Re: [PATCH 3/5] mm/mremap: use pmd_addr_end to calculate next in move_page_tables()

Dmitry Osipenko
2020-01-29T14:21:55Z

29.01.2020 12:47, Russell King - ARM Linux admin пишет:
> On Sun, Jan 26, 2020 at 05:47:57PM +0300, Dmitry Osipenko wrote:
>> 18.01.2020 02:22, Wei Yang пишет:
>>> Use the general helper instead of do it by hand.
>>>
>>> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
>>> ---
>>> mm/mremap.c | 7 ++-----
>>> 1 file changed, 2 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/mm/mremap.c b/mm/mremap.c
>>> index c2af8ba4ba43..a258914f3ee1 100644
>>> --- a/mm/mremap.c
>>> +++ b/mm/mremap.c
>>> @@ -253,11 +253,8 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
>>>
>>> for (; old_addr < old_end; old_addr += extent, new_addr += extent) {
>>> cond_resched();
>>> - next = (old_addr + PMD_SIZE) & PMD_MASK;
>>> - /* even if next overflowed, extent below will be ok */
>>> + next = pmd_addr_end(old_addr, old_end);
>>> extent = next - old_addr;
>>> - if (extent > old_end - old_addr)
>>> - extent = old_end - old_addr;
>>> old_pmd = get_old_pmd(vma->vm_mm, old_addr);
>>> if (!old_pmd)
>>> continue;
>>> @@ -301,7 +298,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
>>>
>>> if (pte_alloc(new_vma->vm_mm, new_pmd))
>>> break;
>>> - next = (new_addr + PMD_SIZE) & PMD_MASK;
>>> + next = pmd_addr_end(new_addr, new_addr + len);
>>> if (extent > next - new_addr)
>>> extent = next - new_addr;
>>> move_ptes(vma, old_pmd, old_addr, old_addr + extent, new_vma,
>>>
>>
>> Hello Wei,
>>
>> Starting with next-20200122, I'm seeing the following in KMSG on NVIDIA
>> Tegra (ARM32):
>>
>> BUG: Bad rss-counter state mm:(ptrval) type:MM_ANONPAGES val:190
>>
>> and eventually kernel hangs.
>>
>> Git's bisection points to this patch and reverting it helps. Please fix,
>> thanks in advance.
>
> The above is definitely wrong - pXX_addr_end() are designed to be used
> with an address index within the pXX table table and the address index
> of either the last entry in the same pXX table or the beginning of the
> _next_ pXX table. Arbitary end address indicies are not allowed.
>
> When page tables are "rolled up" when levels don't exist, it is common
> practice for these macros to just return their end address index.
> Hence, if they are used with arbitary end address indicies, then the
> iteration will fail.
>
> The only way to do this is:
>
> next = pmd_addr_end(old_addr,
> pud_addr_end(old_addr,
> p4d_addr_end(old_addr,
> pgd_addr_end(old_addr, old_end))));
>
> which gives pmd_addr_end() (and each of the intermediate pXX_addr_end())
> the correct end argument. However, that's a more complex and verbose,
> and likely less efficient than the current code.
>
> I'd suggest that there's nothing to "fix" in the v5.5 code wrt this,
> and trying to "clean it up" will just result in less efficient or
> broken code.
>

Hello Russell,

Thank you very much for the extra clarification!