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[91.12.97.29]) by smtp.gmail.com with ESMTPSA id t1sm22639629wrx.28.2021.06.09.02.42.15 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 09 Jun 2021 02:42:15 -0700 (PDT) Subject: Re: [PATCH v2 1/3] mm,page_alloc: Use {get,put}_online_mems() to get stable zone's values From: David Hildenbrand To: Oscar Salvador Cc: Michal Hocko , Andrew Morton , Dave Hansen , Anshuman Khandual , Vlastimil Babka , Pavel Tatashin , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20210602091457.17772-1-osalvador@suse.de> <20210602091457.17772-2-osalvador@suse.de> <39473305-6e91-262d-bcc2-76b745a5b14a@redhat.com> <20210604074140.GA25063@linux> <20210607075147.GA10554@linux> <85984701-55ae-dfa5-2a8d-f637051b612d@redhat.com> <20210607102318.GA12683@linux> <0eadea9c-5af0-d7e6-071e-898b04294dd3@redhat.com> Organization: Red Hat Message-ID: Date: Wed, 9 Jun 2021 11:42:14 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.10.1 MIME-Version: 1.0 In-Reply-To: <0eadea9c-5af0-d7e6-071e-898b04294dd3@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08.06.21 17:00, David Hildenbrand wrote: > On 07.06.21 12:23, Oscar Salvador wrote: >> On Mon, Jun 07, 2021 at 10:49:01AM +0200, David Hildenbrand wrote: >>> I'd like to point out that I think the seqlock is not in place to >>> synchronize with actual growing/shrinking but to get consistent zone ranges >>> -- like using atomics, but we have two inter-dependent values here. >> >> I guess so, at least that's what it should do. >> But the way it is placed right now is misleading. >> >> If we really want to get consistent zone ranges, we should start using >> zone's seqlock where it matters and that is pretty much all those >> places that use zone_spans_pfn(). > > Right, or even only zone_end_pfn() to get a consistent value. > >> Otherwise there is no way you can be sure the pfn you're checking is >> within the limits. Moreover, as Michal pointed out early, if we really >> want to go down that road the locking should be made in the caller >> evolving the operation, otheriwse things might change once the lock >> is dropped and you're working with a wrong assumption. >> >> I can see arguments for both riping it out and doing it right (but none for >> the way it is right now). >> For riping it out, one could say that those races might not be fatal, >> as usually the pfn you're working with (the one you want to check falls >> within a certain range) you know is valid, so the worst can happen is >> you get false positives/negatives and that might or might not be detected >> further down. How bad are false positive/negatives I guess it depends on the >> situation, but we already do that right now. >> The zone_spans_pfn() from page_outside_zone_boundaries() is the only one using >> locking right now, so well, if we survided this long without locks in other places >> using zone_spans_pfn() makes one wonder if it is that bad. >> >> On the other hand, one could argue that for correctness sake, we should be holding >> zone's seqlock whenever checking for zone_spans_pfn() to avoid any inconsistency. >> >> > > IMHO, as we know the race exists and we have a tool to handle it in > place, we should maybe fix the obvious cases if possible. > > Code that uses zone->zone_start_pfn directly is unlikely to be broken on > most architectures. We will usually read/write via single instruction > and won't get inconsistencies, for example, when shrinking or growing > the zone. We most probably don't want to use an atomic for that right now. > > Code that uses zone->spanned_pages to detect the zone end, however, is > more likely to be broken. I don't think we have any relevant around > anymore. Everything was converted to zone_end_pfn(). > > I feel like we should just make zone_end_pfn() take the seqlock in read. > Then, we at least get a consistent value, for example, while growing a zone. > > Just imagine the following case when we grow a section to the front when > onlining memory: > > zone->zone_start_pfn -= new_pages; > zone->spanned_pages += new_pages; > > Note that compilers/CPUs might reshuffle as they like. If someone (e.g., > zone_spans_pfn()) races with that code, it might get new > zone->zone_start_pfn but old zone->spanned_pages. zone_end_pfn() will > report a "too small zone" and trigger false negatives in zone_spans_pfn(). > Thinking again, we could of course also simply convert to zone->zone_start_+ pfn zone->zone_end_pfn. Places that need spanned_pages() would have the same issue, but I think they are rather a concern case. -- Thanks, David / dhildenb