Re: [PATCH v2 1/2] mm: Abstract THP allocation

[Dev Jain <dev.jain@xxxxxxx>]

On 9/5/24 16:38, Ryan Roberts wrote:
> On 04/09/2024 11:09, Dev Jain wrote:
> > In preparation for the second patch, abstract away the THP allocation
> > logic present in the create_huge_pmd() path, which corresponds to the
> > faulting case when no page is present.
> >
> > There should be no functional change as a result of applying
> > this patch.
> >
> > Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
> > ---
> >   mm/huge_memory.c | 110 +++++++++++++++++++++++++++++------------------
> >   1 file changed, 67 insertions(+), 43 deletions(-)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 67c86a5d64a6..58125fbcc532 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -943,47 +943,89 @@ unsigned long thp_get_unmapped_area(struct file *filp, unsigned long addr,
> >   }
> >   EXPORT_SYMBOL_GPL(thp_get_unmapped_area);
> >   
> > -static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
> > -			struct page *page, gfp_t gfp)
> > +static vm_fault_t thp_fault_alloc(gfp_t gfp, int order, struct vm_area_struct *vma,
> Is there a reason for specifying order as a parameter? Previously it was
> hardcoded to HPAGE_PMD_ORDER. But now, thp_fault_alloc() and
> __thp_fault_success_stats() both take order and map_pmd_thp() is still
> implicitly mapping a PMD-sized block. Unless there is a reason you need this
> parameter in the next patch (I don't think there is?) I suggest simplifying.

If I am not wrong, thp_fault_alloc() and __thp_fault_success_stats()
will remain the same in case of mTHP? I chose to pass order so that these
functions can be used by others in the future. Therefore, these two functions
can be generically used in the future while map_pmd_thp() (as the name suggests)
maps only a PMD-mappable THP.

> > +				  unsigned long haddr, struct folio **foliop,
> FWIW, I agree with Kirill's suggestion to just return folio* and drop the output
> param.

As I replied to Kirill, I do not think that is a good idea. If you do a git grep on
the tree for "foliop", you will find several places where that is being used, for
example, check out alloc_charge_folio() in mm/khugepaged.c. The author intends to
do the stat computation and setting *foliop in the function itself, so that the
caller is only concerned with the return value.
Also, if we return the folio instead of VM_FAULT_FALLBACK from thp_fault_alloc(),
then you will have two "if (unlikely(!folio))" branches, the first to do the stat
computation in the function, and the second branch in the caller to set ret = VM_FAULT_FALLBACK
and then goto out/release.
And, it is already inconsistent to break away the stat computation and the return value
setting, when the stat computation name is of the form "count_vm_event(THP_FAULT_FALLBACK)"
and friends, at which point it would just be better to open code the function.

If I am missing something and your suggestion can be implemented neatly, please guide.

