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Subject: Re: [v2 PATCH] mm: shmem: allow split THP when truncating THP
 partially
To:     David Hildenbrand <david@redhat.com>,
        Alexander Duyck <alexander.duyck@gmail.com>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
        Hugh Dickins <hughd@google.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
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From:   Yang Shi <yang.shi@linux.alibaba.com>
Message-ID: <00338ec9-76fc-bc0e-39af-495ff3b5c7e9@linux.alibaba.com>
Date:   Wed, 26 Feb 2020 10:00:55 -0800
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On 2/26/20 9:45 AM, David Hildenbrand wrote:
> On 26.02.20 18:31, Yang Shi wrote:
>>
>> On 2/21/20 4:24 PM, Alexander Duyck wrote:
>>> On Fri, Feb 21, 2020 at 10:24 AM Yang Shi <yang.shi@linux.alibaba.com> wrote:
>>>>
>>>> On 2/20/20 10:16 AM, Alexander Duyck wrote:
>>>>> On Tue, Dec 3, 2019 at 4:43 PM Yang Shi <yang.shi@linux.alibaba.com> wrote:
>>>>>> Currently when truncating shmem file, if the range is partial of THP
>>>>>> (start or end is in the middle of THP), the pages actually will just get
>>>>>> cleared rather than being freed unless the range cover the whole THP.
>>>>>> Even though all the subpages are truncated (randomly or sequentially),
>>>>>> the THP may still be kept in page cache.  This might be fine for some
>>>>>> usecases which prefer preserving THP.
>>>>>>
>>>>>> But, when doing balloon inflation in QEMU, QEMU actually does hole punch
>>>>>> or MADV_DONTNEED in base page size granulairty if hugetlbfs is not used.
>>>>>> So, when using shmem THP as memory backend QEMU inflation actually doesn't
>>>>>> work as expected since it doesn't free memory.  But, the inflation
>>>>>> usecase really needs get the memory freed.  Anonymous THP will not get
>>>>>> freed right away too but it will be freed eventually when all subpages are
>>>>>> unmapped, but shmem THP would still stay in page cache.
>>>>>>
>>>>>> Split THP right away when doing partial hole punch, and if split fails
>>>>>> just clear the page so that read to the hole punched area would return
>>>>>> zero.
>>>>>>
>>>>>> Cc: Hugh Dickins <hughd@google.com>
>>>>>> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>>>>>> Cc: Andrea Arcangeli <aarcange@redhat.com>
>>>>>> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
>>>>> One question I would have is if this is really the desired behavior we
>>>>> are looking for?
>>>>>
>>>>> By proactively splitting the THP you are likely going to see a
>>>>> performance regression with the virtio-balloon driver enabled in QEMU.
>>>>> I would suspect the response to that would be to update the QEMU code
>>>>> to  identify the page size of the shared memory ramblock. At that
>>>>> point I suspect it would start behaving the same as how it currently
>>>>> handles anonymous memory, and the work done here would essentially
>>>>> have been wasted other than triggering the desire to resolve this in
>>>>> QEMU to avoid a performance regression.
>>>>>
>>>>> The code for inflating a the balloon in virtio-balloon in QEMU can be
>>>>> found here:
>>>>> https://github.com/qemu/qemu/blob/master/hw/virtio/virtio-balloon.c#L66
>>>>>
>>>>> If there is a way for us to just populate the value obtained via
>>>>> qemu_ram_pagesize with the THP page size instead of leaving it at 4K,
>>>>> which is the size I am assuming it is at since you indicated that it
>>>>> is just freeing the base page size, then we could address the same
>>>>> issue and likely get the desired outcome of freeing the entire THP
>>>>> page when it is no longer used.
>>>> If qemu could punch hole (this is how qemu free file-backed memory) in
>>>> THP unit, either w/ or w/o the patch the THP won't get split since the
>>>> whole THP will get truncated. But, if qemu has to free memory in sub-THP
>>>> size due to whatever reason (for example, 1MB for every 2MB section),
>>>> then we have to split THP otherwise no memory will be freed actually
>>>> with the current code. It is not about performance, it is about really
>>>> giving memory back to host.
>>> I get that, but at the same time I am not sure if everyone will be
>>> happy with the trade-off. That is my concern.
>>>
>>> You may want to change the patch description above if that is the
>>> case. Based on the description above it makes it sound as if the issue
>>> is that QEMU is using hole punch or MADV_DONTNEED with the wrong
>>> granularity. Based on your comment here it sounds like you want to
>>> have the ability to break up the larger THP page as soon as you want
>>> to push out a single 4K page from it.
>> Yes, you are right. The commit log may be confusing. What I wanted to
>> convey is QEMU has no idea if THP is used or not so it treats memory
>> with base size unless hugetlbfs is used since QEMU is aware huge page is
>> used in this case.
>> This may sounds irrelevant to the problem, I would just remove that.
>>
>>> I am not sure the description for the behavior of anonymous THP with
>>> respect to QEMU makes sense either. Based on the description you made
>>> it sound like it was somehow using the same process used for huge
>>> pages. That isn't the case right? My understanding is that in the case
>>> of an anonymous THP it is getting broken into 4K subpages and then
>>> those are freed individually. That should leave you with the same
>>> performance regression that I had brought up earlier.
>> No, anonymous THP won't get split immediately and those memory also
>> won't get freed immediately if QEMU does MADV_DONTNEED on sub THP range
>> (for example, 1MB range in THP). The THP will get freed when:
>> 1. Host has memory pressure. The THP will get split and unmapped pages
>> will be freed.
>> 2. Other sub pages in the same THP are MADV_DONTNEED'ed (eventually the
>> whole THP get unmapped).
>>
>> The difference between shmem and anonymous page is shmem will not get
>> freed unless hole punch the whole THP, anonymous page will get freed
>> sooner or later.
>>
> As far as I understood Hugh, the "page size" we'll see in QEMU via
> fstatfs() is 4k, not 2MB. IMHO, that's the block size of the "device",
> and breaking up THP is the right think to to obey the documentation of
> "FALLOC_FL_PUNCH_HOLE".

This is what the patch attempts to accomplish.

>
> IMHO THP is called "transparent" because it shouldn't have any such
> visible side effects.

AFAICT, the lazy split is due to locking issue in partial unmap paths. 
Please refer to "Partial unmap and deferred_split_huge_page()" section 
in Documentation/vm/transhuge.rst.

>
> As always, anybody correct me if I am wrong here.
>