linux-kernel - Re: [PATCH v8 2/9] mmap: make mlock_future

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Date:   Thu, 12 Nov 2020 17:22:00 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christopher Lameter <cl@...ux.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tycho Andersen <tycho@...ho.ws>, Will Deacon <will@...nel.org>,
        linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-riscv@...ts.infradead.org,
        x86@...nel.org
Subject: Re: [PATCH v8 2/9] mmap: make mlock_future_check() global

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@...ux.ibm.com>
>>>
>>> It will be used by the upcoming secret memory implementation.
>>>
>>> Signed-off-by: Mike Rapoport <rppt@...ux.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 :) )?

I mean, my concern is not earth shattering, this can be reworked later. 
As I said, it just feels wrong.

-- 
Thanks,

David / dhildenb