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Subject: Re: [PATCH v9 0/8] stg mail -e --version=v9 \
To:     David Hildenbrand <david@redhat.com>,
        Michal Hocko <mhocko@kernel.org>,
        "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        Alexander Duyck <alexander.duyck@gmail.com>,
        virtio-dev@lists.oasis-open.org, kvm list <kvm@vger.kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Dave Hansen <dave.hansen@intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        linux-mm <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>, will@kernel.org,
        linux-arm-kernel@lists.infradead.org,
        Oscar Salvador <osalvador@suse.de>,
        Yang Zhang <yang.zhang.wz@gmail.com>,
        Pankaj Gupta <pagupta@redhat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Rik van Riel <riel@surriel.com>, lcapitulino@redhat.com,
        "Wang, Wei W" <wei.w.wang@intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>, ying.huang@intel.com,
        Paolo Bonzini <pbonzini@redhat.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Fengguang Wu <fengguang.wu@intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
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From:   Nitesh Narayan Lal <nitesh@redhat.com>
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Organization: Red Hat Inc,
Message-ID: <87e0f863-6a49-c07e-49a7-c8a5bdb745ba@redhat.com>
Date:   Wed, 11 Sep 2019 08:55:17 -0400
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On 9/11/19 8:42 AM, David Hildenbrand wrote:
> On 11.09.19 14:25, Michal Hocko wrote:
>> On Wed 11-09-19 14:19:41, Michal Hocko wrote:
>>> On Wed 11-09-19 08:08:38, Michael S. Tsirkin wrote:
>>>> On Wed, Sep 11, 2019 at 01:36:19PM +0200, Michal Hocko wrote:
>>>>> On Tue 10-09-19 14:23:40, Alexander Duyck wrote:
>>>>> [...]
>>>>>> We don't put any limitations on the allocator other then that it needs to
>>>>>> clean up the metadata on allocation, and that it cannot allocate a page
>>>>>> that is in the process of being reported since we pulled it from the
>>>>>> free_list. If the page is a "Reported" page then it decrements the
>>>>>> reported_pages count for the free_area and makes sure the page doesn't
>>>>>> exist in the "Boundary" array pointer value, if it does it moves the
>>>>>> "Boundary" since it is pulling the page.
>>>>> This is still a non-trivial limitation on the page allocation from an
>>>>> external code IMHO. I cannot give any explicit reason why an ordering on
>>>>> the free list might matter (well except for page shuffling which uses it
>>>>> to make physical memory pattern allocation more random) but the
>>>>> architecture seems hacky and dubious to be honest. It shoulds like the
>>>>> whole interface has been developed around a very particular and single
>>>>> purpose optimization.
>>>>>
>>>>> I remember that there was an attempt to report free memory that provided
>>>>> a callback mechanism [1], which was much less intrusive to the internals
>>>>> of the allocator yet it should provide a similar functionality. Did you
>>>>> see that approach? How does this compares to it? Or am I completely off
>>>>> when comparing them?
>>>>>
>>>>> [1] mostly likely not the latest version of the patchset
>>>>> http://lkml.kernel.org/r/1502940416-42944-5-git-send-email-wei.w.wang@intel.com
>>>> Linus nacked that one. He thinks invoking callbacks with lots of
>>>> internal mm locks is too fragile.
>>> I would be really curious how much he would be happy about injecting
>>> other restrictions on the allocator like this patch proposes. This is
>>> more intrusive as it has a higher maintenance cost longterm IMHO.
>> Btw. I do agree that callbacks with internal mm locks are not great
>> either. We do have a model for that in mmu_notifiers and it is something
>> I do consider PITA, on the other hand it is mostly sleepable part of the
>> interface which makes it the real pain. The above callback mechanism was
>> explicitly documented with restrictions and that the context is
>> essentially atomic with no access to particular struct pages and no
>> expensive operations possible. So in the end I've considered it
>> acceptably painful. Not that I want to override Linus' nack but if
>> virtualization usecases really require some form of reporting and no
>> other way to do that push people to invent even more interesting
>> approaches then we should simply give them/you something reasonable
>> and least intrusive to our internals.
>>
> The issue with "[PATCH v14 4/5] mm: support reporting free page blocks"
>  is that it cannot really handle the use case we have here if I am not
> wrong. While a page is getting processed by the hypervisor (e.g.
> MADV_DONTNEED), it must not get reused.
>
> "Some page blocks may
> leave the free list after zone->lock is released, so it is the caller's
> responsibility to either detect or prevent the use of such pages."
>
> If I'm not wrong, this only made sense to speed up migration in the
> hypervisor, where you can deal with false positives differently.

Another difference between the two approaches is the origin from where
the reporting request is getting generated. (If I remember correctly)
In Alexander's series or in my series [1], VM is able to report pages
dynamically without any requirement of host intervention.

[1] https://lkml.org/lkml/2019/8/12/593


-- 
Thanks
Nitesh