like this (BTW, why is the barrier() needed, it
is not used for the other set_huge_pte_at() calls later?):

@@ -733,24 +733,28 @@ static void __init hugetlb_advanced_test
        struct page *page = pfn_to_page(pfn);
        pte_t pte = READ_ONCE(*ptep);
 
-       pte = __pte(pte_val(pte) | RANDOM_ORVALUE);
+       set_huge_pte_at(mm, vaddr, ptep, pte);
+       WARN_ON(!pte_same(pte, huge_ptep_get(ptep)));
+
+       pte = pte_mkhuge(mk_pte(phys_to_page(RANDOM_ORVALUE & PMD_MASK), prot));
        set_huge_pte_at(mm, vaddr, ptep, pte);
        barrier();
        WARN_ON(!pte_same(pte, huge_ptep_get(ptep)));

This would actually add a new test "write empty pte with
set_huge_pte_at(), then verify with huge_ptep_get()", which happens
to trigger a warning on s390 :-)

That (new) warning actually points to misbehavior on s390, we do not
write a correct empty pmd in this case, but one that is empty and also
marked as large. huge_ptep_get() will then not correctly recognize it
as empty and do wrong conversion. It is also not consistent with
huge_ptep_get_and_clear(), where we write the empty pmd w/o marking
as large. Last but not least it would also break our pmd_protnone()
logic (see below). Another nice finding on s390 :-)

I don't think this has any effect in practice (yet), but I will post a
fix for that, just in case you will add / change this test.

> 
> >         set_huge_pte_at(mm, vaddr, ptep, pte);
> >         barrier();
> >         WARN_ON(!pte_same(pte, huge_ptep_get(ptep)));
> >         huge_pte_clear(mm, vaddr, ptep, PMD_SIZE);
> > -       pte = READ_ONCE(*ptep);
> > +       pte = huge_ptep_get(ptep);
> >         WARN_ON(!huge_pte_none(pte));
> >  
> >         pte = mk_huge_pte(page, prot);
> >         set_huge_pte_at(mm, vaddr, ptep, pte);
> >         huge_ptep_set_wrprotect(mm, vaddr, ptep);
> > -       pte = READ_ONCE(*ptep);
> > +       pte = huge_ptep_get(ptep);
> >         WARN_ON(huge_pte_write(pte));
> >  
> >         pte = mk_huge_pte(page, prot);
> >         set_huge_pte_at(mm, vaddr, ptep, pte);
> >         huge_ptep_get_and_clear(mm, vaddr, ptep);
> > -       pte = READ_ONCE(*ptep);
> > +       pte = huge_ptep_get(ptep);
> >         WARN_ON(!huge_pte_none(pte));
> >  
> >         pte = mk_huge_pte(page, prot);
> > @@ -759,7 +759,7 @@ static void __init hugetlb_advanced_test
> >         pte = huge_pte_mkwrite(pte);
> >         pte = huge_pte_mkdirty(pte);
> >         huge_ptep_set_access_flags(vma, vaddr, ptep, pte, 1);
> > -       pte = READ_ONCE(*ptep);
> > +       pte = huge_ptep_get(ptep);
> >         WARN_ON(!(huge_pte_write(pte) && huge_pte_dirty(pte)));
> >  }
> >  #else
> > 
> > 3) The pmd_protnone_tests() has an issue, because it passes a pmd to
> > pmd_protnone() which has not been marked as large. We check for large
> > pmd in the s390 implementation of pmd_protnone(), and will fail if a
> > pmd is not large. We had similar issues before, in other helpers, where
> > I changed the logic on s390 to not require the pmd large check, but I'm
> > not so sure in this case. Is there a valid use case for doing
> > pmd_protnone() on "normal" pmds? Or could this be changed like this:  
> 
> That is a valid question. IIUC, all existing callers for pmd_protnone()
> ensure that it is indeed a huge PMD. But even assuming otherwise should
> not the huge PMD requirement get checked in the caller itself rather than
> in the arch helper which is just supposed to check the existence of the
> dedicated PTE bit(s) for this purpose. Purely from a helper perspective
> pmd_protnone() should not really care about being large even though it
> might never get used without one.
> 
> Also all platforms (except s390) derive the pmd_protnone() from their
> respective pte_protnone(). I wonder why should s390 be any different
> unless it is absolutely necessary.

This is again because of our different page table entry layouts for
pte/pmd and (large) pmd. The bits we check for pmd_protnone() are
not valid for normal pmd/pte, and we would return undefined result for
normal entries.

