Messages in this thread | | | Date | Thu, 05 Jun 2014 11:24:30 +0200 | From | Vlastimil Babka <> | Subject | Re: [RFC PATCH 4/6] mm, compaction: skip buddy pages by their order in the migrate scanner |
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On 06/05/2014 02:02 AM, David Rientjes wrote: > On Wed, 4 Jun 2014, Vlastimil Babka wrote: > >> diff --git a/mm/compaction.c b/mm/compaction.c >> index ae7db5f..3dce5a7 100644 >> --- a/mm/compaction.c >> +++ b/mm/compaction.c >> @@ -640,11 +640,18 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc, >> } >> >> /* >> - * Skip if free. page_order cannot be used without zone->lock >> - * as nothing prevents parallel allocations or buddy merging. >> + * Skip if free. We read page order here without zone lock >> + * which is generally unsafe, but the race window is small and >> + * the worst thing that can happen is that we skip some >> + * potential isolation targets. > > Should we only be doing the low_pfn adjustment based on the order for > MIGRATE_ASYNC? It seems like sync compaction, including compaction that > is triggered from the command line, would prefer to scan over the > following pages.
I thought even sync compaction would benefit from the skipped iterations. I'd say the probability of this race is smaller than probability of somebody allocating what compaction just freed.
>> */ >> - if (PageBuddy(page)) >> + if (PageBuddy(page)) { >> + unsigned long freepage_order = page_order_unsafe(page); > > I don't assume that we want a smp_wmb() in set_page_order() for this > little race and to recheck PageBuddy() here after smp_rmb().
Hm right, barriers didn't came up last time a patch like this was posted. Rechecking PageBuddy() did came up but I thought the range checks on the order are enough for this case.
> I think this is fine for MIGRATE_ASYNC. > >> + >> + if (freepage_order > 0 && freepage_order < MAX_ORDER) >> + low_pfn += (1UL << freepage_order) - 1; >> continue; >> + } >> >> /* >> * Check may be lockless but that's ok as we recheck later. >> @@ -733,6 +740,13 @@ next_pageblock: >> low_pfn = ALIGN(low_pfn + 1, pageblock_nr_pages) - 1; >> } >> >> + /* >> + * The PageBuddy() check could have potentially brought us outside >> + * the range to be scanned. >> + */ >> + if (unlikely(low_pfn > end_pfn)) >> + end_pfn = low_pfn; >> + >> acct_isolated(zone, locked, cc); >> >> if (locked) >> diff --git a/mm/internal.h b/mm/internal.h >> index 1a8a0d4..6aa1f74 100644 >> --- a/mm/internal.h >> +++ b/mm/internal.h >> @@ -164,7 +164,8 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc, >> * general, page_zone(page)->lock must be held by the caller to prevent the >> * page from being allocated in parallel and returning garbage as the order. >> * If a caller does not hold page_zone(page)->lock, it must guarantee that the >> - * page cannot be allocated or merged in parallel. >> + * page cannot be allocated or merged in parallel. Alternatively, it must >> + * handle invalid values gracefully, and use page_order_unsafe() below. >> */ >> static inline unsigned long page_order(struct page *page) >> { >> @@ -172,6 +173,23 @@ static inline unsigned long page_order(struct page *page) >> return page_private(page); >> } >> >> +/* >> + * Like page_order(), but for callers who cannot afford to hold the zone lock, >> + * and handle invalid values gracefully. ACCESS_ONCE is used so that if the >> + * caller assigns the result into a local variable and e.g. tests it for valid >> + * range before using, the compiler cannot decide to remove the variable and >> + * inline the function multiple times, potentially observing different values >> + * in the tests and the actual use of the result. >> + */ >> +static inline unsigned long page_order_unsafe(struct page *page) >> +{ >> + /* >> + * PageBuddy() should be checked by the caller to minimize race window, >> + * and invalid values must be handled gracefully. >> + */ >> + return ACCESS_ONCE(page_private(page)); >> +} >> + >> /* mm/util.c */ >> void __vma_link_list(struct mm_struct *mm, struct vm_area_struct *vma, >> struct vm_area_struct *prev, struct rb_node *rb_parent); > > I don't like this change at all, I don't think we should have header > functions that imply the context in which the function will be called. I > think it would make much more sense to just do > ACCESS_ONCE(page_order(page)) in the migration scanner with a comment.
But that won't compile. It would have to be converted to a #define, unless there's some trick I don't know. Sure I would hope this could be done cleaner somehow.
> These are __attribute__((pure)) semantics for page_order().
Not sure I understand what you mean here. Would adding that attribute change anything?
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