Re: [PATCH v2] mm: hugetlb: independent PMD page table shared count

From: jane . chu
Date: Mon Dec 16 2024 - 19:05:34 EST

On 12/15/2024 11:11 PM, Liu Shixin wrote:

The folio refcount may be increased unexpectly through try_get_folio() by
caller such as split_huge_pages. In huge_pmd_unshare(), we use refcount to
check whether a pmd page table is shared. The check is incorrect if the
refcount is increased by the above caller, and this can cause the page
table leaked:

BUG: Bad page state in process sh pfn:109324
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x66 pfn:0x109324
flags: 0x17ffff800000000(node=0|zone=2|lastcpupid=0xfffff)
page_type: f2(table)
raw: 017ffff800000000 0000000000000000 0000000000000000 0000000000000000
raw: 0000000000000066 0000000000000000 00000000f2000000 0000000000000000
page dumped because: nonzero mapcount
...
CPU: 31 UID: 0 PID: 7515 Comm: sh Kdump: loaded Tainted: G B 6.13.0-rc2master+ #7
Tainted: [B]=BAD_PAGE
Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
Call trace:
show_stack+0x20/0x38 (C)
dump_stack_lvl+0x80/0xf8
dump_stack+0x18/0x28
bad_page+0x8c/0x130
free_page_is_bad_report+0xa4/0xb0
free_unref_page+0x3cc/0x620
__folio_put+0xf4/0x158
split_huge_pages_all+0x1e0/0x3e8
split_huge_pages_write+0x25c/0x2d8
full_proxy_write+0x64/0xd8
vfs_write+0xcc/0x280
ksys_write+0x70/0x110
__arm64_sys_write+0x24/0x38
invoke_syscall+0x50/0x120
el0_svc_common.constprop.0+0xc8/0xf0
do_el0_svc+0x24/0x38
el0_svc+0x34/0x128
el0t_64_sync_handler+0xc8/0xd0
el0t_64_sync+0x190/0x198

The issue may be triggered by damon, offline_page, page_idle, etc, which
will increase the refcount of page table.

Fix it by introducing independent PMD page table shared count.
As described by comment, pt_index/pt_mm/pt_frag_refcount are used for
s390 gmap, x86 pgds and powerpc, pt_share_count is used for x86/arm64/riscv
pmds, so we can reuse the field as pt_share_count.

Fixes: 39dde65c9940 ("[PATCH] shared page table for hugetlb page")
Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx>
---
v1->v2: Fix build error when !CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING and
modify the changelog.

Sorry for having missed v2 when I responded,  the same  question remains -

https://lore.kernel.org/linux-mm/202412151541.SQEZStgy-lkp@xxxxxxxxx/T/#m90c26215594e494f5c65c3fa71979b519f0c8481

