Re: [PATCH 3/9] mm/rmap: refactor lazyfree unmap commit path to commit_ttu_lazyfree_folio()

["Lorenzo Stoakes (Oracle)" <ljs@kernel.org>]

On Tue, Mar 10, 2026 at 02:12:45PM +0530, Dev Jain wrote:
>
>
> On 10/03/26 1:49 pm, Lorenzo Stoakes (Oracle) wrote:
> > On Tue, Mar 10, 2026 at 01:00:07PM +0530, Dev Jain wrote:
> >> Clean up the code by refactoring the post-pte-clearing path of lazyfree
> >> folio unmapping, into commit_ttu_lazyfree_folio().
> >>
> >> No functional change is intended.
> >>
> >> Signed-off-by: Dev Jain <dev.jain@arm.com>
> >
> > This is a good idea, and we need more refactoring like this in the rmap code,
> > but comments/nits below.
> >
> >> ---
> >>  mm/rmap.c | 93 ++++++++++++++++++++++++++++++++-----------------------
> >>  1 file changed, 54 insertions(+), 39 deletions(-)
> >>
> >> diff --git a/mm/rmap.c b/mm/rmap.c
> >> index 1fa020edd954a..a61978141ee3f 100644
> >> --- a/mm/rmap.c
> >> +++ b/mm/rmap.c
> >> @@ -1966,6 +1966,57 @@ static inline unsigned int folio_unmap_pte_batch(struct folio *folio,
> >>  				     FPB_RESPECT_WRITE | FPB_RESPECT_SOFT_DIRTY);
> >>  }
> >>
> >> +static inline int commit_ttu_lazyfree_folio(struct vm_area_struct *vma,
> >
> > Strange name, maybe lazyfree_range()? Not sure what ttu has to do with
>
> ttu means try_to_unmap, just like it is used in TTU_SYNC,
> TTU_SPLIT_HUGE_PMD, etc. So personally I really like the name, it reads
> "commit the try-to-unmap of a lazyfree folio". The "commit" comes because
> the pte clearing has already happened, so now we are deciding if at all
> to back-off and restore the ptes.

I absolutely hate the name, and nobody sane is going to read it like that
sorry :) I think this is a case of being too close to the work.

I also hate the overloading of 'lazyfree' which is actually MADV_FREE,
users don't know what lazyfree is and it's a bit of an overloaded term
(we've had discussions of a new 'lazy free' implementation at conferences
before).

Commit is overloaded everywhere I'm not sure it's even particularly
pertinent here.

Also it's not necessarily a folio is it? It could be a contpte range within
a folio so that's just misleading...

lazyfree_pte_range() or something like that seems better to me.

>
> > anything...
> >
> >> +		struct folio *folio, unsigned long address, pte_t *ptep,
> >> +		pte_t pteval, long nr_pages)
> >
> > That long nr_pages is really grating now...
>
> Reading the discussion on patch 1, I'll convert this to unsigned long.

Thanks!

>
> >
> >> +{
> >
> > Come on Dev, it's 2026, why on earth are you returning an integer and not a
> > bool?
> >
> > Also it would make sense for this to return false if something breaks, otherwise
> > true.
>
> Yes I was confused on which one of the options to choose :). Since the
> function does a lot more than just test some functionality (which is what
> boolean functions usually do) I was feeling weird when returning bool.
> But yeah alright, I'll convert this to bool.

Thanks!

