Re: [RFC PATCH 0/6] mm: thp: use generic THP migration for NUMA hinting fault

[Gerald Schaefer <gerald.schaefer@linux.ibm.com>]

On Tue, 30 Mar 2021 09:51:46 -0700
Yang Shi <shy828301@gmail.com> wrote:

> On Tue, Mar 30, 2021 at 7:42 AM Gerald Schaefer
> <gerald.schaefer@linux.ibm.com> wrote:
> >
> > On Mon, 29 Mar 2021 11:33:06 -0700
> > Yang Shi <shy828301@gmail.com> wrote:
> >  
> > >
> > > When the THP NUMA fault support was added THP migration was not supported yet.
> > > So the ad hoc THP migration was implemented in NUMA fault handling.  Since v4.14
> > > THP migration has been supported so it doesn't make too much sense to still keep
> > > another THP migration implementation rather than using the generic migration
> > > code.  It is definitely a maintenance burden to keep two THP migration
> > > implementation for different code paths and it is more error prone.  Using the
> > > generic THP migration implementation allows us remove the duplicate code and
> > > some hacks needed by the old ad hoc implementation.
> > >
> > > A quick grep shows x86_64, PowerPC (book3s), ARM64 ans S390 support both THP
> > > and NUMA balancing.  The most of them support THP migration except for S390.
> > > Zi Yan tried to add THP migration support for S390 before but it was not
> > > accepted due to the design of S390 PMD.  For the discussion, please see:
> > > https://lkml.org/lkml/2018/4/27/953.
> > >
> > > I'm not expert on S390 so not sure if it is feasible to support THP migration
> > > for S390 or not.  If it is not feasible then the patchset may make THP NUMA
> > > balancing not be functional on S390.  Not sure if this is a show stopper although
> > > the patchset does simplify the code a lot.  Anyway it seems worth posting the
> > > series to the mailing list to get some feedback.  
> >
> > The reason why THP migration cannot work on s390 is because the migration
> > code will establish swap ptes in a pmd. The pmd layout is very different from
> > the pte layout on s390, so you cannot simply write a swap pte into a pmd.
> > There are no separate swp primitives for swap/migration pmds, IIRC. And even
> > if there were, we'd still need to find some space for a present bit in the
> > s390 pmd, and/or possibly move around some other bits.
> >
> > A lot of things can go wrong here, even if it could be possible in theory,
> > by introducing separate swp primitives in common code for pmd entries, along
> > with separate offset, type, shift, etc. I don't see that happening in the
> > near future.  
> 
> Thanks a lot for elaboration. IIUC, implementing migration PMD entry
> is *not* prevented from by hardware, it may be very tricky to
> implement it, right?

Well, it depends. The HW is preventing proper full-blown swap + migration
support for PMD, similar to what we have for PTE, because we simply don't
have enough OS-defined bits in the PMD. A 5-bit swap type for example,
similar to a PTE, plus the PFN would not be possible.

The HW would not prevent a similar mechanism in principle, i.e. we could
mark it as invalid to trigger a fault, and have some magic bits that tell
the fault handler or migration code what it is about.

For handling migration aspects only, w/o any swap device or other support, a
single type bit could already be enough, to indicate read/write migration,
plus a "present" bit similar to PTE. But even those 2 bits would be hard to
find, though I would not entirely rule that out. That would be the tricky
part.

Then of course, common code would need some changes, to reflect the
different swap/migration (type) capabilities of PTE and PMD entries.
Not sure if such an approach would be acceptable for common code.

But this is just some very abstract and optimistic view, I have not
really properly looked into the details. So it might be even more
tricky, or not possible at all.

> 
> >
> > Not sure if this is a show stopper, but I am not familiar enough with
> > NUMA and migration code to judge. E.g., I do not see any swp entry action
> > in your patches, but I assume this is implicitly triggered by the switch
> > to generic THP migration code.  
> 
> Yes, exactly. The migrate_pages() called by migrate_misplaced_page()
> takes care of everything.
> 
> >
> > Could there be a work-around by splitting THP pages instead of marking them
> > as migrate pmds (via pte swap entries), at least when THP migration is not
> > supported? I guess it could also be acceptable if THP pages were simply not
> > migrated for NUMA balancing on s390, but then we might need some extra config
> > option to make that behavior explicit.  
> 
> Yes, it could be. The old behavior of migration was to return -ENOMEM
> if THP migration is not supported then split THP. That behavior was
> not very friendly to some usecases, for example, memory policy and
> migration lieu of reclaim (the upcoming). But I don't mean we restore
> the old behavior. We could split THP if it returns -ENOSYS and the
> page is THP.

OK, as long as we don't get any broken PMD migration entries established
for s390, some extra THP splitting would be acceptable I guess.