The patch titled Subject: fork: only invoke khugepaged, ksm hooks if no error has been added to the -mm mm-hotfixes-unstable branch. Its filename is fork-only-invoke-khugepaged-ksm-hooks-if-no-error.patch
This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...
This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------ From: Lorenzo Stoakes lorenzo.stoakes@oracle.com Subject: fork: only invoke khugepaged, ksm hooks if no error Date: Tue, 15 Oct 2024 18:56:06 +0100
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.
Link: https://lkml.kernel.org/r/e0cb8b840c9d1d5a6e84d4f8eff5f3f2022aa10c.172901437... Fixes: d24062914837 ("fork: use __mt_dup() to duplicate maple tree in dup_mmap()") Signed-off-by: Lorenzo Stoakes lorenzo.stoakes@oracle.com Reported-by: Jann Horn jannh@google.com Reviewed-by: Liam R. Howlett Liam.Howlett@Oracle.com Reviewed-by: Vlastimil Babka vbabka@suse.cz Reviewed-by: Jann 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: Andrew Morton akpm@linux-foundation.org ---
include/linux/ksm.h | 10 ++++------ kernel/fork.c | 7 ++----- 2 files changed, 6 insertions(+), 11 deletions(-)
--- a/include/linux/ksm.h~fork-only-invoke-khugepaged-ksm-hooks-if-no-error +++ a/include/linux/ksm.h @@ -54,12 +54,11 @@ static inline long mm_ksm_zero_pages(str 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_ 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) --- a/kernel/fork.c~fork-only-invoke-khugepaged-ksm-hooks-if-no-error +++ a/kernel/fork.c @@ -653,11 +653,6 @@ static __latent_entropy int dup_mmap(str 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 @@ loop_out: 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 _
Patches currently in -mm which might be from lorenzo.stoakes@oracle.com are
mm-mmap-correct-error-handling-in-mmap_region.patch maple_tree-correct-tree-corruption-on-spanning-store.patch maple_tree-add-regression-test-for-spanning-store-bug.patch maintainers-add-memory-mapping-vma-co-maintainers.patch fork-do-not-invoke-uffd-on-fork-if-error-occurs.patch fork-only-invoke-khugepaged-ksm-hooks-if-no-error.patch selftests-mm-add-pkey_sighandler_xx-hugetlb_dio-to-gitignore.patch mm-refactor-mm_access-to-not-return-null.patch mm-refactor-mm_access-to-not-return-null-fix.patch mm-madvise-unrestrict-process_madvise-for-current-process.patch maple_tree-do-not-hash-pointers-on-dump-in-debug-mode.patch