On Fri 19-01-18 15:49:24, Kirill A. Shutemov wrote:
> Tetsuo reported random crashes under memory pressure on 32-bit x86
> system and tracked down to change that introduced
> page_vma_mapped_walk().
>
> The root cause of the issue is the faulty pointer math in check_pte().
> As ->pte may point to an arbitrary page we have to check that they are
> belong to the section before doing math. Otherwise it may lead to weird
> results.
>
> It wasn't noticed until now as mem_map[] is virtually contiguous on
> flatmem or vmemmap sparsemem. Pointer arithmetic just works against all
> 'struct page' pointers. But with classic sparsemem, it doesn't because
> each section memap is allocated separately and so consecutive pfns
> crossing two sections might have struct pages at completely unrelated
> addresses.
>
> Let's restructure code a bit and replace pointer arithmetic with
> operations on pfns.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Reported-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Fixes: ace71a19cec5 ("mm: introduce page_vma_mapped_walk()")
> Cc: stable@xxxxxxxxxxxxxxx
Much better. Thanks!
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> v2:
> - Do not use uninitialized 'pfn' for !MIGRATION case (Michal)
>
> ---
> include/linux/swapops.h | 21 +++++++++++++++++
> mm/page_vma_mapped.c | 63 +++++++++++++++++++++++++++++--------------------
> 2 files changed, 59 insertions(+), 25 deletions(-)
>
> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> index 9c5a2628d6ce..1d3877c39a00 100644
> --- a/include/linux/swapops.h
> +++ b/include/linux/swapops.h
> @@ -124,6 +124,11 @@ static inline bool is_write_device_private_entry(swp_entry_t entry)
> return unlikely(swp_type(entry) == SWP_DEVICE_WRITE);
> }
>
> +static inline unsigned long device_private_entry_to_pfn(swp_entry_t entry)
> +{
> + return swp_offset(entry);
> +}
> +
> static inline struct page *device_private_entry_to_page(swp_entry_t entry)
> {
> return pfn_to_page(swp_offset(entry));
> @@ -154,6 +159,11 @@ static inline bool is_write_device_private_entry(swp_entry_t entry)
> return false;
> }
>
> +static inline unsigned long device_private_entry_to_pfn(swp_entry_t entry)
> +{
> + return 0;
> +}
> +
> static inline struct page *device_private_entry_to_page(swp_entry_t entry)
> {
> return NULL;
> @@ -189,6 +199,11 @@ static inline int is_write_migration_entry(swp_entry_t entry)
> return unlikely(swp_type(entry) == SWP_MIGRATION_WRITE);
> }
>
> +static inline unsigned long migration_entry_to_pfn(swp_entry_t entry)
> +{
> + return swp_offset(entry);
> +}
> +
> static inline struct page *migration_entry_to_page(swp_entry_t entry)
> {
> struct page *p = pfn_to_page(swp_offset(entry));
> @@ -218,6 +233,12 @@ static inline int is_migration_entry(swp_entry_t swp)
> {
> return 0;
> }
> +
> +static inline unsigned long migration_entry_to_pfn(swp_entry_t entry)
> +{
> + return 0;
> +}
> +
> static inline struct page *migration_entry_to_page(swp_entry_t entry)
> {
> return NULL;
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index d22b84310f6d..956015614395 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -30,10 +30,29 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw)
> return true;
> }
>
> +/**
> + * check_pte - check if @pvmw->page is mapped at the @pvmw->pte
> + *
> + * page_vma_mapped_walk() found a place where @pvmw->page is *potentially*
> + * mapped. check_pte() has to validate this.
> + *
> + * @pvmw->pte may point to empty PTE, swap PTE or PTE pointing to arbitrary
> + * page.
> + *
> + * If PVMW_MIGRATION flag is set, returns true if @pvmw->pte contains migration
> + * entry that points to @pvmw->page or any subpage in case of THP.
> + *
> + * If PVMW_MIGRATION flag is not set, returns true if @pvmw->pte points to
> + * @pvmw->page or any subpage in case of THP.
> + *
> + * Otherwise, return false.
> + *
> + */
> static bool check_pte(struct page_vma_mapped_walk *pvmw)
> {
> + unsigned long pfn;
> +
> if (pvmw->flags & PVMW_MIGRATION) {
> -#ifdef CONFIG_MIGRATION
> swp_entry_t entry;
> if (!is_swap_pte(*pvmw->pte))
> return false;
> @@ -41,37 +60,31 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw)
>
> if (!is_migration_entry(entry))
> return false;
> - if (migration_entry_to_page(entry) - pvmw->page >=
> - hpage_nr_pages(pvmw->page)) {
> - return false;
> - }
> - if (migration_entry_to_page(entry) < pvmw->page)
> - return false;
> -#else
> - WARN_ON_ONCE(1);
> -#endif
> - } else {
> - if (is_swap_pte(*pvmw->pte)) {
> - swp_entry_t entry;
>
> - entry = pte_to_swp_entry(*pvmw->pte);
> - if (is_device_private_entry(entry) &&
> - device_private_entry_to_page(entry) == pvmw->page)
> - return true;
> - }
> + pfn = migration_entry_to_pfn(entry);
> + } else if (is_swap_pte(*pvmw->pte)) {
> + swp_entry_t entry;
>
> - if (!pte_present(*pvmw->pte))
> + /* Handle un-addressable ZONE_DEVICE memory */
> + entry = pte_to_swp_entry(*pvmw->pte);
> + if (!is_device_private_entry(entry))
> return false;
>
> - /* THP can be referenced by any subpage */
> - if (pte_page(*pvmw->pte) - pvmw->page >=
> - hpage_nr_pages(pvmw->page)) {
> - return false;
> - }
> - if (pte_page(*pvmw->pte) < pvmw->page)
> + pfn = device_private_entry_to_pfn(entry);
> + } else {
> + if (!pte_present(*pvmw->pte))
> return false;
> +
> + pfn = pte_pfn(*pvmw->pte);
> }
>
> + if (pfn < page_to_pfn(pvmw->page))
> + return false;
> +
> + /* THP can be referenced by any subpage */
> + if (pfn - page_to_pfn(pvmw->page) >= hpage_nr_pages(pvmw->page))
> + return false;
> +
> return true;
> }
>
> --
> 2.15.1
--
Michal Hocko
SUSE Labs