Messages in this thread | | | Date | Fri, 19 Oct 2012 16:41:17 +0800 | From | Wen Congyang <> | Subject | Re: [PATCH v3 8/9] memory-hotplug: fix NR_FREE_PAGES mismatch |
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At 10/19/2012 03:41 PM, KOSAKI Motohiro Wrote: > On Fri, Oct 19, 2012 at 2:46 AM, <wency@cn.fujitsu.com> wrote: >> From: Wen Congyang <wency@cn.fujitsu.com> >> >> NR_FREE_PAGES will be wrong after offlining pages. We add/dec NR_FREE_PAGES >> like this now: >> 1. mova all pages in buddy system to MIGRATE_ISOLATE, and dec NR_FREE_PAGES > > move?
Yes. __offline_pages() start_isolate_page_range() set_migratetype_isolate() move_freepages_block() // move all pages in buddy system to MIGRATE_ISOLATE __mod_zone_freepage_state() // dec NR_FREE_PAGES
> >> 2. don't add NR_FREE_PAGES when it is freed and the migratetype is MIGRATE_ISOLATE >> 3. dec NR_FREE_PAGES when offlining isolated pages. >> 4. add NR_FREE_PAGES when undoing isolate pages. >> >> When we come to step 3, all pages are in MIGRATE_ISOLATE list, and NR_FREE_PAGES >> are right. When we come to step4, all pages are not in buddy system, so we don't >> change NR_FREE_PAGES in this step, but we change NR_FREE_PAGES in step3. So >> NR_FREE_PAGES is wrong after offlining pages. So there is no need to change >> NR_FREE_PAGES in step3. > > Sorry, I don't understand this two paragraph. Can you please elaborate more?
OK.
If we don't online/offline memory, we add NR_FREE_PAGES when we free a page, and dec it when allocate a page. If we put the page into pcp, we don't add NR_FREE_PAGES. We will add it when the page is moved to buddy system from pcp.
When we offline a memory section, we should dec NR_FREE_PAGES(we will add it when onlining memory section). The pages may be freed or inuse: 1. If the page is freed, and in buddy system. We move it to MIGRATE_ISOLATE, and dec NR_FREE_PAGES 2. If the page is inuse, we will migrate them to other memory section and free them. We don't dec NR_FREE_PAGES when it is freed because we have decreased it when it is allocated. We just put them in MIGRATE_ISOLATE. 3. If the page is in pcp, we call drain_all_pages() to put them to MIGRATE_ISOLATE. We have decreased NR_FREE_PAGES when we allocate a page and put it in pcp. So we just put them in MIGRATE_ISOLATE.
Step1 deals with case1, and step2 deals with case2,3
So NR_FREE_PAGES is right after all pages are put into MIGRATE_ISOLATE list. Now offline_isolated_pages() will be called after all pages are put in MIGRATE_ISOLATE list. So we should not change NR_FREE_PAGES now, but we dec NR_FREE_PAGES in offline_isolated_pages().
> > and one more trivial question: why do we need to call > undo_isolate_page_range() from > __offline_pages()?
We need to restore the page's migrate type to MIGRATE_MOVABLE.
> > >> >> This patch also fixs a problem in step2: if the migratetype is MIGRATE_ISOLATE, >> we should not add NR_FRR_PAGES when we remove pages from pcppages. > > Why drain_all_pages doesn't work? >
drain_all_pages() deals with case3, and it should not touch NR_FREE_PAGES if it put a page to MIGRATE_ISOLATE list. But we touch NR_FREE_PAGES without checking where the page is put.
Thanks Wen Congyang
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