Of course, we could rely on nobody calling pmd_protnone() on normal
pmds, but in this case we also use pmd_large() check in pmd_protnone()
for indication if the pmd is present. W/o that, we would return
true for empty pmds, that doesn't sound right. Not sure if we also
want to rely on nobody calling pmd_protnone() on empty pmds.

Anyway, if in practice it is not correct to use pmd_protnone()
on normal pmds, then I would suggest that your tests should also
not do / test it. And I strongly assume that it is not correct, at
least I cannot think of a valid case, and of course s390 would
already be broken if there was such a case.

Regards,
Gerald

Re: [PATCH V2 0/3] mm/debug: Add more arch page table helper tests

[Anshuman Khandual]

On 04/07/2020 09:24 PM, Gerald Schaefer wrote:
> On Sun, 5 Apr 2020 17:58:14 +0530
> Anshuman Khandual <anshuman.khandual@arm.com> wrote:
> 
> [...]
>>>
>>> Could be fixed like this (the first de-reference is a bit special,
>>> because at that point *ptep does not really point to a large (pmd) entry
>>> yet, it is initially an invalid pte entry, which breaks our huge_ptep_get()  
>>
>> There seems to be an inconsistency on s390 platform. Even though it defines
>> a huge_ptep_get() override, it does not subscribe __HAVE_ARCH_HUGE_PTEP_GET
>> which should have forced it fallback on generic huge_ptep_get() but it does
>> not :) Then I realized that __HAVE_ARCH_HUGE_PTEP_GET only makes sense when
>> an arch uses <asm-generic/hugetlb.h>. s390 does not use that and hence gets
>> away with it's own huge_ptep_get() without __HAVE_ARCH_HUGE_PTEP_GET. Sounds
>> confusing ? But I might not have the entire context here.
> 
> Yes, that sounds very confusing. Also a bit ironic, since huge_ptep_get()
> was initially introduced because of s390, and now we don't select
> __HAVE_ARCH_HUGE_PTEP_GET...
> 
> As you realized, I guess this is because we do not use generic hugetlb.h.
> And when __HAVE_ARCH_HUGE_PTEP_GET was introduced with commit 544db7597ad
> ("hugetlb: introduce generic version of huge_ptep_get"), that was probably
> the reason why we did not get our share of __HAVE_ARCH_HUGE_PTEP_GET.

Understood.

> 
> Nothing really wrong with that, but yes, very confusing. Maybe we could
> also select it for s390, even though it wouldn't have any functional
> impact (so far), just for less confusion. Maybe also thinking about
> using the generic hugetlb.h, not sure if the original reasons for not
> doing so would still apply. Now I only need to find the time...

Seems like something worth to explore if we could remove this confusion.

> 
>>
>>> conversion logic. I also added PMD_MASK alignment for RANDOM_ORVALUE,
>>> because we do have some special bits there in our large pmds. It seems
>>> to also work w/o that alignment, but it feels a bit wrong):  
>>
>> Sure, we can accommodate that.
>>
>>>
>>> @@ -731,26 +731,26 @@ static void __init hugetlb_advanced_test
>>>                                           unsigned long vaddr, pgprot_t prot)
>>>  {
>>>         struct page *page = pfn_to_page(pfn);
>>> -       pte_t pte = READ_ONCE(*ptep);
>>> +       pte_t pte;
>>>
>>> -       pte = __pte(pte_val(pte) | RANDOM_ORVALUE);
>>> +       pte = pte_mkhuge(mk_pte_phys(RANDOM_ORVALUE & PMD_MASK, prot));  
>>
>> So that keeps the existing value in 'ptep' pointer at bay and instead
>> construct a PTE from scratch. I would rather have READ_ONCE(*ptep) at
>> least provide the seed that can be ORed with RANDOM_ORVALUE before
>> being masked with PMD_MASK. Do you see any problem ?
> 
> Yes, unfortunately. The problem is that the resulting pte is not marked
> as present. The conversion pte -> (huge) pmd, which is done in
> set_huge_pte_at() for s390, will establish an empty pmd for non-present
> ptes, all the RANDOM_ORVALUE stuff is lost. And a subsequent
> huge_ptep_get() will not result in the same original pte value. If you

Ohh.

> want to preserve and check the RANDOM_ORVALUE, it has to be a present
> pte, hence the mk_pte(_phys).

