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

On 2025/12/16 14:13, Barry Song wrote:

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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.

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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.