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

[Kirill A. Shutemov]

On Mon, Apr 03, 2023 at 06:50:11PM +0300, Kirill A. Shutemov wrote:
> 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.

efi_fake_mem applied too late. Bitmap that represents unaccepted memory
for kernel created at kernel decompression stage, but efi_fake_mem=
handled in main kernel.

I don't think pushing efi_fake_mem to decompressor makes sesne. I would
rather drom the feature altogether.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov