Commit 985da552 authored by Lorenzo Stoakes's avatar Lorenzo Stoakes Committed by Andrew Morton
Browse files

fork: only invoke khugepaged, ksm hooks if no error 

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

The change in commit d2406291 ("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 d2081b2b ("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.

Link: https://lkml.kernel.org/r/e0cb8b840c9d1d5a6e84d4f8eff5f3f2022aa10c.1729014377.git.lorenzo.stoakes@oracle.com


Fixes: d2406291 ("fork: use __mt_dup() to duplicate maple tree in dup_mmap()")
Signed-off-by: default avatarLorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reported-by: default avatarJann Horn <jannh@google.com>
Reviewed-by: default avatarLiam R. Howlett <Liam.Howlett@Oracle.com>
Reviewed-by: default avatarVlastimil Babka <vbabka@suse.cz>
Reviewed-by: default avatarJann Horn <jannh@google.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Linus Torvalds <torvalds@linuxfoundation.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent f64e67e5

include/linux/ksm.h

+4 −6
Original line number Diff line number Diff line
@@ -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)

kernel/fork.c

+2 −5
Original line number Diff line number Diff line
@@ -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