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Fri, 25 Sep 2020 06:23:24 -0400 X-MC-Unique: 675avHVpP7234P3_DG-bFw-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id BCF931008558; Fri, 25 Sep 2020 10:23:22 +0000 (UTC) Received: from [10.36.112.211] (ovpn-112-211.ams2.redhat.com [10.36.112.211]) by smtp.corp.redhat.com (Postfix) with ESMTP id 2729C5D9FC; Fri, 25 Sep 2020 10:23:20 +0000 (UTC) Subject: Re: [PATCH 4/9] mm, page_alloc: simplify pageset_update() To: Vlastimil Babka , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, Michal Hocko , Pavel Tatashin , Oscar Salvador , Joonsoo Kim References: <20200922143712.12048-1-vbabka@suse.cz> <20200922143712.12048-5-vbabka@suse.cz> From: David Hildenbrand Autocrypt: addr=david@redhat.com; prefer-encrypt=mutual; keydata= mQINBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABtCREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT6JAlgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63W5Ag0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAGJAjwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat GmbH Message-ID: <96cb97df-fe33-5a99-6618-1560467f8ffa@redhat.com> Date: Fri, 25 Sep 2020 12:23:20 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0 MIME-Version: 1.0 In-Reply-To: <20200922143712.12048-5-vbabka@suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 22.09.20 16:37, Vlastimil Babka wrote: > pageset_update() attempts to update pcplist's high and batch values in a way > that readers don't observe batch > high. It uses smp_wmb() to order the updates > in a way to achieve this. However, without proper pairing read barriers in > readers this guarantee doesn't hold, and there are no such barriers in > e.g. free_unref_page_commit(). > > Commit 88e8ac11d2ea ("mm, page_alloc: fix core hung in free_pcppages_bulk()") > already showed this is problematic, and solved this by ultimately only trusing > pcp->count of the current cpu with interrupts disabled. > > The update dance with unpaired write barriers thus makes no sense. Replace > them with plain WRITE_ONCE to prevent store tearing, and document that the > values can change asynchronously and should not be trusted for correctness. > > All current readers appear to be OK after 88e8ac11d2ea. Convert them to > READ_ONCE to prevent unnecessary read tearing, but mainly to alert anybody > making future changes to the code that special care is needed. > > Signed-off-by: Vlastimil Babka > --- > mm/page_alloc.c | 40 ++++++++++++++++++---------------------- > 1 file changed, 18 insertions(+), 22 deletions(-) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 76c2b4578723..99b74c1c2b0a 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -1297,7 +1297,7 @@ static void free_pcppages_bulk(struct zone *zone, int count, > { > int migratetype = 0; > int batch_free = 0; > - int prefetch_nr = 0; > + int prefetch_nr = READ_ONCE(pcp->batch); > bool isolated_pageblocks; > struct page *page, *tmp; > LIST_HEAD(head); > @@ -1348,8 +1348,10 @@ static void free_pcppages_bulk(struct zone *zone, int count, > * avoid excessive prefetching due to large count, only > * prefetch buddy for the first pcp->batch nr of pages. > */ > - if (prefetch_nr++ < pcp->batch) > + if (prefetch_nr) { > prefetch_buddy(page); > + prefetch_nr--; > + } > } while (--count && --batch_free && !list_empty(list)); > } > > @@ -3131,10 +3133,8 @@ 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); > - free_pcppages_bulk(zone, batch, pcp); > - } > + if (pcp->count >= READ_ONCE(pcp->high)) > + free_pcppages_bulk(zone, READ_ONCE(pcp->batch), pcp); > } > > /* > @@ -3318,7 +3318,7 @@ static struct page *__rmqueue_pcplist(struct zone *zone, int migratetype, > do { > if (list_empty(list)) { > pcp->count += rmqueue_bulk(zone, 0, > - pcp->batch, list, > + READ_ONCE(pcp->batch), list, > migratetype, alloc_flags); > if (unlikely(list_empty(list))) > return NULL; > @@ -6174,13 +6174,16 @@ static int zone_batchsize(struct zone *zone) > } > > /* > - * pcp->high and pcp->batch values are related and dependent on one another: > - * ->batch must never be higher then ->high. > - * The following function updates them in a safe manner without read side > - * locking. > + * pcp->high and pcp->batch values are related and generally batch is lower > + * than high. They are also related to pcp->count such that count is lower > + * than high, and as soon as it reaches high, the pcplist is flushed. > * > - * Any new users of pcp->batch and pcp->high should ensure they can cope with > - * those fields changing asynchronously (acording to the above rule). > + * However, guaranteeing these relations at all times would require e.g. write > + * barriers here but also careful usage of read barriers at the read side, and > + * thus be prone to error and bad for performance. Thus the update only prevents > + * store tearing. Any new users of pcp->batch and pcp->high should ensure they > + * can cope with those fields changing asynchronously, and fully trust only the > + * pcp->count field on the local CPU with interrupts disabled. > * > * mutex_is_locked(&pcp_batch_high_lock) required when calling this function > * outside of boot time (or some other assurance that no concurrent updaters > @@ -6189,15 +6192,8 @@ static int zone_batchsize(struct zone *zone) > static void pageset_update(struct per_cpu_pages *pcp, unsigned long high, > unsigned long batch) > { > - /* start with a fail safe value for batch */ > - pcp->batch = 1; > - smp_wmb(); > - > - /* Update high, then batch, in order */ > - pcp->high = high; > - smp_wmb(); > - > - pcp->batch = batch; > + WRITE_ONCE(pcp->batch, batch); > + WRITE_ONCE(pcp->high, high); > } > > static void pageset_init(struct per_cpu_pageset *p) > I *think* this is okay and obviously simplifies things. But to be 100% sure, I'll have to rely on your judgment :) Acked-by: David Hildenbrand -- Thanks, David / dhildenb