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")
Christophe
