Re: [PATCH v1 1/5] mm: pagewalk: Fix walk for hugepage tables
From: Daniel Axtens <hidden>
Date: 2021-04-15 22:43:46
Also in:
linux-arch, linux-arm-kernel, linux-mm, linux-riscv, linux-s390, lkml
Hi Christophe,
quoted hunk ↗ jump to hunk
Pagewalk ignores hugepd entries and walk down the tables as if it was traditionnal entries, leading to crazy result. Add walk_hugepd_range() and use it to walk hugepage tables. Signed-off-by: Christophe Leroy <redacted> --- mm/pagewalk.c | 54 +++++++++++++++++++++++++++++++++++++++++++++------ 1 file changed, 48 insertions(+), 6 deletions(-)diff --git a/mm/pagewalk.c b/mm/pagewalk.c index e81640d9f177..410a9d8f7572 100644 --- a/mm/pagewalk.c +++ b/mm/pagewalk.c@@ -58,6 +58,32 @@ static int walk_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end, return err; } +static int walk_hugepd_range(hugepd_t *phpd, unsigned long addr, + unsigned long end, struct mm_walk *walk, int pdshift) +{ + int err = 0; +#ifdef CONFIG_ARCH_HAS_HUGEPD + const struct mm_walk_ops *ops = walk->ops; + int shift = hugepd_shift(*phpd); + int page_size = 1 << shift; + + if (addr & (page_size - 1)) + return 0; + + for (;;) { + pte_t *pte = hugepte_offset(*phpd, addr, pdshift); + + err = ops->pte_entry(pte, addr, addr + page_size, walk); + if (err) + break; + if (addr >= end - page_size) + break; + addr += page_size; + }
Initially I thought this was a somewhat unintuitive way to structure this loop, but I see it parallels the structure of walk_pte_range_inner, so I think the consistency is worth it. I notice the pte walking code potentially takes some locks: does this code need to do that? arch/powerpc/mm/hugetlbpage.c says that hugepds are protected by the mm->page_table_lock, but I don't think we're taking it in this code.
quoted hunk ↗ jump to hunk
+#endif + return err; +} + static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end, struct mm_walk *walk) {@@ -108,7 +134,10 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end, goto again; } - err = walk_pte_range(pmd, addr, next, walk); + if (is_hugepd(__hugepd(pmd_val(*pmd)))) + err = walk_hugepd_range((hugepd_t *)pmd, addr, next, walk, PMD_SHIFT); + else + err = walk_pte_range(pmd, addr, next, walk); if (err) break; } while (pmd++, addr = next, addr != end);@@ -157,7 +186,10 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end, if (pud_none(*pud)) goto again; - err = walk_pmd_range(pud, addr, next, walk); + if (is_hugepd(__hugepd(pud_val(*pud)))) + err = walk_hugepd_range((hugepd_t *)pud, addr, next, walk, PUD_SHIFT); + else + err = walk_pmd_range(pud, addr, next, walk);
I'm a bit worried you might end up calling into walk_hugepd_range with
ops->pte_entry == NULL, and then jumping to 0.
static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
struct mm_walk *walk)
{
...
pud = pud_offset(p4d, addr);
do {
...
if ((!walk->vma && (pud_leaf(*pud) || !pud_present(*pud))) ||
walk->action == ACTION_CONTINUE ||
!(ops->pmd_entry || ops->pte_entry)) <<< THIS CHECK
continue;
...
if (is_hugepd(__hugepd(pud_val(*pud))))
err = walk_hugepd_range((hugepd_t *)pud, addr, next, walk, PUD_SHIFT);
else
err = walk_pmd_range(pud, addr, next, walk);
if (err)
break;
} while (pud++, addr = next, addr != end);
walk_pud_range will proceed if there is _either_ an ops->pmd_entry _or_
an ops->pte_entry, but walk_hugepd_range will call ops->pte_entry
unconditionally.
The same issue applies to walk_{p4d,pgd}_range...
Kind regards,
Daniel