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21 Feb 2020 11:25:50 -0800 X-IronPort-AV: E=Sophos;i="5.70,469,1574150400"; d="scan'208";a="316143032" Received: from ahduyck-desk1.jf.intel.com ([10.7.198.76]) by orsmga001-auth.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 21 Feb 2020 11:25:50 -0800 Message-ID: <2e4ff237de090dd4760995d948b9a1788c2f351d.camel@linux.intel.com> Subject: Re: [PATCH v17 4/9] mm: Introduce Reported pages From: Alexander Duyck To: Mel Gorman 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 Date: Fri, 21 Feb 2020 11:25:49 -0800 In-Reply-To: <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> <20200220223508.GX3466@techsingularity.net> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.32.5 (3.32.5-1.fc30) MIME-Version: 1.0 Content-Transfer-Encoding: 7bit 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, 2020-02-20 at 22:35 +0000, Mel Gorman wrote: > 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. The comment I can do, that shouldn't be an issue. The point I was getting at is that a separate drain call is expected for this any time the function is not returning an error, and the only way it can return an error is if there was a reporting issue. > > > 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. I would think adding that would confuse things even more. There is a break statement at the end of the loop that will break out if err is set. So we should never hit the VM_WARN_ON_ONCE because err should always be 0 before we even attempt to isolate the page. I think something like the following would probably make more sense: err = page_reporting_cycle(prdev, zone, order, mt, sgl, &offset); if (err) { /* * We should have drained the scatterlist * prior to exiting page_reporting_cycle if * we encountered an error. If we did not * then this could result in a memory leak. * Verify that the end of the scatterlist * was cleared prior to us getting here. */ sgl = &sgl[PAGE_REPORTING_CAPACITY - 1]; VM_WARN_ON_ONCE(sg_page(sgl)); return err; } With that we are more-or-less making certain that they called page_reporting_drain which will zero the scatterlist.