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    SubjectRe: [PATCH v2 2/2] mm/memory-failure.c: Don't access uninitialized memmaps in memory_failure()
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    On Thu, Oct 10, 2019 at 09:17:42AM +0200, David Hildenbrand wrote:
    > On 10.10.19 02:26, Naoya Horiguchi wrote:
    > > On Wed, Oct 09, 2019 at 04:24:35PM +0200, David Hildenbrand wrote:
    > >> We should check for pfn_to_online_page() to not access uninitialized
    > >> memmaps. Reshuffle the code so we don't have to duplicate the error
    > >> message.
    > >>
    > >> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
    > >> Cc: Andrew Morton <akpm@linux-foundation.org>
    > >> Cc: Michal Hocko <mhocko@kernel.org>
    > >> Signed-off-by: David Hildenbrand <david@redhat.com>
    > >> ---
    > >> mm/memory-failure.c | 14 ++++++++------
    > >> 1 file changed, 8 insertions(+), 6 deletions(-)
    > >>
    > >> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
    > >> index 7ef849da8278..e866e6e5660b 100644
    > >> --- a/mm/memory-failure.c
    > >> +++ b/mm/memory-failure.c
    > >> @@ -1253,17 +1253,19 @@ int memory_failure(unsigned long pfn, int flags)
    > >> if (!sysctl_memory_failure_recovery)
    > >> panic("Memory failure on page %lx", pfn);
    > >>
    > >> - if (!pfn_valid(pfn)) {
    > >> + p = pfn_to_online_page(pfn);
    > >> + if (!p) {
    > >> + if (pfn_valid(pfn)) {
    > >> + pgmap = get_dev_pagemap(pfn, NULL);
    > >> + if (pgmap)
    > >> + return memory_failure_dev_pagemap(pfn, flags,
    > >> + pgmap);
    > >> + }
    > >> pr_err("Memory failure: %#lx: memory outside kernel control\n",
    > >> pfn);
    > >> return -ENXIO;
    > >> }
    > >>
    > >> - pgmap = get_dev_pagemap(pfn, NULL);
    > >> - if (pgmap)
    > >> - return memory_failure_dev_pagemap(pfn, flags, pgmap);
    > >> -
    > >> - p = pfn_to_page(pfn);
    > >
    > > This change seems to assume that memory_failure_dev_pagemap() is never
    > > called for online pages. Is it an intended behavior?
    > > Or the concept "online pages" is not applicable to zone device pages?
    >
    > Yes, that's the real culprit. ZONE_DEVICE/devmem pages are never online
    > (SECTION_IS_ONLINE). The terminology "online" only applies to pages that
    > were given to the buddy. And as we support sup-section hotadd for
    > devmem, we cannot easily make use of the section flag it. I already
    > proposed somewhere to convert SECTION_IS_ONLINE to a subsection bitmap
    > and call it something like pfn_active().
    >
    > pfn_online() would then be "pfn_active() && zone != ZONE_DEVICE". And we
    > could use pfn_active() everywhere to test for initialized memmaps (well,
    > besides some special cases like device reserved memory that does not
    > span full sub-sections). Until now, nobody volunteered and I have other
    > things to do.

    Thank you for explanation. I'm not sure how hard now but we try to do this.
    You fix the problem from online/offline viewpoint now, and leave remaining
    part untouched. I agree with that approach, and the suggested change looks
    good to me.

    Acked-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>

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