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    SubjectRe: [PATCH -mm v2] mm/page_isolation: fix potential warning from user
    On Mon 20-01-20 08:19:09, Qian Cai wrote:
    > It makes sense to call the WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE)
    > from start_isolate_page_range(), but should avoid triggering it from
    > userspace, i.e, from is_mem_section_removable() because it could be a
    > DoS if warn_on_panic is set.

    Let's just make it clear that this mostly a pre-cautious because a real
    DoS should be pretty much impossible. But let's see whether somebody
    want to make a CVE out of it ;)

    > While at it, simplify the code a bit by removing an unnecessary jump
    > label and a local variable, so set_migratetype_isolate() could really
    > return a bool.
    >
    > Suggested-by: Michal Hocko <mhocko@kernel.org>
    > Signed-off-by: Qian Cai <cai@lca.pw>

    Acked-by: Michal Hocko <mhocko@suse.com>

    Thanks!

    > ---
    >
    > v2: Improve the commit log.
    > Warn for all start_isolate_page_range() users not just offlining.
    >
    > mm/page_alloc.c | 11 ++++-------
    > mm/page_isolation.c | 30 +++++++++++++++++-------------
    > 2 files changed, 21 insertions(+), 20 deletions(-)
    >
    > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
    > index 621716a25639..3c4eb750a199 100644
    > --- a/mm/page_alloc.c
    > +++ b/mm/page_alloc.c
    > @@ -8231,7 +8231,7 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
    > if (is_migrate_cma(migratetype))
    > return NULL;
    >
    > - goto unmovable;
    > + return page;
    > }
    >
    > for (; iter < pageblock_nr_pages; iter++) {
    > @@ -8241,7 +8241,7 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
    > page = pfn_to_page(pfn + iter);
    >
    > if (PageReserved(page))
    > - goto unmovable;
    > + return page;
    >
    > /*
    > * If the zone is movable and we have ruled out all reserved
    > @@ -8261,7 +8261,7 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
    > unsigned int skip_pages;
    >
    > if (!hugepage_migration_supported(page_hstate(head)))
    > - goto unmovable;
    > + return page;
    >
    > skip_pages = compound_nr(head) - (page - head);
    > iter += skip_pages - 1;
    > @@ -8303,12 +8303,9 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
    > * is set to both of a memory hole page and a _used_ kernel
    > * page at boot.
    > */
    > - goto unmovable;
    > + return page;
    > }
    > return NULL;
    > -unmovable:
    > - WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE);
    > - return pfn_to_page(pfn + iter);
    > }
    >
    > #ifdef CONFIG_CONTIG_ALLOC
    > diff --git a/mm/page_isolation.c b/mm/page_isolation.c
    > index e70586523ca3..31f5516f5d54 100644
    > --- a/mm/page_isolation.c
    > +++ b/mm/page_isolation.c
    > @@ -15,12 +15,12 @@
    > #define CREATE_TRACE_POINTS
    > #include <trace/events/page_isolation.h>
    >
    > -static int set_migratetype_isolate(struct page *page, int migratetype, int isol_flags)
    > +static bool set_migratetype_isolate(struct page *page, int migratetype,
    > + int isol_flags)
    > {
    > - struct page *unmovable = NULL;
    > + struct page *unmovable = ERR_PTR(-EBUSY);
    > struct zone *zone;
    > unsigned long flags;
    > - int ret = -EBUSY;
    >
    > zone = page_zone(page);
    >
    > @@ -49,21 +49,25 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
    > NULL);
    >
    > __mod_zone_freepage_state(zone, -nr_pages, mt);
    > - ret = 0;
    > }
    >
    > out:
    > spin_unlock_irqrestore(&zone->lock, flags);
    > - if (!ret)
    > +
    > + if (!unmovable) {
    > drain_all_pages(zone);
    > - else if ((isol_flags & REPORT_FAILURE) && unmovable)
    > - /*
    > - * printk() with zone->lock held will guarantee to trigger a
    > - * lockdep splat, so defer it here.
    > - */
    > - dump_page(unmovable, "unmovable page");
    > -
    > - return ret;
    > + } else {
    > + WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE);
    > +
    > + if ((isol_flags & REPORT_FAILURE) && !IS_ERR(unmovable))
    > + /*
    > + * printk() with zone->lock held will likely trigger a
    > + * lockdep splat, so defer it here.
    > + */
    > + dump_page(unmovable, "unmovable page");
    > + }
    > +
    > + return !!unmovable;
    > }
    >
    > static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
    > --
    > 2.21.0 (Apple Git-122.2)

    --
    Michal Hocko
    SUSE Labs

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