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
