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Date: Mon, 09 Jun 2014 13:35:45 +0200 From: Vlastimil Babka <vbabka@...e.cz> To: David Rientjes <rientjes@...gle.com> CC: linux-mm@...ck.org, linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>, Greg Thelen <gthelen@...gle.com>, Minchan Kim <minchan@...nel.org>, Mel Gorman <mgorman@...e.de>, Joonsoo Kim <iamjoonsoo.kim@....com>, Michal Nazarewicz <mina86@...a86.com>, Naoya Horiguchi <n-horiguchi@...jp.nec.com>, Christoph Lameter <cl@...ux.com>, Rik van Riel <riel@...hat.com> Subject: Re: [RFC PATCH 4/6] mm, compaction: skip buddy pages by their order in the migrate scanner On 06/09/2014 11:09 AM, David Rientjes wrote: > On Fri, 6 Jun 2014, Vlastimil Babka wrote: > >>>>>> 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. >>>> >>> >>> Sorry, I meant ACCESS_ONCE(page_private(page)) in the migration scanner >> >> Hm but that's breaking the abstraction of page_order(). I don't know if it's >> worse to create a new variant of page_order() or to do this. BTW, seems like >> next_active_pageblock() in memory-hotplug.c should use this variant too. >> > > The compiler seems free to disregard the access of a volatile object above > because the return value of the inline function is unsigned long. What's > the difference between unsigned long order = page_order_unsafe(page) and > unsigned long order = (unsigned long)ACCESS_ONCE(page_private(page)) and I think there's none functionally, but one is abstraction layer violation and the other imply the context of usage as you say (but is that so uncommon?). > the compiler being able to reaccess page_private() because the result is > no longer volatile qualified? You think it will reaccess? That would defeat all current ACCESS_ONCE usages, no? What I'm trying to prevent is that this code: unsigned long freepage_order = page_order(page); if (freepage_order > 0 && freepage_order < MAX_ORDER) low_pfn += (1UL << freepage_order) - 1; could be effectively changed (AFAIK legal for the compiler to do) to: if (page_order(page) > 0 && page_order(page) < MAX_ORDER) low_pfn += (1UL << page_order(page)) - 1; And thus check a different value than it's in the end used to bump low_pfn. I believe that even though freepage_order itself is not volatile, the fact it was assigned through a volatile cast means the compiler won't be able to do this anymore. > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@...ck.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@...ck.org"> email@...ck.org </a> > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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