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SubjectRe: [patch] mm, page_alloc: move_freepages should not examine struct page of reserved memory
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On 8/13/19 5:37 AM, David Rientjes wrote:
> After commit 907ec5fca3dc ("mm: zero remaining unavailable struct pages"),
> struct page of reserved memory is zeroed. This causes page->flags to be 0
> and fixes issues related to reading /proc/kpageflags, for example, of
> reserved memory.
>
> The VM_BUG_ON() in move_freepages_block(), however, assumes that
> page_zone() is meaningful even for reserved memory. That assumption is no
> longer true after the aforementioned commit.

How comes that move_freepages_block() gets called on reserved memory in
the first place?

> There's no reason why move_freepages_block() should be testing the
> legitimacy of page_zone() for reserved memory; its scope is limited only
> to pages on the zone's freelist.
>
> Note that pfn_valid() can be true for reserved memory: there is a backing
> struct page. The check for page_to_nid(page) is also buggy but reserved
> memory normally only appears on node 0 so the zeroing doesn't affect this.
>
> Move the debug checks to after verifying PageBuddy is true. This isolates
> the scope of the checks to only be for buddy pages which are on the zone's
> freelist which move_freepages_block() is operating on. In this case, an
> incorrect node or zone is a bug worthy of being warned about (and the
> examination of struct page is acceptable bcause this memory is not
> reserved).
>
> Signed-off-by: David Rientjes <rientjes@google.com>
> ---
> mm/page_alloc.c | 19 ++++---------------
> 1 file changed, 4 insertions(+), 15 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2238,27 +2238,12 @@ static int move_freepages(struct zone *zone,
> unsigned int order;
> int pages_moved = 0;
>
> -#ifndef CONFIG_HOLES_IN_ZONE
> - /*
> - * page_zone is not safe to call in this context when
> - * CONFIG_HOLES_IN_ZONE is set. This bug check is probably redundant
> - * anyway as we check zone boundaries in move_freepages_block().
> - * Remove at a later date when no bug reports exist related to
> - * grouping pages by mobility
> - */
> - VM_BUG_ON(pfn_valid(page_to_pfn(start_page)) &&
> - pfn_valid(page_to_pfn(end_page)) &&
> - page_zone(start_page) != page_zone(end_page));
> -#endif
> for (page = start_page; page <= end_page;) {
> if (!pfn_valid_within(page_to_pfn(page))) {
> page++;
> continue;
> }
>
> - /* Make sure we are not inadvertently changing nodes */
> - VM_BUG_ON_PAGE(page_to_nid(page) != zone_to_nid(zone), page);
> -
> if (!PageBuddy(page)) {
> /*
> * We assume that pages that could be isolated for
> @@ -2273,6 +2258,10 @@ static int move_freepages(struct zone *zone,
> continue;
> }
>
> + /* Make sure we are not inadvertently changing nodes */
> + VM_BUG_ON_PAGE(page_to_nid(page) != zone_to_nid(zone), page);
> + VM_BUG_ON_PAGE(page_zone(page) != zone, page);

The later check implies the former check, so if it's to stay, the first
one could be removed and comment adjusted s/nodes/zones/

> +
> order = page_order(page);
> move_to_free_area(page, &zone->free_area[order], migratetype);
> page += 1 << order;
>

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