On 30/05/25 7:06 pm, Will Deacon wrote:

On Fri, May 30, 2025 at 02:11:36PM +0100, Ryan Roberts wrote:

quoted

On 30/05/2025 13:35, Will Deacon wrote:

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On Fri, May 30, 2025 at 12:50:40PM +0100, Ryan Roberts wrote:

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On 30/05/2025 10:14, Dev Jain wrote:

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On 30/05/25 2:10 pm, Ryan Roberts wrote:

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On 30/05/2025 09:20, Dev Jain wrote:

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arm64 disables vmalloc-huge when kernel page table dumping is enabled,
because an intermediate table may be removed, potentially causing the
ptdump code to dereference an invalid address. We want to be able to
analyze block vs page mappings for kernel mappings with ptdump, so to
enable vmalloc-huge with ptdump, synchronize between page table removal in
pmd_free_pte_page()/pud_free_pmd_page() and ptdump pagetable walking. We
use mmap_read_lock and not write lock because we don't need to synchronize
between two different vm_structs; two vmalloc objects running this same
code path will point to different page tables, hence there is no race.

My "correction" from race->no problem was incorrect after all :) There will
be no race too since the vm_struct object has exclusive access to whatever
table it is clearing.

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Signed-off-by: Dev Jain <dev.jain@arm.com>
---
   arch/arm64/include/asm/vmalloc.h | 6 ++----
   arch/arm64/mm/mmu.c              | 7 +++++++
   2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/vmalloc.h b/arch/arm64/include/asm/vmalloc.h
index 38fafffe699f..28b7173d8693 100644
--- a/arch/arm64/include/asm/vmalloc.h
+++ b/arch/arm64/include/asm/vmalloc.h

@@ -12,15 +12,13 @@ static inline bool arch_vmap_pud_supported(pgprot_t prot)
       /*
        * SW table walks can't handle removal of intermediate entries.
        */
-    return pud_sect_supported() &&
-           !IS_ENABLED(CONFIG_PTDUMP_DEBUGFS);
+    return pud_sect_supported();
   }
     #define arch_vmap_pmd_supported arch_vmap_pmd_supported
   static inline bool arch_vmap_pmd_supported(pgprot_t prot)
   {
-    /* See arch_vmap_pud_supported() */
-    return !IS_ENABLED(CONFIG_PTDUMP_DEBUGFS);
+    return true;
   }
     #endif

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index ea6695d53fb9..798cebd9e147 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c

@@ -1261,7 +1261,11 @@ int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr)
       }
         table = pte_offset_kernel(pmdp, addr);
+
+    /* Synchronize against ptdump_walk_pgd() */
+    mmap_read_lock(&init_mm);
       pmd_clear(pmdp);
+    mmap_read_unlock(&init_mm);

So this works because ptdump_walk_pgd() takes the write_lock (which is mutually
exclusive with any read_lock holders) for the duration of the table walk, so it
will either consistently see the pgtables before or after this removal. It will
never disappear during the walk, correct?

I guess there is a risk of this showing up as contention with other init_mm
write_lock holders. But I expect that pmd_free_pte_page()/pud_free_pmd_page()
are called sufficiently rarely that the risk is very small. Let's fix any perf
problem if/when we see it.

We can avoid all of that by my initial approach - to wrap the lock around
CONFIG_PTDUMP_DEBUGFS.
I don't have a strong opinion, just putting it out there.

(I wrote then failed to send earlier):

It's ugly though. Personally I'd prefer to keep it simple unless we have clear
evidence that its needed. I was just laying out my justification for not needing
to doing the conditional wrapping in this comment.

I really don't think we should be adding unconditional locking overhead
to core mm routines purely to facilitate a rarely used debug option.

Instead, can we either adopt something like the RCU-like walk used by
fast GUP or stick the locking behind a static key that's only enabled
when a ptdump walk is in progress (a bit like how
hugetlb_vmemmap_optimize_folio() manipulates hugetlb_optimize_vmemmap_key)?

My sense is that the static key will be less effort and can be contained fully
in arm64. I think we would need to enable the key around the call to
ptdump_walk_pgd() in both ptdump_walk() and ptdump_check_wx(). Then where Dev is
currently taking the read lock, that would be contingent on the key being
enabled and the unlock would be contingent on having taken the lock.

Does that sound like an acceptable approach?

Yup, and I think you'll probably need something like a synchronize_rcu()
when flipping the key to deal with any pre-existing page-table freers.

IIUC, you mean to say that we need to ensure any existing readers having
a reference to the isolated table in pmd_free_pte_page and friend, have exited.
But the problem is that we take an mmap_write_lock() around ptdump_walk_pgd() so
this is a sleepable code path.

Will