Understood and mk_pte() is also available on all platforms.

> 
>>
>> Some thing like this instead.
>>
>> pte_t pte = READ_ONCE(*ptep);
>> pte = pte_mkhuge(__pte((pte_val(pte) | RANDOM_ORVALUE) & PMD_MASK));
>>
>> We cannot use mk_pte_phys() as it is defined only on some platforms
>> without any generic fallback for others.
> 
> Oh, didn't know that, sorry. What about using mk_pte() instead, at least
> it would result in a present pte:
> 
> pte = pte_mkhuge(mk_pte(phys_to_page(RANDOM_ORVALUE & PMD_MASK), prot));

Lets use mk_pte() here but can we do this instead

paddr = (__pfn_to_phys(pfn) | RANDOM_ORVALUE) & PMD_MASK;
pte = pte_mkhuge(mk_pte(phys_to_page(paddr), prot));

> 
> And if you also want to do some with the existing value, which seems
> to be an empty pte, then maybe just check if writing and reading that
> value with set_huge_pte_at() / huge_ptep_get() returns the same,
> i.e. initially w/o RANDOM_ORVALUE.
> 
> So, in combination, like this (BTW, why is the barrier() needed, it
> is not used for the other set_huge_pte_at() calls later?):

Ahh missed, will add them. Earlier we faced problem without it after
set_pte_at() for a test on powerpc (64) platform. Hence just added it
here to be extra careful.

> 
> @@ -733,24 +733,28 @@ static void __init hugetlb_advanced_test
>         struct page *page = pfn_to_page(pfn);
>         pte_t pte = READ_ONCE(*ptep);
>  
> -       pte = __pte(pte_val(pte) | RANDOM_ORVALUE);
> +       set_huge_pte_at(mm, vaddr, ptep, pte);
> +       WARN_ON(!pte_same(pte, huge_ptep_get(ptep)));
> +
> +       pte = pte_mkhuge(mk_pte(phys_to_page(RANDOM_ORVALUE & PMD_MASK), prot));
>         set_huge_pte_at(mm, vaddr, ptep, pte);
>         barrier();
>         WARN_ON(!pte_same(pte, huge_ptep_get(ptep)));
> 
> This would actually add a new test "write empty pte with
> set_huge_pte_at(), then verify with huge_ptep_get()", which happens
> to trigger a warning on s390 :-)

On arm64 as well which checks for pte_present() in set_huge_pte_at().
But PTE present check is not really present in each set_huge_pte_at()
implementation especially without __HAVE_ARCH_HUGE_SET_HUGE_PTE_AT.
Hence wondering if we should add this new test here which will keep
giving warnings on s390 and arm64 (at the least).

> 
> That (new) warning actually points to misbehavior on s390, we do not
> write a correct empty pmd in this case, but one that is empty and also
> marked as large. huge_ptep_get() will then not correctly recognize it
> as empty and do wrong conversion. It is also not consistent with
> huge_ptep_get_and_clear(), where we write the empty pmd w/o marking
> as large. Last but not least it would also break our pmd_protnone()
> logic (see below). Another nice finding on s390 :-)

:) 

> 
> I don't think this has any effect in practice (yet), but I will post a
> fix for that, just in case you will add / change this test.

Okay.

