Re: [PATCH v8 2/9] mmap: make mlock_future_check() global

Mike Rapoport
2020-11-15T08:27:06Z

On Thu, Nov 12, 2020 at 09:15:18PM +0100, David Hildenbrand wrote:
>
> > Am 12.11.2020 um 20:08 schrieb Mike Rapoport <rppt@kernel.org>:
> >
> > ﻿On Thu, Nov 12, 2020 at 05:22:00PM +0100, David Hildenbrand wrote:
> >>> On 10.11.20 19:06, Mike Rapoport wrote:
> >>> On Tue, Nov 10, 2020 at 06:17:26PM +0100, David Hildenbrand wrote:
> >>>> On 10.11.20 16:14, Mike Rapoport wrote:
> >>>>> From: Mike Rapoport <rppt@linux.ibm.com>
> >>>>>
> >>>>> It will be used by the upcoming secret memory implementation.
> >>>>>
> >>>>> Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
> >>>>> ---
> >>>>> mm/internal.h | 3 +++
> >>>>> mm/mmap.c | 5 ++---
> >>>>> 2 files changed, 5 insertions(+), 3 deletions(-)
> >>>>>
> >>>>> diff --git a/mm/internal.h b/mm/internal.h
> >>>>> index c43ccdddb0f6..ae146a260b14 100644
> >>>>> --- a/mm/internal.h
> >>>>> +++ b/mm/internal.h
> >>>>> @@ -348,6 +348,9 @@ static inline void munlock_vma_pages_all(struct vm_area_struct *vma)
> >>>>> extern void mlock_vma_page(struct page *page);
> >>>>> extern unsigned int munlock_vma_page(struct page *page);
> >>>>> +extern int mlock_future_check(struct mm_struct *mm, unsigned long flags,
> >>>>> + unsigned long len);
> >>>>> +
> >>>>> /*
> >>>>> * Clear the page's PageMlocked(). This can be useful in a situation where
> >>>>> * we want to unconditionally remove a page from the pagecache -- e.g.,
> >>>>> diff --git a/mm/mmap.c b/mm/mmap.c
> >>>>> index 61f72b09d990..c481f088bd50 100644
> >>>>> --- a/mm/mmap.c
> >>>>> +++ b/mm/mmap.c
> >>>>> @@ -1348,9 +1348,8 @@ static inline unsigned long round_hint_to_min(unsigned long hint)
> >>>>> return hint;
> >>>>> }
> >>>>> -static inline int mlock_future_check(struct mm_struct *mm,
> >>>>> - unsigned long flags,
> >>>>> - unsigned long len)
> >>>>> +int mlock_future_check(struct mm_struct *mm, unsigned long flags,
> >>>>> + unsigned long len)
> >>>>> {
> >>>>> unsigned long locked, lock_limit;
> >>>>>
> >>>>
> >>>> So, an interesting question is if you actually want to charge secretmem
> >>>> pages against mlock now, or if you want a dedicated secretmem cgroup
> >>>> controller instead?
> >>>
> >>> Well, with the current implementation there are three limits an
> >>> administrator can use to control secretmem limits: mlock, memcg and
> >>> kernel parameter.
> >>>
> >>> The kernel parameter puts a global upper limit for secretmem usage,
> >>> memcg accounts all secretmem allocations, including the unused memory in
> >>> large pages caching and mlock allows per task limit for secretmem
> >>> mappings, well, like mlock does.
> >>>
> >>> I didn't consider a dedicated cgroup, as it seems we already have enough
> >>> existing knobs and a new one would be unnecessary.
> >>
> >> To me it feels like the mlock() limit is a wrong fit for secretmem. But
> >> maybe there are other cases of using the mlock() limit without actually
> >> doing mlock() that I am not aware of (most probably :) )?
> >
> > Secretmem does not explicitly calls to mlock() but it does what mlock()
> > does and a bit more. Citing mlock(2):
> >
> > mlock(), mlock2(), and mlockall() lock part or all of the calling
> > process's virtual address space into RAM, preventing that memory from
> > being paged to the swap area.
> >
> > So, based on that secretmem pages are not swappable, I think that
> > RLIMIT_MEMLOCK is appropriate here.
> >
>
> The page explicitly lists mlock() system calls.

Well, it's mlock() man page, isn't it? ;-)

My thinking was that since secretmem does what mlock() does wrt
swapability, it should at least obey the same limit, i.e.
RLIMIT_MEMLOCK.

> E.g., we also don‘t
> account for gigantic pages - which might be allocated from CMA and are
> not swappable.

Do you mean gigantic pages in hugetlbfs?
It seems to me that hugetlbfs accounting is a completely different
story.

> >> I mean, my concern is not earth shattering, this can be reworked later. As I
> >> said, it just feels wrong.
> >>
> >> --
> >> Thanks,
> >>
> >> David / dhildenb
> >>
> >
> > --
> > Sincerely yours,
> > Mike.
> >
>

--
Sincerely yours,
Mike.