On 7/7/22 04:32, Baolin Wang wrote: > On 7/6/2022 11:48 PM, Dave Hansen wrote: >> On 7/6/22 01:59, Baolin Wang wrote: >>> Now we will miss to account the PUD level pagetable and kernel PTE level >>> pagetable, as well as missing to set the PG_table flags for these >>> pagetable >>> pages, which will get an inaccurate pagetable accounting, and miss >>> PageTable() validation in some cases. So this patch set introduces new >>> helpers to help to account PUD and kernel PTE pagetable pages. >> >> Could you explain the motivation for this series a bit more? Is there a >> real-world problem that this fixes? > > Not fix real problem. The motivation is that making the pagetable > accounting more accurate, which helps us to analyse the consumption of > the pagetable pages in some cases, and maybe help to do some empty > pagetable reclaiming in future. This accounting isn't free. It costs storage (and also parts of cachelines) in each mm and CPU time to maintain it, plus maintainer eyeballs to maintain. PUD pages are also fundamentally (on x86 at least) 0.0004% of the overhead of PTE and 0.2% of the overhead of PMD pages unless someone is using gigantic hugetlbfs mappings. Even with 1G gigantic pages, you would need a quarter of a million (well, 262144 or 512*512) mappings of one 1G page to consume 1G of memory on PUD pages. That just doesn't seem like something anyone is likely to actually do in practice. That makes the benefits of the PUD portion of this series rather unclear in the real world. As for the kernel page tables, I'm not really aware of them causing any problems. We have a pretty good idea how much space they consume from the DirectMap* entries in meminfo: DirectMap4k: 2262720 kB DirectMap2M: 40507392 kB DirectMap1G: 24117248 kB as well as our page table debugging infrastructure. I haven't found myself dying for more specific info on them. So, nothing in this series seems like a *BAD* idea, but I'm not sure in the end it solves more problems than it creates.
