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SubjectRe: [PATCH] mm/madvise: allow MADV_DONTNEED to free memory that is MLOCK_ONFAULT
[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@akamai.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
> 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

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