Re: [PATCH v2 3/4] mm: prevent KSM from completely breaking VMA merging

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* Lorenzo Stoakes <lorenzo.stoakes@oracle.com> [250521 14:20]:
> If a user wishes to enable KSM mergeability for an entire process and all
> fork/exec'd processes that come after it, they use the prctl()
> PR_SET_MEMORY_MERGE operation.
> 
> This defaults all newly mapped VMAs to have the VM_MERGEABLE VMA flag set
> (in order to indicate they are KSM mergeable), as well as setting this flag
> for all existing VMAs.
> 
> However it also entirely and completely breaks VMA merging for the process
> and all forked (and fork/exec'd) processes.
> 
> This is because when a new mapping is proposed, the flags specified will
> never have VM_MERGEABLE set. However all adjacent VMAs will already have
> VM_MERGEABLE set, rendering VMAs unmergeable by default.
> 
> To work around this, we try to set the VM_MERGEABLE flag prior to
> attempting a merge. In the case of brk() this can always be done.
> 
> However on mmap() things are more complicated - while KSM is not supported
> for file-backed mappings, it is supported for MAP_PRIVATE file-backed
> mappings.
> 
> And these mappings may have deprecated .mmap() callbacks specified which
> could, in theory, adjust flags and thus KSM eligiblity.
> 
> This is unlikely to cause an issue on merge, as any adjacent file-backed
> mappings would already have the same post-.mmap() callback attributes, and
> thus would naturally not be merged.
> 
> But for the purposes of establishing a VMA as KSM-eligible (as well as
> initially scanning the VMA), this is potentially very problematic.
> 
> So we check to determine whether this at all possible. If not, we set
> VM_MERGEABLE prior to the merge attempt on mmap(), otherwise we retain the
> previous behaviour.
> 
> When .mmap_prepare() is more widely used, we can remove this precaution.
> 
> While this doesn't quite cover all cases, it covers a great many (all
> anonymous memory, for instance), meaning we should already see a
> significant improvement in VMA mergeability.
> 
> Since, when it comes to file-backed mappings (other than shmem) we are
> really only interested in MAP_PRIVATE mappings which have an available anon
> page by default. Therefore, the VM_SPECIAL restriction makes less sense for
> KSM.
> 
> In a future series we therefore intend to remove this limitation, which
> ought to simplify this implementation. However it makes sense to defer
> doing so until a later stage so we can first address this mergeability
> issue.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Fixes: d7597f59d1d3 ("mm: add new api to enable ksm per process") # please no backport!
> Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>

Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>

> ---
>  include/linux/ksm.h |  8 +++++---
>  mm/ksm.c            | 18 +++++++++++------
>  mm/vma.c            | 49 +++++++++++++++++++++++++++++++++++++++++++--
>  3 files changed, 64 insertions(+), 11 deletions(-)
> 
> diff --git a/include/linux/ksm.h b/include/linux/ksm.h
> index d73095b5cd96..51787f0b0208 100644
> --- a/include/linux/ksm.h
> +++ b/include/linux/ksm.h
> @@ -17,8 +17,8 @@
>  #ifdef CONFIG_KSM
>  int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
>  		unsigned long end, int advice, unsigned long *vm_flags);
> -
> -void ksm_add_vma(struct vm_area_struct *vma);
> +vm_flags_t ksm_vma_flags(const struct mm_struct *mm, const struct file *file,
> +			 vm_flags_t vm_flags);
>  int ksm_enable_merge_any(struct mm_struct *mm);
>  int ksm_disable_merge_any(struct mm_struct *mm);
>  int ksm_disable(struct mm_struct *mm);
> @@ -97,8 +97,10 @@ bool ksm_process_mergeable(struct mm_struct *mm);
> 
>  #else  /* !CONFIG_KSM */
> 
> -static inline void ksm_add_vma(struct vm_area_struct *vma)
> +static inline vm_flags_t ksm_vma_flags(const struct mm_struct *mm,
> +		const struct file *file, vm_flags_t vm_flags)
>  {
> +	return vm_flags;
>  }
> 
>  static inline int ksm_disable(struct mm_struct *mm)
> diff --git a/mm/ksm.c b/mm/ksm.c
> index d0c763abd499..18b3690bb69a 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -2731,16 +2731,22 @@ static int __ksm_del_vma(struct vm_area_struct *vma)
>  	return 0;
>  }
>  /**
> - * ksm_add_vma - Mark vma as mergeable if compatible
> + * ksm_vma_flags - Update VMA flags to mark as mergeable if compatible
>   *
> - * @vma:  Pointer to vma
> + * @mm:       Proposed VMA's mm_struct
> + * @file:     Proposed VMA's file-backed mapping, if any.
> + * @vm_flags: Proposed VMA"s flags.
> + *
> + * Returns: @vm_flags possibly updated to mark mergeable.
>   */
> -void ksm_add_vma(struct vm_area_struct *vma)
> +vm_flags_t ksm_vma_flags(const struct mm_struct *mm, const struct file *file,
> +			 vm_flags_t vm_flags)
>  {
> -	struct mm_struct *mm = vma->vm_mm;
> +	if (test_bit(MMF_VM_MERGE_ANY, &mm->flags) &&
> +	    __ksm_should_add_vma(file, vm_flags))
> +		vm_flags |= VM_MERGEABLE;
> 
> -	if (test_bit(MMF_VM_MERGE_ANY, &mm->flags))
> -		__ksm_add_vma(vma);
> +	return vm_flags;
>  }
> 
>  static void ksm_add_vmas(struct mm_struct *mm)
> diff --git a/mm/vma.c b/mm/vma.c
> index 3ff6cfbe3338..5bebe55ea737 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -2482,7 +2482,6 @@ static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap)
>  	 */
>  	if (!vma_is_anonymous(vma))
>  		khugepaged_enter_vma(vma, map->flags);
> -	ksm_add_vma(vma);
>  	*vmap = vma;
>  	return 0;
> 
> @@ -2585,6 +2584,45 @@ static void set_vma_user_defined_fields(struct vm_area_struct *vma,
>  	vma->vm_private_data = map->vm_private_data;
>  }
> 
> +static void update_ksm_flags(struct mmap_state *map)
> +{
> +	map->flags = ksm_vma_flags(map->mm, map->file, map->flags);
> +}
> +
> +/*
> + * Are we guaranteed no driver can change state such as to preclude KSM merging?
> + * If so, let's set the KSM mergeable flag early so we don't break VMA merging.
> + *
> + * This is applicable when PR_SET_MEMORY_MERGE has been set on the mm_struct via
> + * prctl() causing newly mapped VMAs to have the KSM mergeable VMA flag set.
> + *
> + * If this is not the case, then we set the flag after considering mergeability,
> + * which will prevent mergeability as, when PR_SET_MEMORY_MERGE is set, a new
> + * VMA will not have the KSM mergeability VMA flag set, but all other VMAs will,
> + * preventing any merge.
> + */
> +static bool can_set_ksm_flags_early(struct mmap_state *map)
> +{
> +	struct file *file = map->file;
> +
> +	/* Anonymous mappings have no driver which can change them. */
> +	if (!file)
> +		return true;
> +
> +	/* shmem is safe. */
> +	if (shmem_file(file))
> +		return true;
> +
> +	/*
> +	 * If .mmap_prepare() is specified, then the driver will have already
> +	 * manipulated state prior to updating KSM flags.
> +	 */
> +	if (file->f_op->mmap_prepare)
> +		return true;
> +
> +	return false;
> +}
> +
>  static unsigned long __mmap_region(struct file *file, unsigned long addr,
>  		unsigned long len, vm_flags_t vm_flags, unsigned long pgoff,
>  		struct list_head *uf)
> @@ -2595,6 +2633,7 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr,
>  	bool have_mmap_prepare = file && file->f_op->mmap_prepare;
>  	VMA_ITERATOR(vmi, mm, addr);
>  	MMAP_STATE(map, mm, &vmi, addr, len, pgoff, vm_flags, file);
> +	bool check_ksm_early = can_set_ksm_flags_early(&map);
> 
>  	error = __mmap_prepare(&map, uf);
>  	if (!error && have_mmap_prepare)
> @@ -2602,6 +2641,9 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr,
>  	if (error)
>  		goto abort_munmap;
> 
> +	if (check_ksm_early)
> +		update_ksm_flags(&map);
> +
>  	/* Attempt to merge with adjacent VMAs... */
>  	if (map.prev || map.next) {
>  		VMG_MMAP_STATE(vmg, &map, /* vma = */ NULL);
> @@ -2611,6 +2653,9 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr,
> 
>  	/* ...but if we can't, allocate a new VMA. */
>  	if (!vma) {
> +		if (!check_ksm_early)
> +			update_ksm_flags(&map);
> +
>  		error = __mmap_new_vma(&map, &vma);
>  		if (error)
>  			goto unacct_error;
> @@ -2713,6 +2758,7 @@ int do_brk_flags(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  	 * Note: This happens *after* clearing old mappings in some code paths.
>  	 */
>  	flags |= VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;
> +	flags = ksm_vma_flags(mm, NULL, flags);
>  	if (!may_expand_vm(mm, flags, len >> PAGE_SHIFT))
>  		return -ENOMEM;
> 
> @@ -2756,7 +2802,6 @@ int do_brk_flags(struct vma_iterator *vmi, struct vm_area_struct *vma,
> 
>  	mm->map_count++;
>  	validate_mm(mm);
> -	ksm_add_vma(vma);

Well, that was in the wrong place.

>  out:
>  	perf_event_mmap(vma);
>  	mm->total_vm += len >> PAGE_SHIFT;
> --
> 2.49.0