Re: [PATCH] hugetlb: simplify hugetlb handling in follow_page_mask

[David Hildenbrand]

On 03.09.22 09:07, Christophe Leroy wrote:
> +Resending with valid powerpc list address
>
> Le 02/09/2022 à 20:52, David Hildenbrand a écrit :
> > > > > Adding Christophe on Cc:
> > > > >
> > > > > Christophe do you know if is_hugepd is true for all hugetlb entries, not
> > > > > just hugepd?
>
> is_hugepd() is true if and only if the directory entry points to a huge
> page directory and not to the normal lower level directory.
>
> As far as I understand if the directory entry is not pointing to any
> lower directory but is a huge page entry, pXd_leaf() is true.
>
>
> > > > > On systems without hugepd entries, I guess ptdump skips all hugetlb entries.
> > > > > Sigh!
>
> As far as I can see, ptdump_pXd_entry() handles the pXd_leaf() case.
>
> > > > IIUC, the idea of ptdump_walk_pgd() is to dump page tables even outside
> > > > VMAs (for debugging purposes?).
> > > >
> > > > I cannot convince myself that that's a good idea when only holding the
> > > > mmap lock in read mode, because we can just see page tables getting
> > > > freed concurrently e.g., during concurrent munmap() ... while holding
> > > > the mmap lock in read we may only walk inside VMA boundaries.
> > > >
> > > > That then raises the questions if we're only calling this on special MMs
> > > > (e.g., init_mm) whereby we cannot really see concurrent munmap() and
> > > > where we shouldn't have hugetlb mappings or hugepd entries.
>
> At least on powerpc, PTDUMP handles only init_mm.
>
> Hugepage are used at least on powerpc 8xx for linear memory mapping, see
>
> commit 34536d780683 ("powerpc/8xx: Add a function to early map kernel
> via huge pages")
> commit cf209951fa7f ("powerpc/8xx: Map linear memory with huge pages")
>
> hugepds may also be used in the future to use huge pages for vmap and
> vmalloc, see commit a6a8f7c4aa7e ("powerpc/8xx: add support for huge
> pages on VMAP and VMALLOC")
>
> As far as I know, ppc64 also use huge pages for VMAP and VMALLOC, see
>
> commit d909f9109c30 ("powerpc/64s/radix: Enable HAVE_ARCH_HUGE_VMAP")
> commit 8abddd968a30 ("powerpc/64s/radix: Enable huge vmalloc mappings")

There is a difference between an ordinary huge mapping (e.g., as used 
for THP) and a a hugetlb mapping.

Our current understanding is that hugepd only applies to hugetlb. 
Wouldn't vmap/vmalloc user ordinary huge pmd entries instead of hugepd?

--
Thanks,

David / dhildenb