Re: [PATCH v3 00/15] Free user PTE page table pages

[David Hildenbrand]

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.

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

-- 
Thanks,

David / dhildenb