> 
>>
>>>         set_huge_pte_at(mm, vaddr, ptep, pte);
>>>         barrier();
>>>         WARN_ON(!pte_same(pte, huge_ptep_get(ptep)));
>>>         huge_pte_clear(mm, vaddr, ptep, PMD_SIZE);
>>> -       pte = READ_ONCE(*ptep);
>>> +       pte = huge_ptep_get(ptep);
>>>         WARN_ON(!huge_pte_none(pte));
>>>  
>>>         pte = mk_huge_pte(page, prot);
>>>         set_huge_pte_at(mm, vaddr, ptep, pte);
>>>         huge_ptep_set_wrprotect(mm, vaddr, ptep);
>>> -       pte = READ_ONCE(*ptep);
>>> +       pte = huge_ptep_get(ptep);
>>>         WARN_ON(huge_pte_write(pte));
>>>  
>>>         pte = mk_huge_pte(page, prot);
>>>         set_huge_pte_at(mm, vaddr, ptep, pte);
>>>         huge_ptep_get_and_clear(mm, vaddr, ptep);
>>> -       pte = READ_ONCE(*ptep);
>>> +       pte = huge_ptep_get(ptep);
>>>         WARN_ON(!huge_pte_none(pte));
>>>  
>>>         pte = mk_huge_pte(page, prot);
>>> @@ -759,7 +759,7 @@ static void __init hugetlb_advanced_test
>>>         pte = huge_pte_mkwrite(pte);
>>>         pte = huge_pte_mkdirty(pte);
>>>         huge_ptep_set_access_flags(vma, vaddr, ptep, pte, 1);
>>> -       pte = READ_ONCE(*ptep);
>>> +       pte = huge_ptep_get(ptep);
>>>         WARN_ON(!(huge_pte_write(pte) && huge_pte_dirty(pte)));
>>>  }
>>>  #else
>>>
>>> 3) The pmd_protnone_tests() has an issue, because it passes a pmd to
>>> pmd_protnone() which has not been marked as large. We check for large
>>> pmd in the s390 implementation of pmd_protnone(), and will fail if a
>>> pmd is not large. We had similar issues before, in other helpers, where
>>> I changed the logic on s390 to not require the pmd large check, but I'm
>>> not so sure in this case. Is there a valid use case for doing
>>> pmd_protnone() on "normal" pmds? Or could this be changed like this:  
>>
>> That is a valid question. IIUC, all existing callers for pmd_protnone()
>> ensure that it is indeed a huge PMD. But even assuming otherwise should
>> not the huge PMD requirement get checked in the caller itself rather than
>> in the arch helper which is just supposed to check the existence of the
>> dedicated PTE bit(s) for this purpose. Purely from a helper perspective
>> pmd_protnone() should not really care about being large even though it
>> might never get used without one.
>>
>> Also all platforms (except s390) derive the pmd_protnone() from their
>> respective pte_protnone(). I wonder why should s390 be any different
>> unless it is absolutely necessary.
> 
> This is again because of our different page table entry layouts for
> pte/pmd and (large) pmd. The bits we check for pmd_protnone() are
> not valid for normal pmd/pte, and we would return undefined result for
> normal entries.
> 
> Of course, we could rely on nobody calling pmd_protnone() on normal
> pmds, but in this case we also use pmd_large() check in pmd_protnone()
> for indication if the pmd is present. W/o that, we would return
> true for empty pmds, that doesn't sound right. Not sure if we also
> want to rely on nobody calling pmd_protnone() on empty pmds.

That might be problematic.

> 
> Anyway, if in practice it is not correct to use pmd_protnone()
> on normal pmds, then I would suggest that your tests should also
> not do / test it. And I strongly assume that it is not correct, at
> least I cannot think of a valid case, and of course s390 would
> already be broken if there was such a case.

Okay, will use huge PMD here as you had suggested earlier.

> 
> Regards,
> Gerald
> 
> 

Re: [PATCH V2 0/3] mm/debug: Add more arch page table helper tests

[Gerald Schaefer]

On Wed, 8 Apr 2020 12:41:51 +0530
Anshuman Khandual <anshuman.khandual@arm.com> wrote:

[...]
> >   
> >>
> >> Some thing like this instead.
> >>
> >> pte_t pte = READ_ONCE(*ptep);
> >> pte = pte_mkhuge(__pte((pte_val(pte) | RANDOM_ORVALUE) & PMD_MASK));
> >>
> >> We cannot use mk_pte_phys() as it is defined only on some platforms
> >> without any generic fallback for others.  
> > 
> > Oh, didn't know that, sorry. What about using mk_pte() instead, at least
> > it would result in a present pte:
> > 
> > pte = pte_mkhuge(mk_pte(phys_to_page(RANDOM_ORVALUE & PMD_MASK), prot));  
> 
> Lets use mk_pte() here but can we do this instead
> 
> paddr = (__pfn_to_phys(pfn) | RANDOM_ORVALUE) & PMD_MASK;
> pte = pte_mkhuge(mk_pte(phys_to_page(paddr), prot));
> 

Sure, that will also work.

BTW, this RANDOM_ORVALUE is not really very random, the way it is
defined. For s390 we already changed it to mask out some arch bits,
but I guess there are other archs and bits that would always be
set with this "not so random" value, and I wonder if/how that would
affect all the tests using this value, see also below.