> Thanks,
> Ryan
>
> > +				  unsigned long addr)
> >   {
> > -	struct vm_area_struct *vma = vmf->vma;
> > -	struct folio *folio = page_folio(page);
> > -	pgtable_t pgtable;
> > -	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
> > -	vm_fault_t ret = 0;
> > +	struct folio *folio = vma_alloc_folio(gfp, order, vma, haddr, true);
> >   
> > -	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
> > +	*foliop = folio;
> > +	if (unlikely(!folio)) {
> > +		count_vm_event(THP_FAULT_FALLBACK);
> > +		count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK);
> > +		return VM_FAULT_FALLBACK;
> > +	}
> >   
> > +	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
> >   	if (mem_cgroup_charge(folio, vma->vm_mm, gfp)) {
> >   		folio_put(folio);
> >   		count_vm_event(THP_FAULT_FALLBACK);
> >   		count_vm_event(THP_FAULT_FALLBACK_CHARGE);
> > -		count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_FALLBACK);
> > -		count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
> > +		count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK);
> > +		count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
> >   		return VM_FAULT_FALLBACK;
> >   	}
> >   	folio_throttle_swaprate(folio, gfp);
> >   
> > -	pgtable = pte_alloc_one(vma->vm_mm);
> > -	if (unlikely(!pgtable)) {
> > -		ret = VM_FAULT_OOM;
> > -		goto release;
> > -	}
> > -
> > -	folio_zero_user(folio, vmf->address);
> > +	folio_zero_user(folio, addr);
> >   	/*
> >   	 * The memory barrier inside __folio_mark_uptodate makes sure that
> >   	 * folio_zero_user writes become visible before the set_pmd_at()
> >   	 * write.
> >   	 */
> >   	__folio_mark_uptodate(folio);
> > +	return 0;
> > +}
> > +
> > +static void __thp_fault_success_stats(struct vm_area_struct *vma, int order)
> > +{
> > +	count_vm_event(THP_FAULT_ALLOC);
> > +	count_mthp_stat(order, MTHP_STAT_ANON_FAULT_ALLOC);
> > +	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)
> > +{
> > +	pmd_t entry;
> > +
> > +	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);
> > +	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);
> > +}
> > +
> > +static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf)
> > +{
> > +	struct vm_area_struct *vma = vmf->vma;
> > +	struct folio *folio = NULL;
> > +	pgtable_t pgtable;
> > +	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
> > +	vm_fault_t ret = 0;
> > +	gfp_t gfp = vma_thp_gfp_mask(vma);
> > +
> > +	pgtable = pte_alloc_one(vma->vm_mm);
> > +	if (unlikely(!pgtable)) {
> > +		ret = VM_FAULT_OOM;
> > +		goto release;
> > +	}
> > +
> > +	ret = thp_fault_alloc(gfp, HPAGE_PMD_ORDER, vma, haddr, &folio,
> > +			      vmf->address);
> > +	if (ret)
> > +		goto release;
> >   
> >   	vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
> > +
> >   	if (unlikely(!pmd_none(*vmf->pmd))) {
> >   		goto unlock_release;
> >   	} else {
> > -		pmd_t entry;
> > -
> >   		ret = check_stable_address_space(vma->vm_mm);
> >   		if (ret)
> >   			goto unlock_release;
> > @@ -997,20 +1039,9 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
> >   			VM_BUG_ON(ret & VM_FAULT_FALLBACK);
> >   			return ret;
> >   		}
> > -
> > -		entry = mk_huge_pmd(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);
> > -		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);
> > +		map_pmd_thp(folio, vmf, vma, haddr, pgtable);
> >   		spin_unlock(vmf->ptl);
> > -		count_vm_event(THP_FAULT_ALLOC);
> > -		count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_ALLOC);
> > -		count_memcg_event_mm(vma->vm_mm, THP_FAULT_ALLOC);
> > +		__thp_fault_success_stats(vma, HPAGE_PMD_ORDER);
> >   	}
> >   
> >   	return 0;
> > @@ -1019,7 +1050,8 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
> >   release:
> >   	if (pgtable)
> >   		pte_free(vma->vm_mm, pgtable);
> > -	folio_put(folio);
> > +	if (folio)
> > +		folio_put(folio);
> >   	return ret;
> >   
> >   }
> > @@ -1077,8 +1109,6 @@ static void set_huge_zero_folio(pgtable_t pgtable, struct mm_struct *mm,
> >   vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
> >   {
> >   	struct vm_area_struct *vma = vmf->vma;
> > -	gfp_t gfp;
> > -	struct folio *folio;
> >   	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
> >   	vm_fault_t ret;
> >   
> > @@ -1129,14 +1159,8 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
> >   		}
> >   		return ret;
> >   	}
> > -	gfp = vma_thp_gfp_mask(vma);
> > -	folio = vma_alloc_folio(gfp, HPAGE_PMD_ORDER, vma, haddr, true);
> > -	if (unlikely(!folio)) {
> > -		count_vm_event(THP_FAULT_FALLBACK);
> > -		count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_FALLBACK);
> > -		return VM_FAULT_FALLBACK;
> > -	}
> > -	return __do_huge_pmd_anonymous_page(vmf, &folio->page, gfp);
> > +
> > +	return __do_huge_pmd_anonymous_page(vmf);
> >   }
> >   
> >   static void insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,