Hi Vlastimil,
On Fri, Oct 07, 2016 at 02:30:04PM +0200, Vlastimil Babka wrote:
> On 10/07/2016 07:45 AM, Minchan Kim wrote:
> >In page freeing path, migratetype is racy so that a highorderatomic
> >page could free into non-highorderatomic free list.
>
> Yes. If page from a pageblock went to a pcplist before that pageblock was
> reserved as highatomic, free_pcppages_bulk() will misplace it.
As well, high-order freeing path has a problem, too.
CPU 1 CPU 2
__free_pages_ok
/* got highatomic mt */
unreserve_highatomic_pageblock mt = get_pfnblock_migratetype
spin_lock_irqsave(&zone->lock);
move_freepages_block
/* change from highatomic to something
set_pageblock_migratetype(page)
spin_unlock_irqrestore(&zone->lock)
spin_lock(&zone->lock);
/* highatomic mt is stale */
__free_one_page(page, mt);
Acutually, I tried to solve this problem with fixing the free path
but it needs to add a branch to verify highorderatomic mt in
both order-0 and high-order page freeing path. On highorder page freeing
path wouldn't be a problem but I don't want to add the branch in pcp
freeing path which is hot.
>
> >If that page
> >is allocated, VM can change the pageblock from higorderatomic to
> >something.
>
> More specifically, steal_suitable_fallback(). Yes.
As well, __isolate_free_page, too.
>
> >In that case, we should adjust nr_reserved_highatomic.
> >Otherwise, VM cannot reserve highorderatomic pageblocks any more
> >although it doesn't reach 1% limit. It means highorder atomic
> >allocation failure would be higher.
> >
> >So, this patch decreases the account as well as migratetype
> >if it was MIGRATE_HIGHATOMIC.
> >
> >Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
>
> Hm wouldn't it be simpler just to prevent the pageblock's migratetype to be
> changed if it's highatomic? Possibly also not do move_freepages_block() in
It could be. Actually, I did it with modifying can_steal_fallback which returns
false it found the pageblock is highorderatomic but changed to this way again
because I don't have any justification to prevent changing pageblock.
If you give concrete justification so others isn't against on it, I am happy to
do what you suggested.
> that case. Most accurate would be to put such misplaced page on the proper
> freelist and retry the fallback, but that might be overkill.
>
> >---
> > mm/page_alloc.c | 44 ++++++++++++++++++++++++++++++++++++++------
> > 1 file changed, 38 insertions(+), 6 deletions(-)
> >
> >diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> >index 55ad0229ebf3..e7cbb3cc22fa 100644
> >--- a/mm/page_alloc.c
> >+++ b/mm/page_alloc.c
> >@@ -282,6 +282,9 @@ EXPORT_SYMBOL(nr_node_ids);
> > EXPORT_SYMBOL(nr_online_nodes);
> > #endif
> >
> >+static void dec_highatomic_pageblock(struct zone *zone, struct page *page,
> >+ int migratetype);
> >+
> > int page_group_by_mobility_disabled __read_mostly;
> >
> > #ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
> >@@ -1935,7 +1938,14 @@ static void change_pageblock_range(struct page *pageblock_page,
> > int nr_pageblocks = 1 << (start_order - pageblock_order);
> >
> > while (nr_pageblocks--) {
> >- set_pageblock_migratetype(pageblock_page, migratetype);
> >+ if (get_pageblock_migratetype(pageblock_page) !=
> >+ MIGRATE_HIGHATOMIC)
> >+ set_pageblock_migratetype(pageblock_page,
> >+ migratetype);
> >+ else
> >+ dec_highatomic_pageblock(page_zone(pageblock_page),
> >+ pageblock_page,
> >+ migratetype);
> > pageblock_page += pageblock_nr_pages;
> > }
> > }
> >@@ -1996,8 +2006,14 @@ static void steal_suitable_fallback(struct zone *zone, struct page *page,
> >
> > /* Claim the whole block if over half of it is free */
> > if (pages >= (1 << (pageblock_order-1)) ||
> >- page_group_by_mobility_disabled)
> >- set_pageblock_migratetype(page, start_type);
> >+ page_group_by_mobility_disabled) {
> >+ int mt = get_pageblock_migratetype(page);
> >+
> >+ if (mt != MIGRATE_HIGHATOMIC)
> >+ set_pageblock_migratetype(page, start_type);
> >+ else
> >+ dec_highatomic_pageblock(zone, page, start_type);
> >+ }
> > }
> >
> > /*
> >@@ -2037,6 +2053,17 @@ int find_suitable_fallback(struct free_area *area, unsigned int order,
> > return -1;
> > }
> >
> >+static void dec_highatomic_pageblock(struct zone *zone, struct page *page,
> >+ int migratetype)
> >+{
> >+ if (zone->nr_reserved_highatomic <= pageblock_nr_pages)
> >+ return;
> >+
> >+ zone->nr_reserved_highatomic -= min(pageblock_nr_pages,
> >+ zone->nr_reserved_highatomic);
> >+ set_pageblock_migratetype(page, migratetype);
> >+}
> >+
> > /*
> > * Reserve a pageblock for exclusive use of high-order atomic allocations if
> > * there are no empty page blocks that contain a page with a suitable order
> >@@ -2555,9 +2582,14 @@ int __isolate_free_page(struct page *page, unsigned int order)
> > struct page *endpage = page + (1 << order) - 1;
> > for (; page < endpage; page += pageblock_nr_pages) {
> > int mt = get_pageblock_migratetype(page);
> >- if (!is_migrate_isolate(mt) && !is_migrate_cma(mt))
> >- set_pageblock_migratetype(page,
> >- MIGRATE_MOVABLE);
> >+ if (!is_migrate_isolate(mt) && !is_migrate_cma(mt)) {
> >+ if (mt != MIGRATE_HIGHATOMIC)
> >+ set_pageblock_migratetype(page,
> >+ MIGRATE_MOVABLE);
> >+ else
> >+ dec_highatomic_pageblock(zone, page,
> >+ MIGRATE_MOVABLE);
> >+ }
> > }
> > }
> >
> >
>
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