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

[Daniel Axtens]

Andrey Konovalov <andreyknvl at google.com> writes:

> On Tue, Sep 3, 2019 at 4:56 PM Daniel Axtens <dja at 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.

We certainly could. I just wonder if this patch is useful on an ongoing
basis. I wrote it to validate my work on lazy freeing of shadow pages -
which is why I included it - but I'm not sure it has much ongoing value
beyond demonstrating that the freeing code works.

If we think it's worth holding on to this patch, I can certainly adjust
the paths.

Regards,
Daniel

>
>>
>> Signed-off-by: Daniel Axtens <dja at 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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