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SubjectRe: [PATCH] mm: page_mapped: don't assume compound page is huge or THP
On Thu, Nov 29, 2018 at 10:53:48PM +0100, Jan Stancek wrote:
> LTP proc01 testcase has been observed to rarely trigger crashes
> on arm64:
> page_mapped+0x78/0xb4
> stable_page_flags+0x27c/0x338
> kpageflags_read+0xfc/0x164
> proc_reg_read+0x7c/0xb8
> __vfs_read+0x58/0x178
> vfs_read+0x90/0x14c
> SyS_read+0x60/0xc0
>
> Issue is that page_mapped() assumes that if compound page is not
> huge, then it must be THP. But if this is 'normal' compound page
> (COMPOUND_PAGE_DTOR), then following loop can keep running until
> it tries to read from memory that isn't mapped and triggers a panic:
> for (i = 0; i < hpage_nr_pages(page); i++) {
> if (atomic_read(&page[i]._mapcount) >= 0)
> return true;
> }
>
> I could replicate this on x86 (v4.20-rc4-98-g60b548237fed) only
> with a custom kernel module [1] which:
> - allocates compound page (PAGEC) of order 1
> - allocates 2 normal pages (COPY), which are initialized to 0xff
> (to satisfy _mapcount >= 0)
> - 2 PAGEC page structs are copied to address of first COPY page
> - second page of COPY is marked as not present
> - call to page_mapped(COPY) now triggers fault on access to 2nd COPY
> page at offset 0x30 (_mapcount)
>
> [1] https://github.com/jstancek/reproducers/blob/master/kernel/page_mapped_crash/repro.c
>
> This patch modifies page_mapped() to check for 'normal'
> compound pages (COMPOUND_PAGE_DTOR).
>
> Debugged-by: Laszlo Ersek <lersek@redhat.com>
> Signed-off-by: Jan Stancek <jstancek@redhat.com>
> ---
> include/linux/mm.h | 9 +++++++++
> mm/util.c | 2 ++
> 2 files changed, 11 insertions(+)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 5411de93a363..18b0bb953f92 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -700,6 +700,15 @@ static inline compound_page_dtor *get_compound_page_dtor(struct page *page)
> return compound_page_dtors[page[1].compound_dtor];
> }
>
> +static inline int PageNormalCompound(struct page *page)
> +{
> + if (!PageCompound(page))
> + return 0;
> +
> + page = compound_head(page);
> + return page[1].compound_dtor == COMPOUND_PAGE_DTOR;
> +}
> +
> static inline unsigned int compound_order(struct page *page)
> {
> if (!PageHead(page))
> diff --git a/mm/util.c b/mm/util.c
> index 8bf08b5b5760..06c1640cb7b3 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -478,6 +478,8 @@ bool page_mapped(struct page *page)
> return true;
> if (PageHuge(page))
> return false;
> + if (PageNormalCompound(page))
> + return false;
> for (i = 0; i < hpage_nr_pages(page); i++) {
> if (atomic_read(&page[i]._mapcount) >= 0)
> return true;

Thanks for catching this.

But I think the right fix would be to change the loop condition:

for (i = 0; i < (1 << compund_order(page)); i++) {

Non-THP compound page also can be mapped and we need to check mapcount of
subpages.

Any objections?

If not, please update the patch.

--
Kirill A. Shutemov

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