[v3 02/24] mm: thp: propagate split failure from vma_adjust_trans_huge()

Usama Arif
2026-03-27T02:14:53Z

With lazy PTE page table allocation, split_huge_pmd_if_needed() and
thus vma_adjust_trans_huge() can now fail if order-0 allocation
for pagetable fails when trying to split. It is important to check
if this failure occurred to prevent a huge PMD straddling at VMA
boundary.

The vma_adjust_trans_huge() call is moved before vma_prepare() in all
three callers (__split_vma, vma_shrink, commit_merge). Previously it sat
between vma_prepare() and vma_complete(), where there is no mechanism to
abort - once vma_prepare() has been called, we must reach vma_complete().
By moving the call earlier, a split failure can return -ENOMEM cleanly
without needing to undo VMA preparation.

This move is safe because vma_adjust_trans_huge() acquires its own
pmd_lock() internally and does not depend on any locks or state changes
from vma_prepare(). The VMA boundaries are also unchanged at the new
call site, satisfying __split_huge_pmd_locked()'s requirement that the
VMA covers the full PMD range.

All 3 callers (__split_vma, vma_shrink, commit_merge) already return
-ENOMEM if there are allocation failures for other reasons (failure in
vma_iter_prealloc for example), this follows the same pattern.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
include/linux/huge_mm.h | 13 ++++++-----
mm/huge_memory.c | 21 +++++++++++++-----
mm/vma.c | 37 +++++++++++++++++++++----------
tools/testing/vma/include/stubs.h | 9 ++++----
4 files changed, 53 insertions(+), 27 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index b081ce044c735..224965fce4e66 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -483,8 +483,8 @@ int hugepage_madvise(struct vm_area_struct *vma, vm_flags_t *vm_flags,
int advice);
int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
unsigned long end, bool *lock_dropped);
-void vma_adjust_trans_huge(struct vm_area_struct *vma, unsigned long start,
- unsigned long end, struct vm_area_struct *next);
+int vma_adjust_trans_huge(struct vm_area_struct *vma, unsigned long start,
+ unsigned long end, struct vm_area_struct *next);
spinlock_t *__pmd_trans_huge_lock(pmd_t *pmd, struct vm_area_struct *vma);
spinlock_t *__pud_trans_huge_lock(pud_t *pud, struct vm_area_struct *vma);

@@ -685,11 +685,12 @@ static inline int madvise_collapse(struct vm_area_struct *vma,
return -EINVAL;
}

-static inline void vma_adjust_trans_huge(struct vm_area_struct *vma,
- unsigned long start,
- unsigned long end,
- struct vm_area_struct *next)
+static inline int vma_adjust_trans_huge(struct vm_area_struct *vma,
+ unsigned long start,
+ unsigned long end,
+ struct vm_area_struct *next)
{
+ return 0;
}
static inline spinlock_t *pmd_trans_huge_lock(pmd_t *pmd,
struct vm_area_struct *vma)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 976a1c74c0870..99f3b8b24c682 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3326,20 +3326,31 @@ static inline int split_huge_pmd_if_needed(struct vm_area_struct *vma, unsigned
return 0;
}

-void vma_adjust_trans_huge(struct vm_area_struct *vma,
+int vma_adjust_trans_huge(struct vm_area_struct *vma,
unsigned long start,
unsigned long end,
struct vm_area_struct *next)
{
+ int err;
+
/* Check if we need to split start first. */
- split_huge_pmd_if_needed(vma, start);
+ err = split_huge_pmd_if_needed(vma, start);
+ if (err)
+ return err;

/* Check if we need to split end next. */
- split_huge_pmd_if_needed(vma, end);
+ err = split_huge_pmd_if_needed(vma, end);
+ if (err)
+ return err;

/* If we're incrementing next->vm_start, we might need to split it. */
- if (next)
- split_huge_pmd_if_needed(next, end);
+ if (next) {
+ err = split_huge_pmd_if_needed(next, end);
+ if (err)
+ return err;
+ }
+
+ return 0;
}

static void unmap_folio(struct folio *folio)
diff --git a/mm/vma.c b/mm/vma.c
index a43f3c5d4b3dd..b4a3839a8036e 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -513,6 +513,15 @@ __split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
return err;
}

