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

[Dev Jain]

On 9/5/24 13:50, Kirill A. Shutemov wrote:
> On Wed, Sep 04, 2024 at 03:39:22PM +0530, 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,
> > +				  unsigned long haddr, struct folio **foliop,
> > +				  unsigned long addr)
> foliop is awkward.
>
> Why not return folio? NULL would indicate to the caller to fallback.

I took inspiration from other call sites like mfill_atomic_pte_copy() which
have a double pointer to the folio. If we do as you say, then in thp_fault_alloc(),
we will have to do all the fallback stat computation, and return the folio, then
check in the caller whether that is NULL, then set ret = VM_FAULT_FALLBACK, then
goto out/release. Basically, stat computation for fallback, and the actual setting
of return value to fallback would get separated. Currently, I can simultaneously
set the folio pointer and the return value.

> >   {
> > -	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;
> THP page allocation has higher probability to fail than pgtable allocation. It
> is better to allocate it first, before pgtable and do less work on error
> path.

Thanks, makes sense.