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

[Vlastimil Babka <vbabka@suse.cz>]

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() ?

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.

> Best Regards,
> Yan, Zi