Re: [RFC PATCH v2 08/47] hugetlb: add HGM enablement functions

[Mike Kravetz]

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