Messages in this thread | | | Date | Wed, 10 Nov 2021 21:59:38 +0800 | Subject | Re: [PATCH v3 00/15] Free user PTE page table pages | From | Qi Zheng <> |
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On 11/10/21 9:25 PM, David Hildenbrand wrote: > 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. >
total agree!
Thanks, Qi
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