Re: [PATCH 4/6] mm: memory: use a folio in do_cow_page()

[Kefeng Wang]

On 2023/11/8 2:27, Sidhartha Kumar wrote:
> On 11/7/23 5:52 AM, Kefeng Wang wrote:
> > Use folio_prealloc() helper and convert to use a folio in
> > do_cow_page(), which save five compound_head() calls.
> >
> > Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
> > ---
> >   mm/memory.c | 16 ++++++----------
> >   1 file changed, 6 insertions(+), 10 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 062136d25da3..5ecee3eac29d 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4648,6 +4648,7 @@ static vm_fault_t do_read_fault(struct vm_fault 
> > *vmf)
> >   static vm_fault_t do_cow_fault(struct vm_fault *vmf)
> >   {
> >       struct vm_area_struct *vma = vmf->vma;
> > +    struct folio *folio;
> >       vm_fault_t ret;
> >       ret = vmf_can_call_fault(vmf);
> > @@ -4656,16 +4657,11 @@ static vm_fault_t do_cow_fault(struct vm_fault 
> > *vmf)
> >       if (ret)
> >           return ret;
> > -    vmf->cow_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, 
> > vmf->address);
> > -    if (!vmf->cow_page)
> > +    folio = folio_prealloc(vma->vm_mm, vma, vmf->address, false);
> > +    if (!folio)
> >           return VM_FAULT_OOM;
> > -    if (mem_cgroup_charge(page_folio(vmf->cow_page), vma->vm_mm,
> > -                GFP_KERNEL)) {
> > -        put_page(vmf->cow_page);
> > -        return VM_FAULT_OOM;
> > -    }
> > -    folio_throttle_swaprate(page_folio(vmf->cow_page), GFP_KERNEL);
>
> Hi Kefeng,
>
> I was wondering why this block of code is no longer needed after using a 
> folio?

folio_prealloc(previous page_copy_prealloc) already with above code.

> Thanks,
> Sidhartha Kumar
>
> > +    vmf->cow_page = &folio->page;
> >       ret = __do_fault(vmf);
> >       if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | 
> > VM_FAULT_RETRY)))
> > @@ -4674,7 +4670,7 @@ static vm_fault_t do_cow_fault(struct vm_fault 
> > *vmf)
> >           return ret;
> >       copy_user_highpage(vmf->cow_page, vmf->page, vmf->address, vma);
> > -    __SetPageUptodate(vmf->cow_page);
> > +    __folio_mark_uptodate(folio);
> >       ret |= finish_fault(vmf);
> >       unlock_page(vmf->page);
> > @@ -4683,7 +4679,7 @@ static vm_fault_t do_cow_fault(struct vm_fault 
> > *vmf)
> >           goto uncharge_out;
> >       return ret;
> >   uncharge_out:
> > -    put_page(vmf->cow_page);
> > +    folio_put(folio);
> >       return ret;
> >   }