Re: [RFC PATCH 15/39] KVM: guest_memfd: hugetlb: allocate and truncate from hugetlb

[Ackerley Tng <ackerleytng@google.com>]

Elliot Berman <quic_eberman@quicinc.com> writes:

> On Tue, Sep 10, 2024 at 11:43:46PM +0000, Ackerley Tng wrote:
>> If HugeTLB is requested at guest_memfd creation time, HugeTLB pages
>> will be used to back guest_memfd.
>> 
>> Signed-off-by: Ackerley Tng <ackerleytng@google.com>
>>
>> <snip>
>>
>> +/**
>> + * Use the uptodate flag to indicate that the folio is prepared for KVM's usage.
>> + */
>>  static inline void kvm_gmem_mark_prepared(struct folio *folio)
>>  {
>>  	folio_mark_uptodate(folio);
>> @@ -72,13 +84,18 @@ static inline void kvm_gmem_mark_prepared(struct folio *folio)
>>  static int kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slot,
>>  				  gfn_t gfn, struct folio *folio)
>>  {
>> -	unsigned long nr_pages, i;
>>  	pgoff_t index;
>>  	int r;
>>  
>> -	nr_pages = folio_nr_pages(folio);
>> -	for (i = 0; i < nr_pages; i++)
>> -		clear_highpage(folio_page(folio, i));
>> +	if (folio_test_hugetlb(folio)) {
>> +		folio_zero_user(folio, folio->index << PAGE_SHIFT);
>
> Is (folio->index << PAGE_SHIFT) the right address hint to provide?
> I don't think we can say the folio will be mapped at this address since
> this value is an offset into the file.  In most cases, I believe it
> won't be mapped anywhere since we just allocated it.


vaddr in folio_zero_user(folio, vaddr) is eventually passed to
clear_user_page(). clear_user_page() uses vaddr to clean up dcaches on
some architectures, according to Documentation/core-api/cachetlb.rst.

In this patch series, folio_zero_user() is used in 2 places:

+ kvm_gmem_prepare_folio()
+ kvm_gmem_fault()

folio->index is valid by the time folio_zero_user() is called in
kvm_gmem_prepare_folio(), because when kvm_gmem_prepare_folio() is called, the
folio is already in the filemap, and folio->index is set when the folios is
added to the filemap.

In kvm_gmem_fault(), kvm_gmem_get_folio() also returns a folio in the filemap
and so folio->index is valid by the tiem folio_zero_user() is called.

Hence in both cases where folio_zero_user() is called, folio->index <<
PAGE_SHIFT returns the offset in the file.

In hugetlb's fallocate, the offset within the file is passed in the call to
folio_zero_user(), which is why the offset within the file was used here.

In the next revision I will refactor this to something like
kvm_gmem_prepare_folio_shared() and kvm_gmem_prepare_folio_private().

In kvm_gmem_prepare_folio_private(), folio->index << PAGE_SHIFT can still be
passed as addr_hint to align with HugeTLB. When being prepared as a private
folio, the folio will be mapped by KVM: addr_hint won't matter since this folio
isn't going to be mapped into userspace. If the folio was previously used as a
shared page, unmapping would have flushed the dcache.

In kvm_gmem_prepare_folio_shared(), the folio will subsequently be mapped and
vmf->real_address should be passed as addr_hint.

Thanks for this question!