Message-ID: <2e19ad1b-15f3-7508-c5d5-6c31765f26d3@bytedance.com>
Date:   Thu, 11 Nov 2021 20:00:06 +0800
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Subject: Re: [PATCH v3 00/15] Free user PTE page table pages
To:     David Hildenbrand <david@redhat.com>,
        Jason Gunthorpe <jgg@nvidia.com>
Cc:     akpm@linux-foundation.org, tglx@linutronix.de,
        kirill.shutemov@linux.intel.com, mika.penttila@nextfour.com,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, songmuchun@bytedance.com,
        zhouchengming@bytedance.com
References: <20211110105428.32458-1-zhengqi.arch@bytedance.com>
 <20211110125601.GQ1740502@nvidia.com>
 <8d0bc258-58ba-52c5-2e0d-a588489f2572@redhat.com>
 <20211110143859.GS1740502@nvidia.com>
 <6ac9cc0d-7dea-0e19-51b3-625ec6561ac7@redhat.com>
 <20211110163925.GX1740502@nvidia.com>
 <7c97d86f-57f4-f764-3e92-1660690a0f24@redhat.com>
 <60515562-5f93-11cd-6c6a-c7cc92ff3bf8@bytedance.com>
 <a052e0ba-a22c-5df1-80b8-d847efacd66e@redhat.com>
 <9ee06b52-4844-7996-fa34-34fc7d4fdc10@bytedance.com>
 <27d73395-70b4-fe4a-4c8d-415b43ff9c1f@redhat.com>
From:   Qi Zheng <zhengqi.arch@bytedance.com>
In-Reply-To: <27d73395-70b4-fe4a-4c8d-415b43ff9c1f@redhat.com>
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On 11/11/21 7:19 PM, David Hildenbrand wrote:
> On 11.11.21 12:08, Qi Zheng wrote:
>>
>>
>> On 11/11/21 5:22 PM, David Hildenbrand wrote:
>>> On 11.11.21 04:58, Qi Zheng wrote:
>>>>
>>>>
>>>> On 11/11/21 1:37 AM, David Hildenbrand wrote:
>>>>>>> It would still be a fairly coarse-grained locking, I am not sure if that
>>>>>>> is a step into the right direction. If you want to modify *some* page
>>>>>>> table in your process you have exclude each and every page table walker.
>>>>>>> Or did I mis-interpret what you were saying?
>>>>>>
>>>>>> That is one possible design, it favours fast walking and penalizes
>>>>>> mutation. We could also stick a lock in the PMD (instead of a
>>>>>> refcount) and still logically be using a lock instead of a refcount
>>>>>> scheme. Remember modify here is "want to change a table pointer into a
>>>>>> leaf pointer" so it isn't an every day activity..
>>>>>
>>>>> It will be if we somewhat frequent when reclaim an empty PTE page table
>>>>> as soon as it turns empty. This not only happens when zapping, but also
>>>>> during writeback/swapping. So while writing back / swapping you might be
>>>>> left with empty page tables to reclaim.
>>>>>
>>>>> Of course, this is the current approach. Another approach that doesn't
>>>>> require additional refcounts is scanning page tables for empty ones and
>>>>> reclaiming them. This scanning can either be triggered manually from
>>>>> user space or automatically from the kernel.
>>>>
>>>> Whether it is introducing a special rwsem or scanning an empty page
>>>> table, there are two problems as follows:
>>>>
>>>> 	#1. When to trigger the scanning or releasing?
>>>
>>> For example when reclaiming memory, when scanning page tables in
>>> khugepaged, or triggered by user space (note that this is the approach I
>>> originally looked into). But it certainly requires more locking thought
>>> to avoid stopping essentially any page table walker.
>>>
>>>> 	#2. Every time to release a 4K page table page, 512 page table
>>>> 	    entries need to be scanned.
>>>
>>> It would happen only when actually trigger reclaim of page tables
>>> (again, someone has to trigger it), so it's barely an issue.
>>>
>>> For example, khugepaged already scans the page tables either way.
>>>
>>>>
>>>> For #1, if the scanning is triggered manually from user space, the
>>>> kernel is relatively passive, and the user does not fully know the best
