linux-kernel - Re: [PATCH] hugetlb: simplify hugetlb handling in follow_page

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Message-ID: <dca632c6-80f4-b80e-a81f-e2f91a3b2e48@csgroup.eu>
Date:   Sat, 3 Sep 2022 06:59:36 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     David Hildenbrand <david@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "inuxppc-dev@...ts.ozlabs.org" <inuxppc-dev@...ts.ozlabs.org>,
        "linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        Michael Ellerman <mpe@...erman.id.au>,
        Muchun Song <songmuchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] hugetlb: simplify hugetlb handling in follow_page_mask



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