Re: [PATCH v5 02/22] liveupdate: luo_core: integrate with KHO

[Pasha Tatashin <pasha.tatashin@soleen.com>]

> > FLB global objects act similarly to subsystem-wide data, except their
> > data has a clear creation and destruction time tied to preserved
> > files. When the first file of a particular type is added to LUO, this
> > global data is created; when the last file of that type is removed
> > (unpreserved or finished), this global data is destroyed, this is why
> > its life is bound to file lifecycle. Crucially, this global data is
> > accessible at any time while LUO owns the associated files spanning
> > the early boot update boundary.
>
> But there are no files at mm_core_init(). I'm really confused here.

This isn't about the files themselves, but about the subsystem global
data. The files are only used to describe the lifetime of this global
data.

I think mm_core_init() is too late, and the call would need to be
moved earlier to work correctly with subsystems. At the very least, we
will have to add some early FDT parsing to retrieve data during early
boot, but that would be part of the HugeTLB preservation work.

I can move liveupdate_init() inside kho_memory_init(), so we don't
need to modify mm_core_init(). Or, rename kho_memory_init to
kho_and_liveupdate_memory_init() and combine the two calls into a
single function in kexec_handover.c.

> > > So I think for now we can move liveupdate_init() later in boot and we will
> > > solve the problem of hugetlb reservations when we add support for hugetlb.
> >
> > HugeTLB reserves memory early in boot. If we already have preserved
> > HugeTLB pages via LUO/KHO, we must ensure they are counted against the
> > boot-time reservation. For example, if hugetlb_cma_reserve() needs to
> > reserve ten 1G pages, but LUO has already preserved seven, we only
> > need to reserve three new pages and the rest are going to be restored
> > with the files.
> >
> > Since this count is contained in the FLB global object, that data
> > needs to be available during the early reservation phase. (Pratyush is
> > working on HugeTLB preservation and can explain further).
>
> Not sure I really follow the design here, but in my understanding the gist
> here is that hugetlb reservations need to be aware of the preserved state.
> If that's the case, we definitely can move liveupdate_init() to an initcall
> and revisit this when hugetlb support for luo comes along.

This will break the in-kernel tests that ensure FLB data is accessible
and works correctly during early boot, as they use
early_initcall(liveupdate_test_early_init);.

We cannot rely on early_initcall() for liveupdate_init() because it
would compete with the test. We also can't move the test to a later
initcall, as that would break the verification of what FLB is
promising: early access to global data by subsystems that need it
(PCI, IOMMU Core, HugeTLB, etc.).

Thanks,
Pasha