Re: [PATCHv9 03/14] mm/page_alloc: Fake unaccepted memory

[Kirill A. Shutemov]

On Mon, Apr 03, 2023 at 05:39:15PM +0300, Kirill A. Shutemov wrote:
> On Mon, Apr 03, 2023 at 03:39:53PM +0200, Vlastimil Babka wrote:
> > On 3/30/23 13:49, Kirill A. Shutemov wrote:
> > > For testing purposes, it is useful to fake unaccepted memory in the
> > > system. It helps to understand unaccepted memory overhead to the page
> > > allocator.
> > 
> > Ack on being useful for testing, but the question is if we want to also
> > merge this patch into mainline as it is?
> 
> I don't insist on getting it upstream, but it can be handy to debug
> related bugs in the future.
> 
> > > The patch allows to treat memory above the specified physical memory
> > > address as unaccepted.
> > > 
> > > The change only fakes unaccepted memory for page allocator. Memblock is
> > > not affected.
> > > 
> > > It also assumes that arch-provided accept_memory() on already accepted
> > > memory is a nop.
> > 
> > I guess to be in mainline it would have to at least gracefully handle the
> > case of accept_memory actually not being a nop, and running on a system with
> > actual unaccepted memory (probably by ignoring the parameter in such case).
> > Then also the parameter would have to be documented.
> 
> As it is written now, accept_memory() is nop on system with real
> unaccepted memory if the memory is already accepted. Arch-specific code
> will check against own records to see if the memory needs accepting. If
> not, just return.
> 
> And the option will not interfere with unaccepted memory declared by EFI
> memmap. It can extend it, but that's it.
> 
> Looks safe to me.
> 
> > Speaking of documented parameters, I found at least two that seem a more
> > generic variant of this (but I didn't look closely if that makes sense):
> > 
> > efi_fake_mem=   nn[KMG]@ss[KMG]:aa[,nn[KMG]@ss[KMG]:aa,..] [EFI; X86]
> >     Add arbitrary attribute to specific memory range by
> >     updating original EFI memory map.

As of now, efi_fake_mem= can adjust attributes of memory. Unaccepted is
type of memory, not an attribute. I guess we can allow it override type
too. But syntax is going to be fun.

> > memmap=<size>%<offset>-<oldtype>+<newtype>
> >     [KNL,ACPI] Convert memory within the specified region
> >     from <oldtype> to <newtype>. If "-<oldtype>" is left

It overrides E820 map, but unaccepted memory is not represented there.
Unaccepted memory is just RAM in E820.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov