Re: [PATCH v2 00/19] mm: Support huge pfnmaps

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, Aug 28, 2024 at 09:10:34AM -0700, Jiaqi Yan wrote:
> On Wed, Aug 28, 2024 at 7:24 AM Jason Gunthorpe <jgg@nvidia.com> wrote:
> >
> > On Tue, Aug 27, 2024 at 05:42:21PM -0700, Jiaqi Yan wrote:
> >
> > > Instead of removing the whole pud, can driver or memory_failure do
> > > something similar to non-struct-page-version of split_huge_page? So
> > > driver doesn't need to re-fault good pages back?
> >
> > It would be far nicer if we didn't have to poke a hole in a 1G mapping
> > just for memory failure reporting.
> 
> If I follow this, which of the following sounds better? 1. remove pud
> and rely on the driver to re-fault PFNs that it knows are not poisoned
> (what Peter suggested), or 2. keep the pud and allow access to both
> good and bad PFNs.

In practice I think people will need 2, as breaking up a 1G mapping
just because a few bits are bad will destroy the VM performance.

For this the expectation would be for the VM to co-operate and not
keep causing memory failures, or perhaps for the platform to spare in
good memory somehow.

> Or provide some knob (configured by ?) so that kernel + driver can
> switch between the two?

This is also sounding reasonable, especially if we need some
alternative protocol to signal userspace about the failed memory
besides fault and SIGBUS.

Jason