> > 
> > And if you also want to do some with the existing value, which seems
> > to be an empty pte, then maybe just check if writing and reading that
> > value with set_huge_pte_at() / huge_ptep_get() returns the same,
> > i.e. initially w/o RANDOM_ORVALUE.
> > 
> > So, in combination, like this (BTW, why is the barrier() needed, it
> > is not used for the other set_huge_pte_at() calls later?):  
> 
> Ahh missed, will add them. Earlier we faced problem without it after
> set_pte_at() for a test on powerpc (64) platform. Hence just added it
> here to be extra careful.
> 
> > 
> > @@ -733,24 +733,28 @@ static void __init hugetlb_advanced_test
> >         struct page *page = pfn_to_page(pfn);
> >         pte_t pte = READ_ONCE(*ptep);
> >  
> > -       pte = __pte(pte_val(pte) | RANDOM_ORVALUE);
> > +       set_huge_pte_at(mm, vaddr, ptep, pte);
> > +       WARN_ON(!pte_same(pte, huge_ptep_get(ptep)));
> > +
> > +       pte = pte_mkhuge(mk_pte(phys_to_page(RANDOM_ORVALUE & PMD_MASK), prot));
> >         set_huge_pte_at(mm, vaddr, ptep, pte);
> >         barrier();
> >         WARN_ON(!pte_same(pte, huge_ptep_get(ptep)));
> > 
> > This would actually add a new test "write empty pte with
> > set_huge_pte_at(), then verify with huge_ptep_get()", which happens
> > to trigger a warning on s390 :-)  
> 
> On arm64 as well which checks for pte_present() in set_huge_pte_at().
> But PTE present check is not really present in each set_huge_pte_at()
> implementation especially without __HAVE_ARCH_HUGE_SET_HUGE_PTE_AT.
> Hence wondering if we should add this new test here which will keep
> giving warnings on s390 and arm64 (at the least).

Hmm, interesting. I forgot about huge swap / migration, which is not
(and probably cannot be) supported on s390. The pte_present() check
on arm64 seems to check for such huge swap / migration entries,
according to the comment.

The new test "write empty pte with set_huge_pte_at(), then verify
with huge_ptep_get()" would then probably trigger the
WARN_ON(!pte_present(pte)) in arm64 code. So I guess "writing empty
ptes with set_huge_pte_at()" is not really a valid use case in practice,
or else you would have seen this warning before. In that case, it
might not be a good idea to add this test.

I also do wonder now, why the original test with
"pte = __pte(pte_val(pte) | RANDOM_ORVALUE);"
did not also trigger that warning on arm64. On s390 this test failed
exactly because the constructed pte was not present (initially empty,
or'ing RANDOM_ORVALUE does not make it present for s390). I guess this
just worked by chance on arm64, because the bits from RANDOM_ORVALUE
also happened to mark the pte present for arm64.

This brings us back to the question above, regarding the "randomness"
of RANDOM_ORVALUE. Not really sure what the intention behind that was,
but maybe it would make sense to restrict this RANDOM_ORVALUE to
non-arch-specific bits, i.e. only bits that would be part of the
address value within a page table entry? Or was it intentionally
chosen to also mess with other bits?

Regards,
Gerald

Re: [PATCH V2 0/3] mm/debug: Add more arch page table helper tests

[Anshuman Khandual]

On 04/08/2020 05:45 PM, Gerald Schaefer wrote:
> On Wed, 8 Apr 2020 12:41:51 +0530
> Anshuman Khandual <anshuman.khandual@arm.com> wrote:
> 
> [...]
>>>   
>>>>
>>>> Some thing like this instead.
>>>>
>>>> pte_t pte = READ_ONCE(*ptep);
>>>> pte = pte_mkhuge(__pte((pte_val(pte) | RANDOM_ORVALUE) & PMD_MASK));
>>>>
>>>> We cannot use mk_pte_phys() as it is defined only on some platforms
>>>> without any generic fallback for others.  
>>>
>>> Oh, didn't know that, sorry. What about using mk_pte() instead, at least
>>> it would result in a present pte:
>>>
>>> pte = pte_mkhuge(mk_pte(phys_to_page(RANDOM_ORVALUE & PMD_MASK), prot));  
>>
>> Lets use mk_pte() here but can we do this instead
>>
>> paddr = (__pfn_to_phys(pfn) | RANDOM_ORVALUE) & PMD_MASK;
>> pte = pte_mkhuge(mk_pte(phys_to_page(paddr), prot));
>>
> 
> Sure, that will also work.
> 
> BTW, this RANDOM_ORVALUE is not really very random, the way it is
> defined. For s390 we already changed it to mask out some arch bits,
> but I guess there are other archs and bits that would always be
> set with this "not so random" value, and I wonder if/how that would
> affect all the tests using this value, see also below.

