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Tue, 11 Aug 2020 09:13:57 -0400 X-MC-Unique: puKvqKhsMTC0BjO2WMxy2Q-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 3FB918017F4; Tue, 11 Aug 2020 13:13:56 +0000 (UTC) Received: from [10.36.113.73] (ovpn-113-73.ams2.redhat.com [10.36.113.73]) by smtp.corp.redhat.com (Postfix) with ESMTP id 9A01E10013C4; Tue, 11 Aug 2020 13:13:54 +0000 (UTC) Subject: Re: [PATCH] mm, page_alloc: fix core hung in free_pcppages_bulk() To: Charan Teja Kalla , akpm@linux-foundation.org, mhocko@suse.com, vbabka@suse.cz, linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, vinmenon@codeaurora.org References: <1597075833-16736-1-git-send-email-charante@codeaurora.org> <3b07d2a6-8ce7-5957-8ca5-a8d977852e14@redhat.com> <3e16448a-ea3b-99c0-13b0-a5a31c9b5582@codeaurora.org> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <3e16448a-ea3b-99c0-13b0-a5a31c9b5582@codeaurora.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11.08.20 15:11, Charan Teja Kalla wrote: > Thanks David for the comments. > > On 8/11/2020 1:59 PM, David Hildenbrand wrote: >> On 10.08.20 18:10, Charan Teja Reddy wrote: >>> The following race is observed with the repeated online, offline and a >>> delay between two successive online of memory blocks of movable zone. >>> >>> P1 P2 >>> >>> Online the first memory block in >>> the movable zone. The pcp struct >>> values are initialized to default >>> values,i.e., pcp->high = 0 & >>> pcp->batch = 1. >>> >>> Allocate the pages from the >>> movable zone. >>> >>> Try to Online the second memory >>> block in the movable zone thus it >>> entered the online_pages() but yet >>> to call zone_pcp_update(). >>> This process is entered into >>> the exit path thus it tries >>> to release the order-0 pages >>> to pcp lists through >>> free_unref_page_commit(). >>> As pcp->high = 0, pcp->count = 1 >>> proceed to call the function >>> free_pcppages_bulk(). >>> Update the pcp values thus the >>> new pcp values are like, say, >>> pcp->high = 378, pcp->batch = 63. >>> Read the pcp's batch value using >>> READ_ONCE() and pass the same to >>> free_pcppages_bulk(), pcp values >>> passed here are, batch = 63, >>> count = 1. >>> >>> Since num of pages in the pcp >>> lists are less than ->batch, >>> then it will stuck in >>> while(list_empty(list)) loop >>> with interrupts disabled thus >>> a core hung. >>> >>> Avoid this by ensuring free_pcppages_bulk() called with proper count of >>> pcp list pages. >>> >>> The mentioned race is some what easily reproducible without [1] because >>> pcp's are not updated for the first memory block online and thus there >>> is a enough race window for P2 between alloc+free and pcp struct values >>> update through onlining of second memory block. >>> >>> With [1], the race is still exists but it is very much narrow as we >>> update the pcp struct values for the first memory block online itself. >>> >>> [1]: https://patchwork.kernel.org/patch/11696389/ >>> >>> Signed-off-by: Charan Teja Reddy >>> --- >>> mm/page_alloc.c | 16 ++++++++++++++-- >>> 1 file changed, 14 insertions(+), 2 deletions(-) >>> >>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >>> index e4896e6..25e7e12 100644 >>> --- a/mm/page_alloc.c >>> +++ b/mm/page_alloc.c >>> @@ -3106,6 +3106,7 @@ static void free_unref_page_commit(struct page *page, unsigned long pfn) >>> struct zone *zone = page_zone(page); >>> struct per_cpu_pages *pcp; >>> int migratetype; >>> + int high; >>> >>> migratetype = get_pcppage_migratetype(page); >>> __count_vm_event(PGFREE); >>> @@ -3128,8 +3129,19 @@ static void free_unref_page_commit(struct page *page, unsigned long pfn) >>> pcp = &this_cpu_ptr(zone->pageset)->pcp; >>> list_add(&page->lru, &pcp->lists[migratetype]); >>> pcp->count++; >>> - if (pcp->count >= pcp->high) { >>> - unsigned long batch = READ_ONCE(pcp->batch); >>> + high = READ_ONCE(pcp->high); >>> + if (pcp->count >= high) { >>> + int batch; >>> + >>> + batch = READ_ONCE(pcp->batch); >>> + /* >>> + * For non-default pcp struct values, high is always >>> + * greater than the batch. If high < batch then pass >>> + * proper count to free the pcp's list pages. >>> + */ >>> + if (unlikely(high < batch)) >>> + batch = min(pcp->count, batch); >>> + >>> free_pcppages_bulk(zone, batch, pcp); >>> } >>> } >>> >> >> I was wondering if we should rather set all pageblocks to >> MIGRATE_ISOLATE in online_pages() before doing the online_pages_range() >> call, and do undo_isolate_page_range() after onlining is done. >> >> move_pfn_range_to_zone()->memmap_init_zone() marks all pageblocks >> MIGRATE_MOVABLE, and as that function is used also during boot, we could >> supply a parameter to configure this. >> >> This would prevent another race from happening: Having pages exposed to >> the buddy ready for allocation in online_pages_range() before the >> sections are marked online. > > Yeah this is another bug. And idea of isolate first, online and undoing > the isolation after zonelist and pcp struct update should work even for > the mentioned issue. This needs to go as a separate fix? Yeah, also requires more work to be done. Will add it to my list of TODOs. > > However, IMO, issue in free_pcppages_bulk() should be fixed by checking > if sane count value is passed. NO? > Posted V2: https://patchwork.kernel.org/patch/11709225/ Yeah, I'm fine with fixing this issue that can actually be reproduced. -- Thanks, David / dhildenb