Re: [PATCH] mm/memory: use folio lock/ref helpers in do_swap_page

[Hongfu Li <hongfu.li@linux.dev>]

> > From: Hongfu Li <lihongfu@kylinos.cn>
> > 
> > Replace page-level lock/ref operations with their folio equivalents
> > in the device_private migration path of do_swap_page().
> 
> When I do folio conversions, I like to mention the actual advantage
> of the patch.  Here, I'd say something like ...
> 
> mm: Use a folio in the softleaf_is_device_private path
> 
> Use the folio APIs, replacing four calls to compound_head() with one.

Many thanks for your patient guidance.
I will update the commit message per your suggestion for v2.

> > Introduce a local fault_folio to hold page_folio(vmf->page) to avoid
> > repeated page_folio() invocations.
> > 
> > Signed-off-by: Hongfu Li <lihongfu@kylinos.cn>
> > ---
> >  mm/memory.c | 11 +++++++----
> >  1 file changed, 7 insertions(+), 4 deletions(-)
> > 
> > diff --git a/mm/memory.c b/mm/memory.c
> > index d5e87624f692..bd27372a1052 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4852,6 +4852,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >  			vmf->page = softleaf_to_page(entry);
> >  			ret = remove_device_exclusive_entry(vmf);
> >  		} else if (softleaf_is_device_private(entry)) {
> > +			struct folio *fault_folio;
> 
> You can just use the existing 'folio' variable.
> 
> Although this function is far too long (400 lines!  14 function-scope
> variables!) so I don't blame you for not noticing that.  Pulling a
> do_non_swap_page() out of it might be a kindness.

Thanks, will drop the local fault_folio and reuse the existing folio
variable. This change will be included in v2.

Agreed that we can split out helpers from do_swap_page, and I will add
a new patch for this refactoring in v2.

> >  			if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
> >  				/*
> >  				 * migrate_to_ram is not yet ready to operate
> > @@ -4863,6 +4865,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >  			}
> >  
> >  			vmf->page = softleaf_to_page(entry);
> > +			fault_folio = page_folio(vmf->page);
> >  			vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
> >  					vmf->address, &vmf->ptl);
> >  			if (unlikely(!vmf->pte ||
> > @@ -4874,15 +4877,15 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >  			 * Get a page reference while we know the page can't be
> >  			 * freed.
> 
> You should also replace 'page' with 'folio' here.  Pages don't have
> refcounts, folios do.

Right, I'll update the comment to "folio" to match. Will include in v2.

Best regards,
Hongfu