linux-kernel - Re: [PATCH 1/2] mm,sparse: Fix deactivate

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Date:   Mon, 15 Jul 2019 21:33:29 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Michal Hocko <mhocko@...e.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm,sparse: Fix deactivate_section for early sections

On Mon, Jul 15, 2019 at 1:16 AM Oscar Salvador <osalvador@...e.de> wrote:
>
> deactivate_section checks whether a section is early or not
> in order to either call free_map_bootmem() or depopulate_section_memmap().
> Being the former for sections added at boot time, and the latter for
> sections hotplugged.
>
> The problem is that we zero section_mem_map, so the last early_section()
> will always report false and the section will not be removed.
>
> Fix this checking whether a section is early or not at function
> entry.
>
> Fixes: mmotm ("mm/sparsemem: Support sub-section hotplug")
> Signed-off-by: Oscar Salvador <osalvador@...e.de>
> ---
>  mm/sparse.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/mm/sparse.c b/mm/sparse.c
> index 3267c4001c6d..1e224149aab6 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -738,6 +738,7 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
>         DECLARE_BITMAP(map, SUBSECTIONS_PER_SECTION) = { 0 };
>         DECLARE_BITMAP(tmp, SUBSECTIONS_PER_SECTION) = { 0 };
>         struct mem_section *ms = __pfn_to_section(pfn);
> +       bool section_is_early = early_section(ms);
>         struct page *memmap = NULL;
>         unsigned long *subsection_map = ms->usage
>                 ? &ms->usage->subsection_map[0] : NULL;
> @@ -772,7 +773,7 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
>         if (bitmap_empty(subsection_map, SUBSECTIONS_PER_SECTION)) {
>                 unsigned long section_nr = pfn_to_section_nr(pfn);
>
> -               if (!early_section(ms)) {
> +               if (!section_is_early) {
>                         kfree(ms->usage);
>                         ms->usage = NULL;
>                 }
> @@ -780,7 +781,7 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
>                 ms->section_mem_map = sparse_encode_mem_map(NULL, section_nr);
>         }
>
> -       if (early_section(ms) && memmap)
> +       if (section_is_early && memmap)
>                 free_map_bootmem(memmap);
>         else
>                 depopulate_section_memmap(pfn, nr_pages, altmap);

Reviewed-by: Dan Williams <dan.j.wiliams@...el.com>

In fact, this bug was re-introduced between v9 and v10 as I had seen
this bug before, but did not write a reproducer for the unit test.