[PATCH v1 09/16] mm/memory: convert details->even_cows into details->skip_cows

David Hildenbrand (Arm)
2026-02-27T20:11:41Z

The current semantics are confusing: simply because someone specifies an
empty zap_detail struct suddenly makes should_zap_cows() behave
differently. The default should be to also zap CoW'ed anonymous pages.

Really only unmap_mapping_pages() and friends want to skip zapping of
these anon folios.

So let's invert the meaning; turn the confusing "reclaim_pt" check that
overrides other properties in should_zap_cows() into a safety check.

Note that the only caller that sets reclaim_pt=true is
madvise_dontneed_single_vma(), which wants to zap any pages.

Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
include/linux/mm.h | 2 +-
mm/madvise.c | 1 -
mm/memory.c | 12 ++++++------
3 files changed, 7 insertions(+), 8 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index d3ef586ee1c0..21b67c203e62 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2798,7 +2798,7 @@ extern void pagefault_out_of_memory(void);
*/
struct zap_details {
struct folio *single_folio; /* Locked folio to be unmapped */
- bool even_cows; /* Zap COWed private pages too? */
+ bool skip_cows; /* Do not zap COWed private pages */
bool reclaim_pt; /* Need reclaim page tables? */
zap_flags_t zap_flags; /* Extra flags for zapping */
};
diff --git a/mm/madvise.c b/mm/madvise.c
index 557a360f7919..b51f216934f3 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -853,7 +853,6 @@ static long madvise_dontneed_single_vma(struct madvise_behavior *madv_behavior)
struct madvise_behavior_range *range = &madv_behavior->range;
struct zap_details details = {
.reclaim_pt = true,
- .even_cows = true,
};

zap_page_range_single_batched(
diff --git a/mm/memory.c b/mm/memory.c
index fdcd2abf29c2..7d7c24c6917c 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1554,11 +1554,13 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
static inline bool should_zap_cows(struct zap_details *details)
{
/* By default, zap all pages */
- if (!details || details->reclaim_pt)
+ if (!details)
return true;

+ VM_WARN_ON_ONCE(details->skip_cows && details->reclaim_pt);
+
/* Or, we zap COWed pages only if the caller wants to */
- return details->even_cows;
+ return !details->skip_cows;
}

/* Decides whether we should zap this folio with the folio pointer specified */
@@ -2149,8 +2151,6 @@ void unmap_vmas(struct mmu_gather *tlb, struct unmap_desc *unmap)
struct mmu_notifier_range range;
struct zap_details details = {
.zap_flags = ZAP_FLAG_DROP_MARKER | ZAP_FLAG_UNMAP,
- /* Careful - we need to zap private pages too! */
- .even_cows = true,
};

vma = unmap->first;
@@ -4282,7 +4282,7 @@ void unmap_mapping_folio(struct folio *folio)
first_index = folio->index;
last_index = folio_next_index(folio) - 1;

- details.even_cows = false;
+ details.skip_cows = true;
details.single_folio = folio;
details.zap_flags = ZAP_FLAG_DROP_MARKER;

@@ -4312,7 +4312,7 @@ void unmap_mapping_pages(struct address_space *mapping, pgoff_t start,
pgoff_t first_index = start;
pgoff_t last_index = start + nr - 1;

- details.even_cows = even_cows;
+ details.skip_cows = !even_cows;
if (last_index < first_index)
last_index = ULONG_MAX;

--
2.43.0