On 12/13/22 10:49, James Houghton wrote: > On Mon, Dec 12, 2022 at 7:14 PM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote: > > > > On 10/21/22 16:36, James Houghton wrote: > > > Currently it is possible for all shared VMAs to use HGM, but it must be > > > enabled first. This is because with HGM, we lose PMD sharing, and page > > > table walks require additional synchronization (we need to take the VMA > > > lock). > > > > Not sure yet, but I expect Peter's series will help with locking for > > hugetlb specific page table walks. > > It should make things a little bit cleaner in this series; I'll rebase > HGM on top of those patches this week (and hopefully get a v1 out > soon). > > I don't think it's possible to implement MADV_COLLAPSE with RCU alone > (as implemented in Peter's series anyway); we still need the VMA lock. As I continue going through the series, I realize that I am not exactly sure what synchronization by the vma lock is required by HGM. As you are aware, it was originally designed to protect against someone doing a pmd_unshare and effectively removing part of the page table. However, since pmd sharing is disabled for vmas with HGM enabled (I think?), then it might be a good idea to explicitly say somewhere the reason for using the lock. -- Mike Kravetz
