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

[Barry Song <21cnbao@gmail.com>]

> >>
> >> 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.

>
> > [0]:https://lore.kernel.org/all/20250214093015.51024-4-21cnbao@gmail.com/T/#u
>
> [1] https://lore.kernel.org/all/20250513084620.58231-1-21cnbao@gmail.com/

Thanks
Barry