Re: [PATCH v2 2/2] mm: Allocate THP on hugezeropage wp-fault

[Dev Jain <dev.jain@arm.com>]

On 9/5/24 18:44, Ryan Roberts wrote:
> On 04/09/2024 11:09, Dev Jain wrote:
>> Introduce do_huge_zero_wp_pmd() to handle wp-fault on a hugezeropage and
>> replace it with a PMD-mapped THP. Change the helpers introduced in the
>> previous patch to flush TLB entry corresponding to the hugezeropage,
>> and preserve PMD uffd-wp marker. In case of failure, fallback to
>> splitting the PMD.
>>
>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>> ---
>>   include/linux/huge_mm.h |  6 ++++
>>   mm/huge_memory.c        | 79 +++++++++++++++++++++++++++++++++++------
>>   mm/memory.c             |  5 +--
>>   3 files changed, 78 insertions(+), 12 deletions(-)
>>
>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>> index e25d9ebfdf89..fdd2cf473a3c 100644
>> --- a/include/linux/huge_mm.h
>> +++ b/include/linux/huge_mm.h
>> @@ -9,6 +9,12 @@
>>   #include <linux/kobject.h>
>>   
>>   vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf);
>> +vm_fault_t thp_fault_alloc(gfp_t gfp, int order, struct vm_area_struct *vma,
>> +			   unsigned long haddr, struct folio **foliop,
>> +			   unsigned long addr);
>> +void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
>> +		 struct vm_area_struct *vma, unsigned long haddr,
>> +		 pgtable_t pgtable);
> I don't think you are using either of these outside of huge_memory.c, so not
> sure you need to declare them here or make them non-static?

As pointed out by Kirill, you are right, I forgot to drop these from my previous
approach.

>
>>   int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>>   		  pmd_t *dst_pmd, pmd_t *src_pmd, unsigned long addr,
>>   		  struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma);
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 58125fbcc532..150163ad77d3 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -943,9 +943,9 @@ unsigned long thp_get_unmapped_area(struct file *filp, unsigned long addr,
>>   }
>>   EXPORT_SYMBOL_GPL(thp_get_unmapped_area);
>>   
>> -static vm_fault_t thp_fault_alloc(gfp_t gfp, int order, struct vm_area_struct *vma,
>> -				  unsigned long haddr, struct folio **foliop,
>> -				  unsigned long addr)
>> +vm_fault_t thp_fault_alloc(gfp_t gfp, int order, struct vm_area_struct *vma,
>> +			   unsigned long haddr, struct folio **foliop,
>> +			   unsigned long addr)
>>   {
>>   	struct folio *folio = vma_alloc_folio(gfp, order, vma, haddr, true);
>>   
>> @@ -984,21 +984,29 @@ static void __thp_fault_success_stats(struct vm_area_struct *vma, int order)
>>   	count_memcg_event_mm(vma->vm_mm, THP_FAULT_ALLOC);
>>   }
>>   
>> -static void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
>> -			struct vm_area_struct *vma, unsigned long haddr,
>> -			pgtable_t pgtable)
>> +void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
>> +		 struct vm_area_struct *vma, unsigned long haddr,
>> +		 pgtable_t pgtable)
>>   {
>> -	pmd_t entry;
>> +	pmd_t entry, old_pmd;
>> +	bool is_pmd_none = pmd_none(*vmf->pmd);
>>   
>>   	entry = mk_huge_pmd(&folio->page, vma->vm_page_prot);
>>   	entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
>>   	folio_add_new_anon_rmap(folio, vma, haddr, RMAP_EXCLUSIVE);
>>   	folio_add_lru_vma(folio, vma);
>> -	pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
>> +	if (!is_pmd_none) {
>> +		old_pmd = pmdp_huge_clear_flush(vma, haddr, vmf->pmd);
>> +		if (pmd_uffd_wp(old_pmd))
>> +			entry = pmd_mkuffd_wp(entry);
> I don't really get this; entry is writable, so I wouldn't expect to also be
> setting uffd-wp here? That combination is not allowed and is checked for in
> page_table_check_pte_flags().
>
> It looks like you expect to get here in the uffd-wp-async case, which used to
> cause the pmd to be split to ptes. I'm guessing (but don't know for sure) that
> would cause the uffd-wp bit to be set in each of the new ptes, then during
> fallback to handling the wp fault on the pte, uffd would handle it?

I guess you are correct; I missed the WARN_ON() in page_table_check_pmd_flags(),
but I did see, if I read the uffd code correctly, that mfill_atomic() will just
return in case of pmd_trans_huge(*dst_pmd) while doing a uffd_copy to the destination
location. So preserving pmd_uffd_wp is useless in case a THP is mapped, but I did not
know that in fact it is supposed to be an invalid combination. So, I will drop it,
unless someone has some other objection.

>
>> +	}
>> +	if (pgtable)
>> +		pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
> Should this call be moved outside of here? It doesn't really feel like it
> belongs. Could it be called before calling map_pmd_thp() for the site that has a
> pgtable?

