Re: [PATCH v1 1/2] mm/userfaultfd: don't place zeropages when zeropages are disallowed

[David Hildenbrand <david@redhat.com>]

On 21.03.24 23:20, Peter Xu wrote:
> On Thu, Mar 21, 2024 at 10:59:53PM +0100, David Hildenbrand wrote:
>> s390x must disable shared zeropages for processes running VMs, because
>> the VMs could end up making use of "storage keys" or protected
>> virtualization, which are incompatible with shared zeropages.
>>
>> Yet, with userfaultfd it is possible to insert shared zeropages into
>> such processes. Let's fallback to simply allocating a fresh zeroed
>> anonymous folio and insert that instead.
>>
>> mm_forbids_zeropage() was introduced in commit 593befa6ab74 ("mm: introduce
>> mm_forbids_zeropage function"), briefly before userfaultfd went
>> upstream.
>>
>> Note that we don't want to fail the UFFDIO_ZEROPAGE request like we do
>> for hugetlb, it would be rather unexpected. Further, we also
>> cannot really indicated "not supported" to user space ahead of time: it
>> could be that the MM disallows zeropages after userfaultfd was already
>> registered.
>>
>> Fixes: c1a4de99fada ("userfaultfd: mcopy_atomic|mfill_zeropage: UFFDIO_COPY|UFFDIO_ZEROPAGE preparation")
>> Signed-off-by: David Hildenbrand <david@redhat.com>
> 
> Reviewed-by: Peter Xu <peterx@redhat.com>
> 
> Still, a few comments below.
> 
>> ---
>>   mm/userfaultfd.c | 35 +++++++++++++++++++++++++++++++++++
>>   1 file changed, 35 insertions(+)
>>
>> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
>> index 712160cd41eca..1d1061ccd1dea 100644
>> --- a/mm/userfaultfd.c
>> +++ b/mm/userfaultfd.c
>> @@ -316,6 +316,38 @@ static int mfill_atomic_pte_copy(pmd_t *dst_pmd,
>>   	goto out;
>>   }
>>   
>> +static int mfill_atomic_pte_zeroed_folio(pmd_t *dst_pmd,
>> +		 struct vm_area_struct *dst_vma, unsigned long dst_addr)
>> +{
>> +	struct folio *folio;
>> +	int ret;
> 
> nitpick: we can set -ENOMEM here, then
> 
>> +
>> +	folio = vma_alloc_zeroed_movable_folio(dst_vma, dst_addr);
>> +	if (!folio)
>> +		return -ENOMEM;
> 
> return ret;
> 
>> +
>> +	ret = -ENOMEM;
> 
> drop.

Sure!

> 
>> +	if (mem_cgroup_charge(folio, dst_vma->vm_mm, GFP_KERNEL))
>> +		goto out_put;
>> +
>> +	/*
>> +	 * The memory barrier inside __folio_mark_uptodate makes sure that
>> +	 * preceding stores to the page contents become visible before
>> +	 * the set_pte_at() write.
>> +	 */
> 
> This comment doesn't apply.  We can drop it.
> 

I thought the same until I spotted that comment (where uffd originally 
copied this from I strongly assume) in do_anonymous_page().

"Preceding stores" here are: zeroing out the memory.


Thanks for the fast review!

-- 
Cheers,

David / dhildenb