Re: [PATCH v7 5/5] kasan debug: track pages allocated for vmalloc shadow

[Andrey Konovalov <andreyknvl@google.com>]

On Tue, Sep 3, 2019 at 4:56 PM Daniel Axtens <dja@axtens.net> wrote:
>
> Provide the current number of vmalloc shadow pages in
> /sys/kernel/debug/kasan_vmalloc/shadow_pages.

Maybe it makes sense to put this into /sys/kernel/debug/kasan/
(without _vmalloc) and name e.g. vmalloc_shadow_pages? In case we want
to expose more generic KASAN debugging info later.

>
> Signed-off-by: Daniel Axtens <dja@axtens.net>
>
> ---
>
> Merging this is probably overkill, but I leave it to the discretion
> of the broader community.
>
> On v4 (no dynamic freeing), I saw the following approximate figures
> on my test VM:
>
>  - fresh boot: 720
>  - after test_vmalloc: ~14000
>
> With v5 (lazy dynamic freeing):
>
>  - boot: ~490-500
>  - running modprobe test_vmalloc pushes the figures up to sometimes
>     as high as ~14000, but they drop down to ~560 after the test ends.
>     I'm not sure where the extra sixty pages are from, but running the
>     test repeately doesn't cause the number to keep growing, so I don't
>     think we're leaking.
>  - with vmap_stack, spawning tasks pushes the figure up to ~4200, then
>     some clearing kicks in and drops it down to previous levels again.
> ---
>  mm/kasan/common.c | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
>
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index e33cbab83309..e40854512417 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -35,6 +35,7 @@
>  #include <linux/vmalloc.h>
>  #include <linux/bug.h>
>  #include <linux/uaccess.h>
> +#include <linux/debugfs.h>
>
>  #include <asm/tlbflush.h>
>
> @@ -750,6 +751,8 @@ core_initcall(kasan_memhotplug_init);
>  #endif
>
>  #ifdef CONFIG_KASAN_VMALLOC
> +static u64 vmalloc_shadow_pages;
> +
>  static int kasan_populate_vmalloc_pte(pte_t *ptep, unsigned long addr,
>                                       void *unused)
>  {
> @@ -776,6 +779,7 @@ static int kasan_populate_vmalloc_pte(pte_t *ptep, unsigned long addr,
>         if (likely(pte_none(*ptep))) {
>                 set_pte_at(&init_mm, addr, ptep, pte);
>                 page = 0;
> +               vmalloc_shadow_pages++;
>         }
>         spin_unlock(&init_mm.page_table_lock);
>         if (page)
> @@ -829,6 +833,7 @@ static int kasan_depopulate_vmalloc_pte(pte_t *ptep, unsigned long addr,
>         if (likely(!pte_none(*ptep))) {
>                 pte_clear(&init_mm, addr, ptep);
>                 free_page(page);
> +               vmalloc_shadow_pages--;
>         }
>         spin_unlock(&init_mm.page_table_lock);
>
> @@ -947,4 +952,25 @@ void kasan_release_vmalloc(unsigned long start, unsigned long end,
>                                        (unsigned long)shadow_end);
>         }
>  }
> +
> +static __init int kasan_init_vmalloc_debugfs(void)
> +{
> +       struct dentry *root, *count;
> +
> +       root = debugfs_create_dir("kasan_vmalloc", NULL);
> +       if (IS_ERR(root)) {
> +               if (PTR_ERR(root) == -ENODEV)
> +                       return 0;
> +               return PTR_ERR(root);
> +       }
> +
> +       count = debugfs_create_u64("shadow_pages", 0444, root,
> +                                  &vmalloc_shadow_pages);
> +
> +       if (IS_ERR(count))
> +               return PTR_ERR(root);
> +
> +       return 0;
> +}
> +late_initcall(kasan_init_vmalloc_debugfs);
>  #endif
> --
> 2.20.1
>
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