Re: [PATCH] mm: avoid zeroing user movable page twice with init_on_alloc=1

[Zi Yan <ziy@nvidia.com>]

On 4 Dec 2024, at 13:13, Zi Yan wrote:

> On 4 Dec 2024, at 12:46, Vlastimil Babka wrote:
>
>> On 12/4/24 18:33, Zi Yan wrote:
>>> On 4 Dec 2024, at 11:29, Matthew Wilcox wrote:
>>>
>>>> On Wed, Dec 04, 2024 at 11:16:51AM -0500, Zi Yan wrote:
>>>>>> So maybe the clearing done as part of page allocator isn't enough here.
>>>>>>
>>>>> Basically, mips needs to flush data cache if kmap address is aliased to
>>>>
>>>> People use "aliased" in contronym ways.  Do you mean "has a
>>>> non-congruent alias" or "has a congruent alias"?
>>>>
>>>>> userspace address. This means when mips has THP on, the patch below
>>>>> is not enough to fix the issue.
>>>>>
>>>>> In post_alloc_hook(), it does not make sense to pass userspace address
>>>>> in to determine whether to flush dcache or not.
>>>>>
>>>>> One way to fix it is to add something like arch_userpage_post_alloc()
>>>>> to flush dcache if kmap address is aliased to userspace address.
>>>>> But my questions are that
>>>>> 1) if kmap address will always be the same for two separate kmap_local() calls,
>>>>
>>>> No.  It just takes the next address in the stack.
>>>
>>> Hmm, if kmap_local() gives different addresses, wouldn’t init_on_alloc be
>>> causing issues before my patch? In the page allocator, the page is zeroed
>>> from one kmap address without flush, then clear_user_highpage() clears
>>> it again with another kmap address with flush. After returning to userspace,
>>> the user application works on the page but when the cache line used by
>>> init_on_alloc is written back (with 0s) at eviction, user data is corrupted.
>>> Am I missing anything? Or all arch with cache aliasing never enables
>>> init_on_alloc?
>>
>> Maybe the arch also defines some hooks like arch_kmap_local_post_unmap() ?
>
> But this does not solve the possible init_on_alloc issue, since init_on_alloc
> is done in mm/page_alloc.c without userspace address and has no knowledge of
> whether the zeroed page will be used in userspace nor the cache line will
> be the same as the userspace page cache line. If dcache is flushed
> unconditionally for kmap_local, that could degrade performance.
>
>> As for the fix, could it rely on e.g. __HAVE_ARCH_COPY_USER_HIGHPAGE instead
>> of CONFIG_MIPS? That affects more arches, I don't know if we broke only mips
>> or others too.
>
> Yes, this is much better, since this issue affects any arch with cache aliasing.
> Let me update my fix. Thanks.

I notice that arm64 has __HAVE_ARCH_COPY_USER_HIGHPAGE defined, so I will
need to look for an alternative.

Best Regards,
Yan, Zi