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Thu, 20 Feb 2020 22:35:11 +0000 (GMT) Received: (qmail 15339 invoked from network); 20 Feb 2020 22:35:11 -0000 Received: from unknown (HELO techsingularity.net) (mgorman@techsingularity.net@[84.203.18.57]) by 81.17.254.9 with ESMTPSA (AES256-SHA encrypted, authenticated); 20 Feb 2020 22:35:10 -0000 Date: Thu, 20 Feb 2020 22:35:08 +0000 From: Mel Gorman To: Alexander Duyck Cc: Alexander Duyck , kvm@vger.kernel.org, david@redhat.com, mst@redhat.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, yang.zhang.wz@gmail.com, pagupta@redhat.com, konrad.wilk@oracle.com, nitesh@redhat.com, riel@surriel.com, willy@infradead.org, lcapitulino@redhat.com, dave.hansen@intel.com, wei.w.wang@intel.com, aarcange@redhat.com, pbonzini@redhat.com, dan.j.williams@intel.com, mhocko@kernel.org, vbabka@suse.cz, osalvador@suse.de Subject: Re: [PATCH v17 4/9] mm: Introduce Reported pages Message-ID: <20200220223508.GX3466@techsingularity.net> References: <20200211224416.29318.44077.stgit@localhost.localdomain> <20200211224635.29318.19750.stgit@localhost.localdomain> <20200219145511.GS3466@techsingularity.net> <7d3c732d9ec7725dcb5a90c1dc8e9859fbe6ccc0.camel@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <7d3c732d9ec7725dcb5a90c1dc8e9859fbe6ccc0.camel@linux.intel.com> User-Agent: Mutt/1.10.1 (2018-07-13) 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 Thu, Feb 20, 2020 at 10:44:21AM -0800, Alexander Duyck wrote: > > > +static int > > > +page_reporting_cycle(struct page_reporting_dev_info *prdev, struct zone *zone, > > > + unsigned int order, unsigned int mt, > > > + struct scatterlist *sgl, unsigned int *offset) > > > +{ > > > + struct free_area *area = &zone->free_area[order]; > > > + struct list_head *list = &area->free_list[mt]; > > > + unsigned int page_len = PAGE_SIZE << order; > > > + struct page *page, *next; > > > + int err = 0; > > > + > > > + /* > > > + * Perform early check, if free area is empty there is > > > + * nothing to process so we can skip this free_list. > > > + */ > > > + if (list_empty(list)) > > > + return err; > > > + > > > + spin_lock_irq(&zone->lock); > > > + > > > + /* loop through free list adding unreported pages to sg list */ > > > + list_for_each_entry_safe(page, next, list, lru) { > > > + /* We are going to skip over the reported pages. */ > > > + if (PageReported(page)) > > > + continue; > > > + > > > + /* Attempt to pull page from list */ > > > + if (!__isolate_free_page(page, order)) > > > + break; > > > + > > > > Might want to note that you are breaking because the only reason to fail > > the isolation is that watermarks are not met and we are likely under > > memory pressure. It's not a big issue. > > > > However, while I think this is correct, it's hard to follow. This loop can > > be broken out of with pages still on the scatter gather list. The current > > flow guarantees that err will not be set at this point so the caller > > cleans it up so we always drain the list either here or in the caller. > > I can probably submit a follow-up patch to update the comments. The reason > for not returning an error is because I didn't consider it an error that > we encountered the watermark and were not able to pull any more pages. > Instead I considered that the "stop" point for this pass and have it just > exit out of the loop and flush the data. > I don't consider it an error and I don't think you should return an error. The comment just needs to explain that the draining happens in the caller in this case. That should be enough of a warning to a future developer to double check the flow after any changes to make sure the drain is reached. > > While I think it works, it's a bit fragile. I recommend putting a comment > > above this noting why it's safe and put a VM_WARN_ON_ONCE(err) before the > > break in case someone tries to change this in a years time and does not > > spot that the flow to reach page_reporting_drain *somewhere* is critical. > > I assume this isn't about this section, but the section below? > I meant something like if (!__isolate_free_page(page, order)) { VM_WARN_ON_ONCE(err); break; } Because at this point it's possible there are entries that should go through page_reporting_drain() but the caller will not call page_reporting_drain() in the event of an error. -- Mel Gorman SUSE Labs