Every other place I checked, they are doing this: make the entry -> deposit pgtable ->
set_pmd_at(). I guess the general flow is to do the deposit based on the old pmd, before
setting the new one. Which implies: I should at least move this check before I call
pmdp_huge_clear_flush(). And, since vmf->pmd and creating the new entry has no relation,
I am inclined to do what you are saying.

>
>>   	set_pmd_at(vma->vm_mm, haddr, vmf->pmd, entry);
>>   	update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);
>>   	add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
>> -	mm_inc_nr_ptes(vma->vm_mm);
>> +	if (is_pmd_none)
>> +		mm_inc_nr_ptes(vma->vm_mm);
>>   }
>>   
>>   static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf)
>> @@ -1576,6 +1584,50 @@ void huge_pmd_set_accessed(struct vm_fault *vmf)
>>   	spin_unlock(vmf->ptl);
>>   }
>>   
>> +static vm_fault_t do_huge_zero_wp_pmd_locked(struct vm_fault *vmf,
>> +					     unsigned long haddr,
>> +					     struct folio *folio)
>> +{
>> +	struct vm_area_struct *vma = vmf->vma;
>> +	vm_fault_t ret = 0;
>> +
>> +	ret = check_stable_address_space(vma->vm_mm);
>> +	if (ret)
>> +		goto out;
>> +	map_pmd_thp(folio, vmf, vma, haddr, NULL);
>> +out:
>> +	return ret;
>> +}
>> +
>> +static vm_fault_t do_huge_zero_wp_pmd(struct vm_fault *vmf, unsigned long haddr)
>> +{
>> +	struct vm_area_struct *vma = vmf->vma;
>> +	gfp_t gfp = vma_thp_gfp_mask(vma);
>> +	struct mmu_notifier_range range;
>> +	struct folio *folio = NULL;
>> +	vm_fault_t ret = 0;
>> +
>> +	ret = thp_fault_alloc(gfp, HPAGE_PMD_ORDER, vma, haddr, &folio,
>> +			      vmf->address);
> Just checking: the PTE table was already allocated during the read fault, right?
> So we don't have to allocate it here.

Correct, that happens in set_huge_zero_folio(). Thanks for checking.

>
>> +	if (ret)
>> +		goto out;
>> +
>> +	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma->vm_mm, haddr,
>> +				haddr + HPAGE_PMD_SIZE);
>> +	mmu_notifier_invalidate_range_start(&range);
>> +	vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
>> +	if (unlikely(!pmd_same(pmdp_get(vmf->pmd), vmf->orig_pmd)))
>> +		goto unlock;
>> +	ret = do_huge_zero_wp_pmd_locked(vmf, haddr, folio);
>> +	if (!ret)
>> +		__thp_fault_success_stats(vma, HPAGE_PMD_ORDER);
>> +unlock:
>> +	spin_unlock(vmf->ptl);
>> +	mmu_notifier_invalidate_range_end(&range);
> I'll confess I don't understand all the mmu notifier rules.

I confess the same :)

> But the doc at
> Documentation/mm/mmu_notifier.rst implies that the notification must be done
> while holding the PTL. Although that's not how wp_page_copy(). Are you confident
> what you have done is correct?

Everywhere else, invalidate_range_end() is getting called after dropping the lock,
one reason is that it has a might_sleep(), and therefore we cannot call it while
holding a spinlock. I still don't know what exactly these calls mean...but I think
what I have done is correct.

>
> Thanks,
> Ryan
>
>> +out:
>> +	return ret;
>> +}
>> +
>>   vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
>>   {
>>   	const bool unshare = vmf->flags & FAULT_FLAG_UNSHARE;
>> @@ -1588,8 +1640,15 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
>>   	vmf->ptl = pmd_lockptr(vma->vm_mm, vmf->pmd);
>>   	VM_BUG_ON_VMA(!vma->anon_vma, vma);
>>   
>> -	if (is_huge_zero_pmd(orig_pmd))
>> +	if (is_huge_zero_pmd(orig_pmd)) {
>> +		vm_fault_t ret = do_huge_zero_wp_pmd(vmf, haddr);
>> +
>> +		if (!(ret & VM_FAULT_FALLBACK))
>> +			return ret;
>> +
>> +		/* Fallback to splitting PMD if THP cannot be allocated */
>>   		goto fallback;
>> +	}
>>   
>>   	spin_lock(vmf->ptl);
>>   
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 3c01d68065be..c081a25f5173 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -5409,9 +5409,10 @@ static inline vm_fault_t wp_huge_pmd(struct vm_fault *vmf)
>>   	if (vma_is_anonymous(vma)) {
>>   		if (likely(!unshare) &&
>>   		    userfaultfd_huge_pmd_wp(vma, vmf->orig_pmd)) {
>> -			if (userfaultfd_wp_async(vmf->vma))
>> +			if (!userfaultfd_wp_async(vmf->vma))
>> +				return handle_userfault(vmf, VM_UFFD_WP);
>> +			if (!is_huge_zero_pmd(vmf->orig_pmd))
>>   				goto split;
>> -			return handle_userfault(vmf, VM_UFFD_WP);
>>   		}
>>   		return do_huge_pmd_wp_page(vmf);
>>   	}