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Mon, 07 Jun 2021 07:52:18 +0000 Date: Mon, 7 Jun 2021 09:52:16 +0200 From: Oscar Salvador To: Michal Hocko Cc: David Hildenbrand , Andrew Morton , Dave Hansen , Anshuman Khandual , Vlastimil Babka , Pavel Tatashin , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 1/3] mm,page_alloc: Use {get,put}_online_mems() to get stable zone's values Message-ID: <20210607075147.GA10554@linux> References: <20210602091457.17772-1-osalvador@suse.de> <20210602091457.17772-2-osalvador@suse.de> <39473305-6e91-262d-bcc2-76b745a5b14a@redhat.com> <20210604074140.GA25063@linux> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210604074140.GA25063@linux> User-Agent: Mutt/1.10.1 (2018-07-13) Authentication-Results: imf27.hostedemail.com; dkim=pass header.d=suse.de header.s=susede2_rsa header.b="KfpD/cLC"; dkim=pass header.d=suse.de header.s=susede2_ed25519 header.b="BIO/RR9W"; dkim=pass header.d=suse.de header.s=susede2_rsa header.b="KfpD/cLC"; dkim=pass header.d=suse.de header.s=susede2_ed25519 header.b="BIO/RR9W"; spf=pass (imf27.hostedemail.com: domain of osalvador@suse.de designates 195.135.220.28 as permitted sender) smtp.mailfrom=osalvador@suse.de; dmarc=none X-Stat-Signature: cusiq7izqokyd737rh3cjmczmtnwaqjk X-Rspamd-Queue-Id: 23736801934C X-Rspamd-Server: rspam06 X-HE-Tag: 1623052337-51822 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Jun 04, 2021 at 09:41:45AM +0200, Oscar Salvador wrote: > On Thu, Jun 03, 2021 at 02:45:13PM +0200, Michal Hocko wrote: > > I believe we need to define the purpose of the locking first. The > > If you ask me, this locking would be meant to make sure zone's zone_start_pfn > or spanned_pages do not change under us, in case we __need__ the value to be > stable. > > > existing locking doesn't serve much purpose, does it? The state might > > Well, half-way. Currently, the locking is taken in write mode whenever > the zone is expanded or shrinked, and in read mode when called from > bad_range()->page_outside_zone_boundaries() (only on VM_DEBUG). > > But as you pointed out, such state might change right after the locking is > released and all the work would be for nothing. > So indeed, the __whole__ operation should be envolved by the lock in the caller > The way that stands right now is not optimal. > > > change right after the lock is released and the caller cannot really > > rely on the result. So aside of the current implementation, I would > > argue that any locking has to be be done on the caller layer. > > > > But the primary question is whether anybody actually cares about > > potential races in the first place. > > I have been checking move_freepages_block() and alloc_contig_pages(), which > are two of the functions that call zone_spans_pfn(). > > move_freepages_block() uses it in a way to align the given pfn to pageblock > top and bottom, and then check that aligned pfns are still within the same zone. > From a memory-hotplug perspective that's ok as we know that we are offlining > PAGES_PER_SECTION (which implies whole pageblocks). > > alloc_contig_pages() (used by the hugetlb gigantic allocator) runs through a > node's zonelist and checks whether zone->zone_start_pfn + nr_pages stays within > the same zone. > IMHO, the race with zone_spans_last_pfn() vs mem-hotplug would not be that bad, > as it will be caught afters by e.g: __alloc_contig_pages when pages cannot be > isolated because they are offline etc. > > So, I would say we do not really need the lock, but I might be missing something. > But if we chose to care about this, then the locking should be done right, not > half-way as it is right now. Any thoughts on this? :-) -- Oscar Salvador SUSE L3