Re: [PATCH 1/3] powerpc/32: Fix hugepage allocation on 8xx at hint address

[Aneesh Kumar K.V]

On 01/16/2018 10:01 PM, Christophe LEROY wrote:
> Le 16/01/2018 Ã 16:49, Aneesh Kumar K.V a ÃcritÂ:
> > Christophe Leroy <christophe.leroy@xxxxxx> writes:
> >
> > > When an app has some regular pages allocated (e.g. see below) and tries
> > > to mmap() a huge page at a hint address covered by the same PMD entry,
> > > the kernel accepts the hint allthough the 8xx cannot handle different
> > > page sizes in the same PMD entry.
> >
> >
> > So that is a bug in get_unmapped_area function that you are using and
> > you want to fix that by using the slice code. Can you describe here what
> > the allocation restrictions are w.r.t 8xx? Do they have segments and
> > base page size like hash64?
>
> I don't think it is a bug in get_unmapped_area() that is used by 
> default. It is that some HW do support mixing any page size in the same 
> page table (eg BOOK3E ?), but the 8xx doesn't.
> In the 8xx, the page size is defined in the PGD entry, then all pages 
> defined in a given page table pointed by a PGD entry have the same size.
>
> So it is similar to segments if you consider each PGD entry as a kind of 
> segment

so IIUC, hugepd format encodes the page size details and that require us 
to ensure that all the address range mapped at that hupge_pd entry is of 
same page size? Hence we want to avoid mmap handing over an address in 
that range when we already have a hugetlb mapping in that range?


-aneesh