Re: [RFC PATCH 19/26] hugetlb: add HGM support for copy_hugetlb_page_range

[Axel Rasmussen]

On Tue, Jul 12, 2022 at 11:07 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> On 07/12/22 10:19, James Houghton wrote:
> > On Mon, Jul 11, 2022 at 4:41 PM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
> > >
> > > On 06/24/22 17:36, James Houghton wrote:
> > > > This allows fork() to work with high-granularity mappings. The page
> > > > table structure is copied such that partially mapped regions will remain
> > > > partially mapped in the same way for the new process.
> > > >
> > > > Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
> > > > ---
> > > >  mm/hugetlb.c | 74 +++++++++++++++++++++++++++++++++++++++++-----------
> > > >  1 file changed, 59 insertions(+), 15 deletions(-)
> > >
> > > FYI -
> > > With https://lore.kernel.org/linux-mm/20220621235620.291305-5-mike.kravetz@xxxxxxxxxx/
> > > copy_hugetlb_page_range() should never be called for shared mappings.
> > > Since HGM only works on shared mappings, code in this patch will never
> > > be executed.
> > >
> > > I have a TODO to remove shared mapping support from copy_hugetlb_page_range.
> >
> > Thanks Mike. If I understand things correctly, it seems like I don't
> > have to do anything to support fork() then; we just don't copy the
> > page table structure from the old VMA to the new one.
>
> Yes, for now.  We will not copy the page tables for shared mappings.
> When adding support for private mapping, we will need to handle the
> HGM case.
>
> >                                                       That is, as
> > opposed to having the same bits of the old VMA being mapped in the new
> > one, the new VMA will have an empty page table. This would slightly
> > change how userfaultfd's behavior on the new VMA, but that seems fine
> > to me.
>
> Right.  Since the 'mapping size information' is essentially carried in
> the page tables, it will be lost if page tables are not copied.
>
> Not sure if anyone would depend on that behavior.
>
> Axel, this may also impact minor fault processing.  Any concerns?
> Patch is sitting in Andrew's tree for next merge window.

Sorry for the slow response, just catching up a bit here. :)

If I understand correctly, let's say we have a process where some
hugetlb pages are fully mapped (pages are in page cache, page table
entries exist). Once we fork(), we in the future won't copy the page
table entries, but I assume we do setup the underlying pages for CoW
still. So I guess this means in the old process no fault would happen
if the memory was touched, but in the forked process it would generate
a minor fault?

To me that seems fine. When userspace gets a minor fault it's always
fine for it to just say "don't care, just UFFDIO_CONTINUE, no work
needed". For VM migration I don't think it's unreasonable to expect
userspace to remember whether or not the page is clean (it already
does this anyway) and whether or not a fork (without exec) had
happened. It seems to me it should work fine.

> --
> Mike Kravetz