Cheers, Lorenzo

>
> >
> >> +	struct mm_struct *mm = vma->vm_mm;
> >> +	int ref_count, map_count;
> >> +
> >> +	/*
> >> +	 * Synchronize with gup_pte_range():
> >> +	 * - clear PTE; barrier; read refcount
> >> +	 * - inc refcount; barrier; read PTE
> >> +	 */
> >> +	smp_mb();
> >> +
> >> +	ref_count = folio_ref_count(folio);
> >> +	map_count = folio_mapcount(folio);
> >> +
> >> +	/*
> >> +	 * Order reads for page refcount and dirty flag
> >> +	 * (see comments in __remove_mapping()).
> >> +	 */
> >> +	smp_rmb();
> >> +
> >> +	if (folio_test_dirty(folio) && !(vma->vm_flags & VM_DROPPABLE)) {
> >> +		/*
> >> +		 * redirtied either using the page table or a previously
> >> +		 * obtained GUP reference.
> >> +		 */
> >> +		set_ptes(mm, address, ptep, pteval, nr_pages);
> >> +		folio_set_swapbacked(folio);
> >> +		return 1;
> >> +	}
> >> +
> >> +	if (ref_count != 1 + map_count) {
> >> +		/*
> >> +		 * Additional reference. Could be a GUP reference or any
> >> +		 * speculative reference. GUP users must mark the folio
> >> +		 * dirty if there was a modification. This folio cannot be
> >> +		 * reclaimed right now either way, so act just like nothing
> >> +		 * happened.
> >> +		 * We'll come back here later and detect if the folio was
> >> +		 * dirtied when the additional reference is gone.
> >> +		 */
> >> +		set_ptes(mm, address, ptep, pteval, nr_pages);
> >> +		return 1;
> >> +	}
> >> +
> >> +	add_mm_counter(mm, MM_ANONPAGES, -nr_pages);
> >> +	return 0;
> >> +}
> >> +
> >>  /*
> >>   * @arg: enum ttu_flags will be passed to this argument
> >>   */
> >> @@ -2227,46 +2278,10 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> >>
> >>  			/* MADV_FREE page check */
> >>  			if (!folio_test_swapbacked(folio)) {
> >> -				int ref_count, map_count;
> >> -
> >> -				/*
> >> -				 * Synchronize with gup_pte_range():
> >> -				 * - clear PTE; barrier; read refcount
> >> -				 * - inc refcount; barrier; read PTE
> >> -				 */
> >> -				smp_mb();
> >> -
> >> -				ref_count = folio_ref_count(folio);
> >> -				map_count = folio_mapcount(folio);
> >> -
> >> -				/*
> >> -				 * Order reads for page refcount and dirty flag
> >> -				 * (see comments in __remove_mapping()).
> >> -				 */
> >> -				smp_rmb();
> >> -
> >> -				if (folio_test_dirty(folio) && !(vma->vm_flags & VM_DROPPABLE)) {
> >> -					/*
> >> -					 * redirtied either using the page table or a previously
> >> -					 * obtained GUP reference.
> >> -					 */
> >> -					set_ptes(mm, address, pvmw.pte, pteval, nr_pages);
> >> -					folio_set_swapbacked(folio);
> >> +				if (commit_ttu_lazyfree_folio(vma, folio, address,
> >> +							      pvmw.pte, pteval,
> >> +							      nr_pages))
> >
> > With above corrections this would be:
> >
> > 	if (!lazyfree_range(vma, folio, address, pvme.pte, pteval, nr_pages))
> > 	   ...
> >
> >>  					goto walk_abort;
> >> -				} else if (ref_count != 1 + map_count) {
> >> -					/*
> >> -					 * Additional reference. Could be a GUP reference or any
> >> -					 * speculative reference. GUP users must mark the folio
> >> -					 * dirty if there was a modification. This folio cannot be
> >> -					 * reclaimed right now either way, so act just like nothing
> >> -					 * happened.
> >> -					 * We'll come back here later and detect if the folio was
> >> -					 * dirtied when the additional reference is gone.
> >> -					 */
> >> -					set_ptes(mm, address, pvmw.pte, pteval, nr_pages);
> >> -					goto walk_abort;
> >> -				}
> >> -				add_mm_counter(mm, MM_ANONPAGES, -nr_pages);
> >>  				goto discard;
> >>  			}
> >>
> >> --
> >> 2.34.1
> >>
> >
> > Thanks, Lorenzo
> >
>