On 10.11.21 13:56, Jason Gunthorpe wrote: > On Wed, Nov 10, 2021 at 06:54:13PM +0800, Qi Zheng wrote: > >> In this patch series, we add a pte_refcount field to the struct page of page >> table to track how many users of PTE page table. Similar to the mechanism of >> page refcount, the user of PTE page table should hold a refcount to it before >> accessing. The PTE page table page will be freed when the last refcount is >> dropped. > > So, this approach basically adds two atomics on every PTE map > > If I have it right the reason that zap cannot clean the PTEs today is > because zap cannot obtain the mmap lock due to a lock ordering issue > with the inode lock vs mmap lock. There are different ways to zap: madvise(DONTNEED) vs fallocate(PUNCH_HOLE). It depends on "from where" we're actually comming: a process page table walker or the rmap. The way locking currently works doesn't allow to remove a page table just by holding the mmap lock, not even in write mode. You'll also need to hold the respective rmap locks -- which implies that reclaiming apge tables crossing VMAs is "problematic". Take a look at khugepaged which has to play quite some tricks to remove a page table. And there are other ways we can create empty page tables via the rmap, like reclaim/writeback, although they are rather a secondary concern mostly. > > If it could obtain the mmap lock then it could do the zap using the > write side as unmapping a vma does. > > Rather than adding a new "lock" to ever PTE I wonder if it would be > more efficient to break up the mmap lock and introduce a specific > rwsem for the page table itself, in addition to the PTL. Currently the > mmap lock is protecting both the vma list and the page table. There is the rmap side of things as well. At least the rmap won't reclaim alloc/free page tables, but it will walk page tables while holding the respective rmap lock. > > I think that would allow the lock ordering issue to be resolved and > zap could obtain a page table rwsem. > > Compared to two atomics per PTE this would just be two atomic per > page table walk operation, it is conceptually a lot simpler, and would > allow freeing all the page table levels, not just PTEs. Another alternative is to not do it in the kernel automatically, but instead have a madvise(MADV_CLEANUP_PGTABLE) mechanism that will get called by user space explicitly once it's reasonable. While this will work for the obvious madvise(DONTNEED) users -- like memory allocators -- that zap memory, it's a bit more complicated once shared memory is involved and we're fallocate(PUNCH_HOLE) memory. But it would at least work for many use cases that want to optimize memory consumption for sparse memory mappings. Note that PTEs are the biggest memory consumer. On x86-64, a 1 TiB area will consume 2 GiB of PTE tables and only 4 MiB of PMD tables. So PTEs are most certainly the most important part piece. -- Thanks, David / dhildenb
