[PATCH hotfix 6.12 2/2] fork: only invoke khugepaged, ksm hooks if no error

[Lorenzo Stoakes]

There is no reason to invoke these hooks early against an mm that is in an
incomplete state.

The change in commit d24062914837 ("fork: use __mt_dup() to duplicate maple
tree in dup_mmap()") makes this more pertinent as we may be in a state
where entries in the maple tree are not yet consistent.

Their placement early in dup_mmap() only appears to have been meaningful
for early error checking, and since functionally it'd require a very small
allocation to fail (in practice 'too small to fail') that'd only occur in
the most dire circumstances, meaning the fork would fail or be OOM'd in any
case.

Since both khugepaged and KSM tracking are there to provide optimisations
to memory performance rather than critical functionality, it doesn't really
matter all that much if, under such dire memory pressure, we fail to
register an mm with these.

As a result, we follow the example of commit d2081b2bf819 ("mm: khugepaged:
make khugepaged_enter() void function") and make ksm_fork() a void function
also.

We only expose the mm to these functions once we are done with them and
only if no error occurred in the fork operation.

Reported-by: Jann Horn <jannh@xxxxxxxxxx>
Fixes: d24062914837 ("fork: use __mt_dup() to duplicate maple tree in dup_mmap()")
Cc: stable@xxxxxxxxxxxxxxx
Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
Reviewed-by: Jann Horn <jannh@xxxxxxxxxx>
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
---
 include/linux/ksm.h | 10 ++++------
 kernel/fork.c       |  7 ++-----
 2 files changed, 6 insertions(+), 11 deletions(-)

diff --git a/include/linux/ksm.h b/include/linux/ksm.h
index 11690dacd986..ec9c05044d4f 100644
--- a/include/linux/ksm.h
+++ b/include/linux/ksm.h
@@ -54,12 +54,11 @@ static inline long mm_ksm_zero_pages(struct mm_struct *mm)
 	return atomic_long_read(&mm->ksm_zero_pages);
 }

-static inline int ksm_fork(struct mm_struct *mm, struct mm_struct *oldmm)
+static inline void ksm_fork(struct mm_struct *mm, struct mm_struct *oldmm)
 {
+	/* Adding mm to ksm is best effort on fork. */
 	if (test_bit(MMF_VM_MERGEABLE, &oldmm->flags))
-		return __ksm_enter(mm);
-
-	return 0;
+		__ksm_enter(mm);
 }

 static inline int ksm_execve(struct mm_struct *mm)
@@ -107,9 +106,8 @@ static inline int ksm_disable(struct mm_struct *mm)
 	return 0;
 }

-static inline int ksm_fork(struct mm_struct *mm, struct mm_struct *oldmm)
+static inline void ksm_fork(struct mm_struct *mm, struct mm_struct *oldmm)
 {
-	return 0;
 }

 static inline int ksm_execve(struct mm_struct *mm)
diff --git a/kernel/fork.c b/kernel/fork.c
index 597b477dd491..3bf38d260cb3 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -653,11 +653,6 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
 	mm->exec_vm = oldmm->exec_vm;
 	mm->stack_vm = oldmm->stack_vm;

-	retval = ksm_fork(mm, oldmm);
-	if (retval)
-		goto out;
-	khugepaged_fork(mm, oldmm);
-
 	/* Use __mt_dup() to efficiently build an identical maple tree. */
 	retval = __mt_dup(&oldmm->mm_mt, &mm->mm_mt, GFP_KERNEL);
 	if (unlikely(retval))
@@ -760,6 +755,8 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
 	vma_iter_free(&vmi);
 	if (!retval) {
 		mt_set_in_rcu(vmi.mas.tree);
+		ksm_fork(mm, oldmm);
+		khugepaged_fork(mm, oldmm);
 	} else if (mpnt) {
 		/*
 		 * The entire maple tree has already been duplicated. If the
--
2.46.2