From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-15.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E1BA2C433B4 for ; Thu, 8 Apr 2021 02:44:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B1ECC610F9 for ; Thu, 8 Apr 2021 02:44:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229640AbhDHCoc (ORCPT ); Wed, 7 Apr 2021 22:44:32 -0400 Received: from szxga04-in.huawei.com ([45.249.212.190]:16070 "EHLO szxga04-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229510AbhDHCob (ORCPT ); Wed, 7 Apr 2021 22:44:31 -0400 Received: from DGGEMS414-HUB.china.huawei.com (unknown [172.30.72.59]) by szxga04-in.huawei.com (SkyGuard) with ESMTP id 4FG5Bl4KGzz1BGPn; Thu, 8 Apr 2021 10:42:07 +0800 (CST) Received: from [10.174.179.9] (10.174.179.9) by DGGEMS414-HUB.china.huawei.com (10.3.19.214) with Microsoft SMTP Server id 14.3.498.0; Thu, 8 Apr 2021 10:44:17 +0800 Subject: Re: [PATCH 2/4] mm/hugeltb: simplify the return code of __vma_reservation_common() To: Mike Kravetz , CC: , , , References: <20210402093249.25137-1-linmiaohe@huawei.com> <20210402093249.25137-3-linmiaohe@huawei.com> <40114ff5-ba3d-ca66-3338-25db80a015da@huawei.com> <1926967f-3805-2baf-6b86-24039c6513ca@huawei.com> From: Miaohe Lin Message-ID: <178a2b05-ab9b-3d38-36c5-3950a3859322@huawei.com> Date: Thu, 8 Apr 2021 10:44:17 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.179.9] X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021/4/8 5:23, Mike Kravetz wrote: > On 4/6/21 8:09 PM, Miaohe Lin wrote: >> On 2021/4/7 10:37, Mike Kravetz wrote: >>> On 4/6/21 7:05 PM, Miaohe Lin wrote: >>>> Hi: >>>> On 2021/4/7 8:53, Mike Kravetz wrote: >>>>> On 4/2/21 2:32 AM, Miaohe Lin wrote: >>>>>> It's guaranteed that the vma is associated with a resv_map, i.e. either >>>>>> VM_MAYSHARE or HPAGE_RESV_OWNER, when the code reaches here or we would >>>>>> have returned via !resv check above. So ret must be less than 0 in the >>>>>> 'else' case. Simplify the return code to make this clear. >>>>> >>>>> I believe we still neeed that ternary operator in the return statement. >>>>> Why? >>>>> >>>>> There are two basic types of mappings to be concerned with: >>>>> shared and private. >>>>> For private mappings, a task can 'own' the mapping as indicated by >>>>> HPAGE_RESV_OWNER. Or, it may not own the mapping. The most common way >>>>> to create a non-owner private mapping is to have a task with a private >>>>> mapping fork. The parent process will have HPAGE_RESV_OWNER set, the >>>>> child process will not. The idea is that since the child has a COW copy >>>>> of the mapping it should not consume reservations made by the parent. >>>> >>>> The child process will not have HPAGE_RESV_OWNER set because at fork time, we do: >>>> /* >>>> * Clear hugetlb-related page reserves for children. This only >>>> * affects MAP_PRIVATE mappings. Faults generated by the child >>>> * are not guaranteed to succeed, even if read-only >>>> */ >>>> if (is_vm_hugetlb_page(tmp)) >>>> reset_vma_resv_huge_pages(tmp); >>>> i.e. we have vma->vm_private_data = (void *)0; for child process and vma_resv_map() will >>>> return NULL in this case. >>>> Or am I missed something? >>>> >>>>> Only the parent (HPAGE_RESV_OWNER) is allowed to consume the >>>>> reservations. >>>>> Hope that makens sense? >>>>> >>>>>> >>>>>> Signed-off-by: Miaohe Lin >>>>>> --- >>>>>> mm/hugetlb.c | 2 +- >>>>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>>>> >>>>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >>>>>> index a03a50b7c410..b7864abded3d 100644 >>>>>> --- a/mm/hugetlb.c >>>>>> +++ b/mm/hugetlb.c >>>>>> @@ -2183,7 +2183,7 @@ static long __vma_reservation_common(struct hstate *h, >>>>>> return 1; >>>>>> } >>>>>> else >>>>> >>>>> This else also handles the case !HPAGE_RESV_OWNER. In this case, we >>>> >>>> IMO, for the case !HPAGE_RESV_OWNER, we won't reach here. What do you think? >>>> >>> >>> I think you are correct. >>> >>> However, if this is true we should be able to simply the code even >>> further. There is no need to check for HPAGE_RESV_OWNER because we know >>> it must be set. Correct? If so, the code could look something like: >>> >>> if (vma->vm_flags & VM_MAYSHARE) >>> return ret; >>> >>> /* We know private mapping with HPAGE_RESV_OWNER */ >>> * ... * >>> * Add that existing comment */ >>> >>> if (ret > 0) >>> return 0; >>> if (ret == 0) >>> return 1; >>> return ret; >>> >> >> Many thanks for good suggestion! What do you mean is this ? > > I think the below changes would work fine. > > However, this patch/discussion has made me ask the question. Do we need > the HPAGE_RESV_OWNER flag? Is the followng true? > !(vm_flags & VM_MAYSHARE) && vma_resv_map() ===> HPAGE_RESV_OWNER > !(vm_flags & VM_MAYSHARE) && !vma_resv_map() ===> !HPAGE_RESV_OWNER > I agree with you. HPAGE_RESV_OWNER is set in hugetlb_reserve_pages() and there's no way to clear it in the owner process. The child process can not inherit both HPAGE_RESV_OWNER and resv_map. So for !HPAGE_RESV_OWNER vma, it knows nothing about resv_map. IMO, in !(vm_flags & VM_MAYSHARE) case, we must have: !!vma_resv_map() == !!HPAGE_RESV_OWNER > I am not suggesting we eliminate the flag and make corresponding > changes. Just curious if you believe we 'could' remove the flag and > depend on the above conditions. > > One reason for NOT removing the flag is that that flag itself and > supporting code and commnets help explain what happens with hugetlb > reserves for COW mappings. That code is hard to understand and the > existing code and coments around HPAGE_RESV_OWNER help with > understanding. Agree. These codes took me several days to understand... > Thanks.