Re: [kvm-unit-tests PATCH 1/3] lib/vmalloc: Treat virt_to_pte_phys() as returning a physical address

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi,

On Thu, Oct 06, 2022 at 01:35:52PM +0200, Claudio Imbrenda wrote:
> On Thu,  6 Oct 2022 12:12:39 +0100
> Alexandru Elisei <alexandru.elisei@arm.com> wrote:
> 
> > All architectures that implements virt_to_pte_phys() (s390x, x86, arm and
> > arm64) return a physical address from the function. Teach vmalloc to treat
> > it as such, instead of confusing the return value with a page table entry.
> 
> I'm not sure I understand what you mean

I thought that vmalloc uses PAGE_MASK because it expects virt_to_pte_phys()
to return a pteval (because of the "pte' part in the virt_to_pte_phys()
function name), which might have the [PAGE_SHIFT-1:0] bits used to store
page metadata by an architecture (like permissions), but like you've
explained below it uses PAGE_MASK to align the page address (which is
identically mapped) before passing it to the page allocator to be freed.

> 
> > Changing things the other way around (having the function return a page
> > table entry instead) is not feasible, because it is possible for an
> > architecture to use the upper bits of the table entry to store metadata
> > about the page.
> > 
> > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > Cc: Thomas Huth <thuth@redhat.com>
> > Cc: Andrew Jones <andrew.jones@linux.dev>
> > Cc: Laurent Vivier <lvivier@redhat.com>
> > Cc: Janosch Frank <frankja@linux.ibm.com>
> > Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
> > Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
> > ---
> >  lib/vmalloc.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/lib/vmalloc.c b/lib/vmalloc.c
> > index 572682576cc3..0696b5da8190 100644
> > --- a/lib/vmalloc.c
> > +++ b/lib/vmalloc.c
> > @@ -169,7 +169,7 @@ static void vm_free(void *mem)
> >  	/* the pointer is not page-aligned, it was a single-page allocation */
> >  	if (!IS_ALIGNED((uintptr_t)mem, PAGE_SIZE)) {
> >  		assert(GET_MAGIC(mem) == VM_MAGIC);
> > -		page = virt_to_pte_phys(page_root, mem) & PAGE_MASK;
> > +		page = virt_to_pte_phys(page_root, mem);
> 
> this will break things for small allocations, though. if the pointer is
> not aligned, then the result of virt_to_pte_phys will also not be
> aligned....

I agree, I missed that part. Would be nice if it were written using
PAGE_ALIGN to avoid mistakes like mine in the future, but that's
unimportant.

> 
> >  		assert(page);
> >  		free_page(phys_to_virt(page));
> 
> ...and phys_to_virt will also return an unaligned address, and
> free_page will complain about it.
> 
> >  		return;
> > @@ -183,7 +183,7 @@ static void vm_free(void *mem)
> >  	/* free all the pages including the metadata page */
> >  	ptr = (uintptr_t)m & PAGE_MASK;
> 
> ptr gets page aligned here
> 
> >  	for (i = 0 ; i < m->npages + 1; i++, ptr += PAGE_SIZE) {
> > -		page = virt_to_pte_phys(page_root, (void *)ptr) & PAGE_MASK;
> > +		page = virt_to_pte_phys(page_root, (void *)ptr);
> 
> so virt_to_pte_phys will also return an aligned address;
> I agree that & PAGE_MASK is redundant here

You are correct, if we've ended up here it means that the pointer is
already page aligned, and it will be incremented by PAGE_SIZE each
iteration, hence the virt_to_pte_phys() will also be paged aligned.

I don't see much point in writing a patch just to remove the unnecessary
alignment here, so I'll drop this patch entirely.

Thank you for the prompt explanation!

Alex

> 
> >  		assert(page);
> >  		free_page(phys_to_virt(page));
> >  	}
> 
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