Re: [PATCH v2 3/3] mm: rmap: support batched unmapping for file large folios

[Baolin Wang]

On 2025/12/16 14:13, Barry Song wrote:
> > > > diff --git a/mm/rmap.c b/mm/rmap.c
> > > > index ec232165c47d..4c9d5777c8da 100644
> > > > --- a/mm/rmap.c
> > > > +++ b/mm/rmap.c
> > > > @@ -1855,9 +1855,10 @@ static inline unsigned int folio_unmap_pte_batch(struct folio *folio,
> > > >       end_addr = pmd_addr_end(addr, vma->vm_end);
> > > >       max_nr = (end_addr - addr) >> PAGE_SHIFT;
> > > >
> > > > -    /* We only support lazyfree batching for now ... */
> > > > -    if (!folio_test_anon(folio) || folio_test_swapbacked(folio))
> > > > +    /* We only support lazyfree or file folios batching for now ... */
> > > > +    if (folio_test_anon(folio) && folio_test_swapbacked(folio))
> > >
> > > Why is it now ok to support file-backed batched unmapping when it wasn't in
> > > Barry's series (see [0])? You don't seem to be justifying this?
> >
> > Barry's series[0] is merely aimed at optimizing lazyfree anonymous large
> > folios and does not continue to optimize anonymous large folios or
> > file-backed large folios at that point.
>
> Yep. At that time, I didn’t have an Android machine with a filesystem
> that supported large folios, so I focused on lazyfree. But I
> agree that lazyfree anon folios and file folios are quite
> similar.
>
> > Subsequently, Barry sent out a new patch (see [1]) to optimize anonymous
> > large folios. As for file-backed large folios, the batched unmapping
> > support is relatively simple, since we only need to clear the PTE
> > entries for file-backed large folios.
>
> Yep. It is actually quite straightforward to go from lazyfree
> anon folios to file folios. Swap-backed anon folios are much
> more tricky, though.

Agree. Thanks Barry for reviewing and confirming.