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

["Kirill A. Shutemov" <kirill@shutemov.name>]

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.
> 
> memmap=<size>%<offset>-<oldtype>+<newtype>
>     [KNL,ACPI] Convert memory within the specified region
>     from <oldtype> to <newtype>. If "-<oldtype>" is left
> 
> Would any of those be usable for this usecase?

Oh. I missed them. Will take a closer look.

> 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > ---
> >  mm/page_alloc.c | 21 +++++++++++++++++++++
> >  1 file changed, 21 insertions(+)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index d62fcb2f28bd..509a93b7e5af 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -7213,6 +7213,8 @@ static DEFINE_STATIC_KEY_FALSE(zones_with_unaccepted_pages);
> >  
> >  static bool lazy_accept = true;
> >  
> > +static unsigned long fake_unaccepted_start = -1UL;
> > +
> >  static int __init accept_memory_parse(char *p)
> >  {
> >  	if (!strcmp(p, "lazy")) {
> > @@ -7227,11 +7229,30 @@ static int __init accept_memory_parse(char *p)
> >  }
> >  early_param("accept_memory", accept_memory_parse);
> >  
> > +static int __init fake_unaccepted_start_parse(char *p)
> > +{
> > +	if (!p)
> > +		return -EINVAL;
> > +
> > +	fake_unaccepted_start = memparse(p, &p);
> > +
> > +	if (*p != '\0') {
> > +		fake_unaccepted_start = -1UL;
> > +		return -EINVAL;
> > +	}
> > +
> > +	return 0;
> > +}
> > +early_param("fake_unaccepted_start", fake_unaccepted_start_parse);
> > +
> >  static bool page_contains_unaccepted(struct page *page, unsigned int order)
> >  {
> >  	phys_addr_t start = page_to_phys(page);
> >  	phys_addr_t end = start + (PAGE_SIZE << order);
> >  
> > +	if (start >= fake_unaccepted_start)
> > +		return true;
> > +
> >  	return range_contains_unaccepted_memory(start, end);
> >  }
> >  
> 

-- 
  Kiryl Shutsemau / Kirill A. Shutemov