[PATCH] mm/damon: use a page-aligned sampling address

[Nathan Gao]

__damon_va_prepare_access_check() picks a random byte address within the
region and stores it in r->sampling_addr. There are two users of
r->sampling_addr in vaddr.c that pass it into a page table walk, and
both use it as the address of a page.

  damon_va_mkold(mm, r->sampling_addr)
    damon_va_walk_page_range(mm, addr, addr + 1)
      damon_mkold_pmd_entry()
        damon_ptep_mkold(pte, vma, addr)
          ptep_test_and_clear_young(vma, addr, pte)
          mmu_notifier_clear_young(mm, addr, addr + PAGE_SIZE)

  damon_va_young(mm, r->sampling_addr, &folio_sz)
    damon_va_walk_page_range(mm, addr, addr + 1)
      damon_young_pmd_entry()
        ptep_get(pte)
        mmu_notifier_test_young(walk->mm, addr)

test_and_clear_young_ptes(), which backs ptep_test_and_clear_young() on
arm64, documents @addr as "Address the first page is mapped at".

For arm64, before commit 6f0e1142173a ("arm64: mm: support batch
clearing of the young flag for large folios"), the contpte helper walked
exactly CONT_PTES entries from the aligned-down page table pointer and
used @addr only to pass down to each entry, so an unaligned value was
harmless:

        ptep = contpte_align_down(ptep);
        addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
        for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE)

Now the range to walk is derived from @addr instead: end = addr +
nr * PAGE_SIZE, rounded up to CONT_PTE_SIZE. For a sample in the last
page of a contpte block, the sub-page offset puts end just past the
block boundary, so the round-up lands a whole block further and the
walk clears PTE_AF in CONT_PTES entries beyond the sampled block. Seen
on an arm64 guest running the DAMON selftests as random slab and page
table corruption.

Align the sampled address down to a page boundary. It is the address of
the page to sample, so this matches its intended meaning and fixes both
users in vaddr.c.

Fixes: 3f49584b262c ("mm/damon: implement primitives for the virtual memory address spaces")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Nathan Gao <zcgao@xxxxxxxxxx>
---
 mm/damon/vaddr.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
index 2c1c1952c008d..e26e426a56af2 100644
--- a/mm/damon/vaddr.c
+++ b/mm/damon/vaddr.c
@@ -360,7 +360,8 @@ static void __damon_va_prepare_access_check(struct mm_struct *mm,
 					struct damon_region *r,
 					struct damon_ctx *ctx)
 {
-	r->sampling_addr = damon_rand(ctx, r->ar.start, r->ar.end);
+	r->sampling_addr = PAGE_ALIGN_DOWN(damon_rand(ctx, r->ar.start,
+						      r->ar.end));
 
 	damon_va_mkold(mm, r->sampling_addr);
 }
-- 
2.50.1