>>>> timing to scan. If the scanning is triggered automatically from the
>>>> kernel, that is great. But the timing is not easy to confirm, is it
>>>> scanned and reclaimed every time zap or try_to_unmap?
>>>>
>>>> For #2, refcount has advantages.
>>>>
>>>>>
>>>>>>
>>>>>> There is some advantage with this thinking because it harmonizes well
>>>>>> with the other stuff that wants to convert tables into leafs, but has
>>>>>> to deal with complicated locking.
>>>>>>
>>>>>> On the other hand, refcounts are a degenerate kind of rwsem and only
>>>>>> help with freeing pages. It also puts more atomics in normal fast
>>>>>> paths since we are refcounting each PTE, not read locking the PMD.
>>>>>>
>>>>>> Perhaps the ideal thing would be to stick a rwsem in the PMD. read
>>>>>> means a table cannot be come a leaf. I don't know if there is space
>>>>>> for another atomic in the PMD level, and we'd have to use a hitching
>>>>>> post/hashed waitq scheme too since there surely isn't room for a waitq
>>>>>> too..
>>>>>>
>>>>>> I wouldn't be so quick to say one is better than the other, but at
>>>>>> least let's have thought about a locking solution before merging
>>>>>> refcounts :)
>>>>>
>>>>> Yes, absolutely. I can see the beauty in the current approach, because
>>>>> it just reclaims "automatically" once possible -- page table empty and
>>>>> nobody is walking it. The downside is that it doesn't always make sense
>>>>> to reclaim an empty page table immediately once it turns empty.
>>>>>
>>>>> Also, it adds complexity for something that is only a problem in some
>>>>> corner cases -- sparse memory mappings, especially relevant for some
>>>>> memory allocators after freeing a lot of memory or running VMs with
>>>>> memory ballooning after inflating the balloon. Some of these use cases
>>>>> might be good with just triggering page table reclaim manually from user
>>>>> space.
>>>>>
>>>>
>>>> Yes, this is indeed a problem. Perhaps some flags can be introduced so
>>>> that the release of page table pages can be delayed in some cases.
>>>> Similar to the lazyfree mechanism in MADV_FREE?
>>>
>>> The issue AFAIU is that once your refcount hits 0 (no more references,
>>> no more entries), the longer you wait with reclaim, the longer others
>>> have to wait for populating a fresh page table because the "page table
>>> to be reclaimed" is still stuck around. You'd have to keep the refcount
>>> increased for a while, and only drop it after a while. But when? And
>>> how? IMHO it's not trivial, but maybe there is an easy way to achieve it.
>>>
>>
>> For running VMs with memory ballooning after inflating the balloon, is
>> this a hot behavior? Even if it is, it is already facing the release and
>> reallocation of physical pages. The overhead after introducing
>> pte_refcount is that we need to release and re-allocate page table page.
>> But 2MB physical pages only corresponds to 4KiB of PTE page table page.
>> So maybe the overhead is not big.
> 
> The cases that come to my mind are
> 
> a) Swapping on shared memory with concurrent access
> b) Reclaim on file-backed memory with concurrent access
> c) Free page reporting as implemented by virtio-balloon
> 
> In all of these cases, you can have someone immediately re-access the
> page table and re-populate it.

In the performance test shown on the cover, we repeatedly performed
touch and madvise(MADV_DONTNEED) actions, which simulated the case
you said above.

We did find a small amount of performance regression, but I think it is
acceptable, and no new perf hotspots have been added.

> 
> For something mostly static (balloon inflation, memory allocator), it's
> not that big of a deal I guess.
>