+ /*
+ * Split any THP straddling the split boundary before splitting
+ * the VMA itself. Do this before vma_prepare() so we can
+ * cleanly fail without undoing VMA preparation.
+ */
+ err = vma_adjust_trans_huge(vma, vma->vm_start, addr, NULL);
+ if (err)
+ return err;
+
new = vm_area_dup(vma);
if (!new)
return -ENOMEM;
@@ -550,11 +559,6 @@ __split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
vp.insert = new;
vma_prepare(&vp);

- /*
- * Get rid of huge pages and shared page tables straddling the split
- * boundary.
- */
- vma_adjust_trans_huge(vma, vma->vm_start, addr, NULL);
if (is_vm_hugetlb_page(vma))
hugetlb_split(vma, addr);

@@ -732,6 +736,7 @@ static int commit_merge(struct vma_merge_struct *vmg)
{
struct vm_area_struct *vma;
struct vma_prepare vp;
+ int err;

if (vmg->__adjust_next_start) {
/* We manipulate middle and adjust next, which is the target. */
@@ -743,6 +748,16 @@ static int commit_merge(struct vma_merge_struct *vmg)
vma_iter_config(vmg->vmi, vmg->start, vmg->end);
}

+ /*
+ * THP pages may need to do additional splits if we increase
+ * middle->vm_start. Do this before vma_prepare() so we can
+ * cleanly fail without undoing VMA preparation.
+ */
+ err = vma_adjust_trans_huge(vma, vmg->start, vmg->end,
+ vmg->__adjust_middle_start ? vmg->middle : NULL);
+ if (err)
+ return err;
+
init_multi_vma_prep(&vp, vma, vmg);

/*
@@ -755,12 +770,6 @@ static int commit_merge(struct vma_merge_struct *vmg)
return -ENOMEM;

vma_prepare(&vp);
- /*
- * THP pages may need to do additional splits if we increase
- * middle->vm_start.
- */
- vma_adjust_trans_huge(vma, vmg->start, vmg->end,
- vmg->__adjust_middle_start ? vmg->middle : NULL);
vma_set_range(vma, vmg->start, vmg->end, vmg->pgoff);
vmg_adjust_set_range(vmg);
vma_iter_store_overwrite(vmg->vmi, vmg->target);
@@ -1248,9 +1257,14 @@ int vma_shrink(struct vma_iterator *vmi, struct vm_area_struct *vma,
unsigned long start, unsigned long end, pgoff_t pgoff)
{
struct vma_prepare vp;
+ int err;

WARN_ON((vma->vm_start != start) && (vma->vm_end != end));

+ err = vma_adjust_trans_huge(vma, start, end, NULL);
+ if (err)
+ return err;
+
if (vma->vm_start < start)
vma_iter_config(vmi, vma->vm_start, start);
else
@@ -1263,7 +1277,6 @@ int vma_shrink(struct vma_iterator *vmi, struct vm_area_struct *vma,

init_vma_prep(&vp, vma);
vma_prepare(&vp);
- vma_adjust_trans_huge(vma, start, end, NULL);

vma_iter_clear(vmi);
vma_set_range(vma, start, end, pgoff);
diff --git a/tools/testing/vma/include/stubs.h b/tools/testing/vma/include/stubs.h
index a30b8bc849557..952e3cc88ef10 100644
--- a/tools/testing/vma/include/stubs.h
+++ b/tools/testing/vma/include/stubs.h
@@ -419,11 +419,12 @@ static inline int vma_dup_policy(struct vm_area_struct *src, struct vm_area_stru
return 0;
}

-static inline void vma_adjust_trans_huge(struct vm_area_struct *vma,
- unsigned long start,
- unsigned long end,
- struct vm_area_struct *next)
+static inline int vma_adjust_trans_huge(struct vm_area_struct *vma,
+ unsigned long start,
+ unsigned long end,
+ struct vm_area_struct *next)
{
+ return 0;
}

static inline void hugetlb_split(struct vm_area_struct *, unsigned long) {}
--
2.52.0