Re: [PATCH] mm/madvise: allow MADV_DONTNEED to free memory that is MLOCK_ONFAULT

[Michal Hocko]

[CCing linux-api - please make sure to CC this mailing list anytime you
 are touching user visible apis]

On Fri 08-06-18 14:56:52, Jason Baron wrote:
> In order to free memory that is marked MLOCK_ONFAULT, the memory region
> needs to be first unlocked, before calling MADV_DONTNEED. And if the region
> is to be reused as MLOCK_ONFAULT, we require another call to mlock2() with
> the MLOCK_ONFAULT flag.
> 
> Let's simplify freeing memory that is set MLOCK_ONFAULT, by allowing
> MADV_DONTNEED to work directly for memory that is set MLOCK_ONFAULT.

I do not understand the point here. How is MLOCK_ONFAULT any different
from the regular mlock here? If you want to free mlocked memory then
fine but the behavior should be consistent. MLOCK_ONFAULT is just a way
to say that we do not want to pre-populate the mlocked area and do that
lazily on the page fault time. madvise should make any difference here.

That being said we do not allow MADV_DONTNEED on VM_LOCKED since ever. I
do not really see why but this would be a user visible change. Can we do
that? What was the original motivation for exclusion?

[keeping the rest of email for linux-api]

> The
> locked memory limits, tracked by mm->locked_vm do not need to be adjusted
> in this case, since they were charged to the entire region when
> MLOCK_ONFAULT was initially set.
> 
> Further, I don't think allowing MADV_FREE for MLOCK_ONFAULT regions makes
> sense, since the point of MLOCK_ONFAULT is for userspace to know when pages
> are locked in memory and thus to know when page faults will occur.
> 
> Signed-off-by: Jason Baron <jbaron@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
>  mm/internal.h | 18 ++++++++++++++++++
>  mm/madvise.c  |  4 ++--
>  mm/oom_kill.c |  2 +-
>  3 files changed, 21 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index 9e3654d..16c0041 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -15,6 +15,7 @@
>  #include <linux/mm.h>
>  #include <linux/pagemap.h>
>  #include <linux/tracepoint-defs.h>
> +#include <uapi/asm-generic/mman-common.h>
>  
>  /*
>   * The set of flags that only affect watermark checking and reclaim
> @@ -45,9 +46,26 @@ void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
>  
>  static inline bool can_madv_dontneed_vma(struct vm_area_struct *vma)
>  {
> +	return !(((vma->vm_flags & (VM_LOCKED|VM_LOCKONFAULT)) == VM_LOCKED) ||
> +		 (vma->vm_flags & (VM_HUGETLB|VM_PFNMAP)));
> +}
> +
> +static inline bool can_madv_free_vma(struct vm_area_struct *vma)
> +{
>  	return !(vma->vm_flags & (VM_LOCKED|VM_HUGETLB|VM_PFNMAP));
>  }
>  
> +static inline bool can_madv_dontneed_or_free_vma(struct vm_area_struct *vma,
> +						 int behavior)
> +{
> +	if (behavior == MADV_DONTNEED)
> +		return can_madv_dontneed_vma(vma);
> +	else if (behavior == MADV_FREE)
> +		return can_madv_free_vma(vma);
> +	else
> +		return 0;
> +}
> +
>  void unmap_page_range(struct mmu_gather *tlb,
>  			     struct vm_area_struct *vma,
>  			     unsigned long addr, unsigned long end,
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 4d3c922..61ff306 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -517,7 +517,7 @@ static long madvise_dontneed_free(struct vm_area_struct *vma,
>  				  int behavior)
>  {
>  	*prev = vma;
> -	if (!can_madv_dontneed_vma(vma))
> +	if (!can_madv_dontneed_or_free_vma(vma, behavior))
>  		return -EINVAL;
>  
>  	if (!userfaultfd_remove(vma, start, end)) {
> @@ -539,7 +539,7 @@ static long madvise_dontneed_free(struct vm_area_struct *vma,
>  			 */
>  			return -ENOMEM;
>  		}
> -		if (!can_madv_dontneed_vma(vma))
> +		if (!can_madv_dontneed_or_free_vma(vma, behavior))
>  			return -EINVAL;
>  		if (end > vma->vm_end) {
>  			/*
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 8ba6cb8..9817d15 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -492,7 +492,7 @@ void __oom_reap_task_mm(struct mm_struct *mm)
>  	set_bit(MMF_UNSTABLE, &mm->flags);
>  
>  	for (vma = mm->mmap ; vma; vma = vma->vm_next) {
> -		if (!can_madv_dontneed_vma(vma))
> +		if (!can_madv_free_vma(vma))
>  			continue;
>  
>  		/*
> -- 
> 2.7.4
> 

-- 
Michal Hocko
SUSE Labs