RANDOM_ORVALUE is a constant which was added in order to make sure
that the page table entries should have some non-zero value before
getting called with pxx_clear() and followed by a pxx_none() check.
This is currently used only in pxx_clear_tests() tests. Hence there
is no impact for the existing tests.

> 
>>>
>>> And if you also want to do some with the existing value, which seems
>>> to be an empty pte, then maybe just check if writing and reading that
>>> value with set_huge_pte_at() / huge_ptep_get() returns the same,
>>> i.e. initially w/o RANDOM_ORVALUE.
>>>
>>> So, in combination, like this (BTW, why is the barrier() needed, it
>>> is not used for the other set_huge_pte_at() calls later?):  
>>
>> Ahh missed, will add them. Earlier we faced problem without it after
>> set_pte_at() for a test on powerpc (64) platform. Hence just added it
>> here to be extra careful.
>>
>>>
>>> @@ -733,24 +733,28 @@ static void __init hugetlb_advanced_test
>>>         struct page *page = pfn_to_page(pfn);
>>>         pte_t pte = READ_ONCE(*ptep);
>>>  
>>> -       pte = __pte(pte_val(pte) | RANDOM_ORVALUE);
>>> +       set_huge_pte_at(mm, vaddr, ptep, pte);
>>> +       WARN_ON(!pte_same(pte, huge_ptep_get(ptep)));
>>> +
>>> +       pte = pte_mkhuge(mk_pte(phys_to_page(RANDOM_ORVALUE & PMD_MASK), prot));
>>>         set_huge_pte_at(mm, vaddr, ptep, pte);
>>>         barrier();
>>>         WARN_ON(!pte_same(pte, huge_ptep_get(ptep)));
>>>
>>> This would actually add a new test "write empty pte with
>>> set_huge_pte_at(), then verify with huge_ptep_get()", which happens
>>> to trigger a warning on s390 :-)  
>>
>> On arm64 as well which checks for pte_present() in set_huge_pte_at().
>> But PTE present check is not really present in each set_huge_pte_at()
>> implementation especially without __HAVE_ARCH_HUGE_SET_HUGE_PTE_AT.
>> Hence wondering if we should add this new test here which will keep
>> giving warnings on s390 and arm64 (at the least).
> 
> Hmm, interesting. I forgot about huge swap / migration, which is not
> (and probably cannot be) supported on s390. The pte_present() check
> on arm64 seems to check for such huge swap / migration entries,
> according to the comment.
> 
> The new test "write empty pte with set_huge_pte_at(), then verify
> with huge_ptep_get()" would then probably trigger the
> WARN_ON(!pte_present(pte)) in arm64 code. So I guess "writing empty
> ptes with set_huge_pte_at()" is not really a valid use case in practice,
> or else you would have seen this warning before. In that case, it
> might not be a good idea to add this test.

Got it.

> 
> I also do wonder now, why the original test with
> "pte = __pte(pte_val(pte) | RANDOM_ORVALUE);"
> did not also trigger that warning on arm64. On s390 this test failed
> exactly because the constructed pte was not present (initially empty,
> or'ing RANDOM_ORVALUE does not make it present for s390). I guess this
> just worked by chance on arm64, because the bits from RANDOM_ORVALUE
> also happened to mark the pte present for arm64.

That is correct. RANDOM_ORVALUE has got PTE_PROT_NONE bit set that makes
the PTE test for pte_present().

On arm64 platform,

#define pte_present(pte)  (!!(pte_val(pte) & (PTE_VALID | PTE_PROT_NONE)))

> 
> This brings us back to the question above, regarding the "randomness"
> of RANDOM_ORVALUE. Not really sure what the intention behind that was,
> but maybe it would make sense to restrict this RANDOM_ORVALUE to
> non-arch-specific bits, i.e. only bits that would be part of the
> address value within a page table entry? Or was it intentionally
> chosen to also mess with other bits?

As mentioned before, RANDOM_ORVALUE just helped make a given page table
entry contain non-zero values before getting cleared. AFAICS we should
not need RANDOM_ORVALUE for HugeTLB test here. I believe the following
'paddr' construct will just be fine instead.

paddr = __pfn_to_phys(pfn) & PMD_MASK;
pte = pte_mkhuge(mk_pte(phys_to_page(paddr), prot));

> 
> Regards,
> Gerald
> 
>