Re: [PATCH] mm: memory: check userfaultfd_wp() in vmf_orig_pte_uffd_wp()

[Peter Xu <peterx@redhat.com>]

Hi, Kefeng,

On Wed, Apr 17, 2024 at 05:30:40PM +0800, Kefeng Wang wrote:
> 
> 
> On 2024/4/17 16:23, Kefeng Wang wrote:
> > Directly call vmf_orig_pte_uffd_wp() in do_anonymous_page() and
> > set_pte_range() to save a uffd_wp and add userfaultfd_wp() check
> > in vmf_orig_pte_uffd_wp() to avoid the unnecessary function calls
> > in the most sense, lat_pagefault testcase does show improvement
> > though very small(~1%).

I'm ok with the change if that helps as big as 1%, but I'm a bit surprised
to see such a difference, because for file pte_marker_uffd_wp() should
check first on pte_none() then it should return already if uffd not even
registered for the vma, while orig_pte should be hot too if valid.

> > 
> > Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> > ---
> >   mm/memory.c | 9 +++++----
> >   1 file changed, 5 insertions(+), 4 deletions(-)
> > 
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 5ae2409d3cb9..a6afc96001e6 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -117,6 +117,9 @@ static bool vmf_orig_pte_uffd_wp(struct vm_fault *vmf)
> 
> 
> 	 if (!IS_ENABLED(CONFIG_PTE_MARKER_UFFD_WP))
>         	return false;
> 
> Will add config check too,

pte_marker_uffd_wp() returns false when !PTE_MARKER_UFFD_WP, so kind of
imply this.  I assume you meant to avoid checking ORIG_PTE_VALID flag, but
the flags is pretty hot too.  Again, just want to double check with you on
whether it can have such a huge difference, e.g., how that compares with
the current code v.s. original patch v.s. this squashed.

Thanks,

> 
> >   	if (!(vmf->flags & FAULT_FLAG_ORIG_PTE_VALID))
> >   		return false;
> > +	if (!userfaultfd_wp(vmf->vma))
> > +		return false;
> > +
> 
> but wait for review.
> 
> >   	return pte_marker_uffd_wp(vmf->orig_pte);
> >   }
> > @@ -4388,7 +4391,6 @@ static struct folio *alloc_anon_folio(struct vm_fault *vmf)
> >    */
> >   static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
> >   {
> > -	bool uffd_wp = vmf_orig_pte_uffd_wp(vmf);
> >   	struct vm_area_struct *vma = vmf->vma;
> >   	unsigned long addr = vmf->address;
> >   	struct folio *folio;
> > @@ -4488,7 +4490,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
> >   	folio_add_new_anon_rmap(folio, vma, addr);
> >   	folio_add_lru_vma(folio, vma);
> >   setpte:
> > -	if (uffd_wp)
> > +	if (vmf_orig_pte_uffd_wp(vmf))
> >   		entry = pte_mkuffd_wp(entry);
> >   	set_ptes(vma->vm_mm, addr, vmf->pte, entry, nr_pages);
> > @@ -4663,7 +4665,6 @@ void set_pte_range(struct vm_fault *vmf, struct folio *folio,
> >   		struct page *page, unsigned int nr, unsigned long addr)
> >   {
> >   	struct vm_area_struct *vma = vmf->vma;
> > -	bool uffd_wp = vmf_orig_pte_uffd_wp(vmf);
> >   	bool write = vmf->flags & FAULT_FLAG_WRITE;
> >   	bool prefault = in_range(vmf->address, addr, nr * PAGE_SIZE);
> >   	pte_t entry;
> > @@ -4678,7 +4679,7 @@ void set_pte_range(struct vm_fault *vmf, struct folio *folio,
> >   	if (write)
> >   		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> > -	if (unlikely(uffd_wp))
> > +	if (unlikely(vmf_orig_pte_uffd_wp(vmf)))
> >   		entry = pte_mkuffd_wp(entry);
> >   	/* copy-on-write page */
> >   	if (write && !(vma->vm_flags & VM_SHARED)) {
> 

-- 
Peter Xu