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

[Baolin Wang]

On 2025/12/15 20:38, Lorenzo Stoakes wrote:
> On Thu, Dec 11, 2025 at 04:16:56PM +0800, Baolin Wang wrote:
> > Similar to folio_referenced_one(), we can apply batched unmapping for file
> > large folios to optimize the performance of file folios reclamation.
> >
> > Performance testing:
> > Allocate 10G clean file-backed folios by mmap() in a memory cgroup, and try to
> > reclaim 8G file-backed folios via the memory.reclaim interface. I can observe
> > 75% performance improvement on my Arm64 32-core server.
>
> Again, you must test on non-arm64 architectures and report the numbers for this
> also.

Yes, I've tested on the x86 machine, and will add the data in the commit 
message.

> > W/o patch:
> > real    0m1.018s
> > user    0m0.000s
> > sys     0m1.018s
> >
> > W/ patch:
> > real	0m0.249s
> > user	0m0.000s
> > sys	0m0.249s
> >
> > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > ---
> >   mm/rmap.c | 7 ++++---
> >   1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > 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.

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.

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

[1] https://lore.kernel.org/all/20250513084620.58231-1-21cnbao@xxxxxxxxx/

> >   		return 1;
> > +
> >   	if (pte_unused(pte))
> >   		return 1;
> >
> > @@ -2223,7 +2224,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> >   			 *
> >   			 * See Documentation/mm/mmu_notifier.rst
> >   			 */
> > -			dec_mm_counter(mm, mm_counter_file(folio));
> > +			add_mm_counter(mm, mm_counter_file(folio), -nr_pages);
>
> Was this just a bug before?

Nope. Before this patch, we never supported batched unmapping for 
file-backed large folios, so the 'nr_pages' was always 1. After this 
patch, we should use the number of pages in this file-backed large folio.