thanks,

-jane


include/linux/mm.h | 1 +
include/linux/mm_types.h | 30 ++++++++++++++++++++++++++++++
mm/hugetlb.c | 16 +++++++---------
3 files changed, 38 insertions(+), 9 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index c39c4945946c..50fbf2a1b0ad 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3115,6 +3115,7 @@ static inline bool pagetable_pmd_ctor(struct ptdesc *ptdesc)
if (!pmd_ptlock_init(ptdesc))
return false;
__folio_set_pgtable(folio);
+ ptdesc_pmd_pts_init(ptdesc);
lruvec_stat_add_folio(folio, NR_PAGETABLE);
return true;
}
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 7361a8f3ab68..332cee285662 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -445,6 +445,7 @@ FOLIO_MATCH(compound_head, _head_2a);
* @pt_index: Used for s390 gmap.
* @pt_mm: Used for x86 pgds.
* @pt_frag_refcount: For fragmented page table tracking. Powerpc only.
+ * @pt_share_count: Used for HugeTLB PMD page table share count.
* @_pt_pad_2: Padding to ensure proper alignment.
* @ptl: Lock for the page table.
* @__page_type: Same as page->page_type. Unused for page tables.
@@ -471,6 +472,9 @@ struct ptdesc {
pgoff_t pt_index;
struct mm_struct *pt_mm;
atomic_t pt_frag_refcount;
+#ifdef CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING
+ atomic_t pt_share_count;
+#endif
};
union {
@@ -516,6 +520,32 @@ static_assert(sizeof(struct ptdesc) <= sizeof(struct page));
const struct page *: (const struct ptdesc *)(p), \
struct page *: (struct ptdesc *)(p)))
+#ifdef CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING
+static inline void ptdesc_pmd_pts_init(struct ptdesc *ptdesc)
+{
+ atomic_set(&ptdesc->pt_share_count, 0);
+}
+
+static inline void ptdesc_pmd_pts_inc(struct ptdesc *ptdesc)
+{
+ atomic_inc(&ptdesc->pt_share_count);
+}
+
+static inline void ptdesc_pmd_pts_dec(struct ptdesc *ptdesc)
+{
+ atomic_dec(&ptdesc->pt_share_count);
+}
+
+static inline int ptdesc_pmd_pts_count(struct ptdesc *ptdesc)
+{
+ return atomic_read(&ptdesc->pt_share_count);
+}
+#else
+static inline void ptdesc_pmd_pts_init(struct ptdesc *ptdesc)
+{
+}
+#endif
+
/*
* Used for sizing the vmemmap region on some architectures
*/
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index ea2ed8e301ef..60846b060b87 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -7212,7 +7212,7 @@ pte_t *huge_pmd_share(struct mm_struct *mm, struct vm_area_struct *vma,
spte = hugetlb_walk(svma, saddr,
vma_mmu_pagesize(svma));
if (spte) {
- get_page(virt_to_page(spte));
+ ptdesc_pmd_pts_inc(virt_to_ptdesc(spte));
break;
}
}
@@ -7227,7 +7227,7 @@ pte_t *huge_pmd_share(struct mm_struct *mm, struct vm_area_struct *vma,
(pmd_t *)((unsigned long)spte & PAGE_MASK));
mm_inc_nr_pmds(mm);
} else {
- put_page(virt_to_page(spte));
+ ptdesc_pmd_pts_dec(virt_to_ptdesc(spte));
}
spin_unlock(&mm->page_table_lock);
out:
@@ -7239,10 +7239,6 @@ pte_t *huge_pmd_share(struct mm_struct *mm, struct vm_area_struct *vma,
/*
* unmap huge page backed by shared pte.
*
- * Hugetlb pte page is ref counted at the time of mapping. If pte is shared
- * indicated by page_count > 1, unmap is achieved by clearing pud and
- * decrementing the ref count. If count == 1, the pte page is not shared.
- *
* Called with page table lock held.
*
* returns: 1 successfully unmapped a shared pte page
@@ -7251,18 +7247,20 @@ pte_t *huge_pmd_share(struct mm_struct *mm, struct vm_area_struct *vma,
int huge_pmd_unshare(struct mm_struct *mm, struct vm_area_struct *vma,
unsigned long addr, pte_t *ptep)
{
+ unsigned long sz = huge_page_size(hstate_vma(vma));
pgd_t *pgd = pgd_offset(mm, addr);
p4d_t *p4d = p4d_offset(pgd, addr);
pud_t *pud = pud_offset(p4d, addr);
i_mmap_assert_write_locked(vma->vm_file->f_mapping);
hugetlb_vma_assert_locked(vma);
- BUG_ON(page_count(virt_to_page(ptep)) == 0);
- if (page_count(virt_to_page(ptep)) == 1)
+ if (sz != PMD_SIZE)
+ return 0;
+ if (!ptdesc_pmd_pts_count(virt_to_ptdesc(ptep)))
return 0;
pud_clear(pud);
- put_page(virt_to_page(ptep));
+ ptdesc_pmd_pts_dec(virt_to_ptdesc(ptep));
mm_dec_nr_pmds(mm);
return 1;
}