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

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Tue, Dec 16, 2025 at 01:48:52PM +0800, Baolin Wang wrote:
>
>
> 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@linux.alibaba.com>
> > > ---
> > >   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.

Yeah, but he sent an entire patch changing a bunch of logic to accommodate this,
you're just changing the conditional and not really justifying it in the commit
message?

It really needs a 'it is safe to allow this for file-backed because blah blah
blah'.

Is this relying on the prior commits you've added? If so you should say so.

If it was as easy as just changing the conditional then it begs the question as
to why Barry didn't do that in the first place :)

>
> > [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/
>
> > >   		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.

Right ok :)

Cheers, Lorenzo