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[37.188.180.223]) by smtp.gmail.com with ESMTPSA id r5sm2851950wmr.15.2020.04.07.08.40.05 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 07 Apr 2020 08:40:06 -0700 (PDT) Date: Tue, 7 Apr 2020 17:40:05 +0200 From: Michal Hocko To: Roman Gushchin Cc: Andrew Morton , Aslan Bakirov , linux-mm@kvack.org, kernel-team@fb.com, linux-kernel@vger.kernel.org, Rik van Riel , Mike Kravetz , Andreas Schaufler , Randy Dunlap , Joonsoo Kim Subject: Re: [PATCH v4 2/2] mm: hugetlb: optionally allocate gigantic hugepages using cma Message-ID: <20200407154005.GT18914@dhcp22.suse.cz> References: <20200407010431.1286488-1-guro@fb.com> <20200407010431.1286488-3-guro@fb.com> <20200407070331.GD18914@dhcp22.suse.cz> <20200407152544.GA9557@carbon.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200407152544.GA9557@carbon.lan> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 07-04-20 08:25:44, Roman Gushchin wrote: > On Tue, Apr 07, 2020 at 09:03:31AM +0200, Michal Hocko wrote: > > On Mon 06-04-20 18:04:31, Roman Gushchin wrote: > > [...] > > My ack still applies but I have only noticed two minor things now. > > Hello, Michal! > > > > > [...] > > > @@ -1281,8 +1308,14 @@ static void update_and_free_page(struct hstate *h, struct page *page) > > > set_compound_page_dtor(page, NULL_COMPOUND_DTOR); > > > set_page_refcounted(page); > > > if (hstate_is_gigantic(h)) { > > > + /* > > > + * Temporarily drop the hugetlb_lock, because > > > + * we might block in free_gigantic_page(). > > > + */ > > > + spin_unlock(&hugetlb_lock); > > > destroy_compound_gigantic_page(page, huge_page_order(h)); > > > free_gigantic_page(page, huge_page_order(h)); > > > + spin_lock(&hugetlb_lock); > > > > This is OK with the current code because existing paths do not have to > > revalidate the state AFAICS but it is a bit subtle. I have checked the > > cma_free path and it can only sleep on the cma->lock unless I am missing > > something. This lock is only used for cma bitmap manipulation and the > > mutex sounds like an overkill there and it can be replaced by a > > spinlock. > > > > Sounds like a follow up patch material to me. > > I had the same idea and even posted a patch: > https://lore.kernel.org/linux-mm/20200403174559.GC220160@carbon.lan/T/#m87be98bdacda02cea3dd6759b48a28bd23f29ff0 > > However, Joonsoo pointed out that in some cases the bitmap operation might > be too long for a spinlock. I was not aware of this email thread. I will have a look. Thanks! > Alternatively, we can implement an asynchronous delayed release on the cma side, > I just don't know if it's worth it (I mean adding code/complexity). > > > > > [...] > > > + for_each_node_state(nid, N_ONLINE) { > > > + int res; > > > + > > > + size = min(per_node, hugetlb_cma_size - reserved); > > > + size = round_up(size, PAGE_SIZE << order); > > > + > > > + res = cma_declare_contiguous_nid(0, size, 0, PAGE_SIZE << order, > > > + 0, false, "hugetlb", > > > + &hugetlb_cma[nid], nid); > > > + if (res) { > > > + pr_warn("hugetlb_cma: reservation failed: err %d, node %d", > > > + res, nid); > > > + break; > > > > Do we really have to break out after a single node failure? There might > > be other nodes that can satisfy the allocation. You are not cleaning up > > previous allocations so there is a partial state and then it would make > > more sense to me to simply s@break@continue@ here. > > But then we should iterate over all nodes in alloc_gigantic_page()? OK, I've managed to miss the early break on hugetlb_cma[node] == NULL there as well. I do not think this makes much sense. Just consider a setup with one node much smaller than others (not unseen on LPAR configurations) and then you are potentially using CMA areas on some nodes without a good reason. > Currently if hugetlb_cma[0] is NULL it will immediately switch back > to the fallback approach. > > Actually, Idk how realistic are use cases with complex node configuration, > so that we can hugetlb_cma areas can be allocated only on some of them. > I'd leave it up to the moment when we'll have a real world example. > Then we probably want something more sophisticated anyway... I do not follow. Isn't the s@break@continue@ in this and alloc_gigantic_page path enough to make it work? -- Michal Hocko SUSE Labs