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=-13.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable 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 E5AE0C433E6 for ; Tue, 23 Feb 2021 05:37:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9F00761493 for ; Tue, 23 Feb 2021 05:37:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230439AbhBWFhP (ORCPT ); Tue, 23 Feb 2021 00:37:15 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56234 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229961AbhBWFhL (ORCPT ); Tue, 23 Feb 2021 00:37:11 -0500 Received: from mail-pl1-x62d.google.com (mail-pl1-x62d.google.com [IPv6:2607:f8b0:4864:20::62d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 43E74C061574 for ; Mon, 22 Feb 2021 21:36:31 -0800 (PST) Received: by mail-pl1-x62d.google.com with SMTP id ba1so9170190plb.1 for ; Mon, 22 Feb 2021 21:36:31 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bytedance-com.20150623.gappssmtp.com; s=20150623; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=aYv0eQvZ2yLg52Ysz8rEXt1oFGhcA0fO7nVu09GhKJo=; b=VattKy++i+crfizvJnT73A/PHg1AtsrF5LZKhRwrP1CZNswQmtI1W9/4DUf8jf6i2G zEMSevQvsTShQLwkFgFPZYH90PTBzEgydaZt4L5P+QCclNIA/apqgEpKRAQ76OK57B8x MTfWZrGzOMGawr83aStTDvgb5GT7Y5hwhEUhAWPS2umyeZu/mr5nPyN+aHgaCWO/w51j SVj12JJohl6IjYxMyCFVaAQQHQf0wS9Wnj1SW/j1ojzvSs16Nd/Y+tyqJK4/JFdfpdpr y5hsC7AwbQsZ0H8bJ/pGjOHX+7SRS8gI7N9sVSl67fLMxAFt5SDNPI4GO2UJpyCQkBri iLZA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=aYv0eQvZ2yLg52Ysz8rEXt1oFGhcA0fO7nVu09GhKJo=; b=BIsTGLg1S+/ra1sJgly4QHIixQYY4K43On1TLu5LefXoViV4pSgfJfseu9Jayy4OFh jCzl7R6N4ZJ5ZvtdEtEn/Z7apeCeScNUeaNSD6Cc3537dpV4rJ2wE74xtmZ2oSJ4lcLM AVPPRf866nY+dJz2iGQXY29md8Rk4F3hlLoyNH/WCKBAfMSUQChY5q9t9eNeE2CFJTBY iIl83iuohos9c8A/wv8nPSd6GBK/GQmyhKqXfcwMG3uwTfV1zI1S52QlPnj+FeNEZsK+ eAeq+p3vVvyNZE3K6ci5Be212ys9EW6XFC8eTDf2suIemh3lIMQzY6YPnQL3CgLQhFIj LofQ== X-Gm-Message-State: AOAM533/+85YDxVsrvvCWPOtc6nrlw6H2bRfFHawpCNst17/4/YyYGYS j1HsAndHWQd3yRtRVaojHBZiyxvq49fEcJNWCr8bn7kf0PMiZxJQ X-Google-Smtp-Source: ABdhPJwgy93EhnypmyOsAuzu+K13tuFOTwwVMC+wgLV+s9q0JC7Q5wYzJvFv7InURvV6eIQwIvRUymiZpeNaJcG+gQk= X-Received: by 2002:a17:902:9341:b029:e1:7b4e:57a8 with SMTP id g1-20020a1709029341b02900e17b4e57a8mr25102009plp.34.1614058590565; Mon, 22 Feb 2021 21:36:30 -0800 (PST) MIME-Version: 1.0 References: <20210219104954.67390-1-songmuchun@bytedance.com> <20210219104954.67390-5-songmuchun@bytedance.com> <13a5363c-6af4-1e1f-9a18-972ca18278b5@oracle.com> In-Reply-To: <13a5363c-6af4-1e1f-9a18-972ca18278b5@oracle.com> From: Muchun Song Date: Tue, 23 Feb 2021 13:35:53 +0800 Message-ID: Subject: Re: [External] Re: [PATCH v16 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page To: Mike Kravetz , Michal Hocko , Oscar Salvador Cc: Jonathan Corbet , Thomas Gleixner , mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com, dave.hansen@linux.intel.com, luto@kernel.org, Peter Zijlstra , viro@zeniv.linux.org.uk, Andrew Morton , paulmck@kernel.org, mchehab+huawei@kernel.org, pawan.kumar.gupta@linux.intel.com, Randy Dunlap , oneukum@suse.com, anshuman.khandual@arm.com, jroedel@suse.de, Mina Almasry , David Rientjes , Matthew Wilcox , "Song Bao Hua (Barry Song)" , David Hildenbrand , =?UTF-8?B?SE9SSUdVQ0hJIE5BT1lBKOWggOWPoyDnm7TkuZ8p?= , Joao Martins , Xiongchun duan , linux-doc@vger.kernel.org, LKML , Linux Memory Management List , linux-fsdevel Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Tue, Feb 23, 2021 at 8:01 AM Mike Kravetz wrote: > > On 2/19/21 2:49 AM, Muchun Song wrote: > > When we free a HugeTLB page to the buddy allocator, we should allocate > > the vmemmap pages associated with it. But we may cannot allocate vmemmap > > pages when the system is under memory pressure, in this case, we just > > refuse to free the HugeTLB page instead of looping forever trying to > > allocate the pages. This changes some behavior (list below) on some > > corner cases. > > Thank you for listing changes in behavior and possible side effects of > not being able to allocate vmemmmap and free huge page to buddy! > > I will not repeat Michal's comment about the check for an atomic context > in free_huge_page path. > > > > > 1) Failing to free a huge page triggered by the user (decrease nr_pages). > > > > Need try again later by the user. > > > > 2) Failing to free a surplus huge page when freed by the application. > > > > Try again later when freeing a huge page next time. > > > > 3) Failing to dissolve a free huge page on ZONE_MOVABLE via > > offline_pages(). > > > > This is a bit unfortunate if we have plenty of ZONE_MOVABLE memory > > but are low on kernel memory. For example, migration of huge pages > > would still work, however, dissolving the free page does not work. > > This is a corner cases. When the system is that much under memory > > pressure, offlining/unplug can be expected to fail. > > > > 4) Failing to dissolve a huge page on CMA/ZONE_MOVABLE via > > alloc_contig_range() - once we have that handling in place. Mainly > > affects CMA and virtio-mem. > > > > Similar to 3). virito-mem will handle migration errors gracefully. > > CMA might be able to fallback on other free areas within the CMA > > region. > > > > We do not want to use GFP_ATOMIC to allocate vmemmap pages. Because it > > grants access to memory reserves and we do not think it is reasonable > > to use memory reserves. We use GFP_KERNEL in alloc_huge_page_vmemmap(). > > > > Signed-off-by: Muchun Song > > --- > > Documentation/admin-guide/mm/hugetlbpage.rst | 8 +++ > > include/linux/mm.h | 2 + > > mm/hugetlb.c | 81 ++++++++++++++++++++-------- > > mm/hugetlb_vmemmap.c | 22 ++++++++ > > mm/hugetlb_vmemmap.h | 6 +++ > > mm/sparse-vmemmap.c | 75 +++++++++++++++++++++++++- > > 6 files changed, 171 insertions(+), 23 deletions(-) > > > > diff --git a/Documentation/admin-guide/mm/hugetlbpage.rst b/Documentation/admin-guide/mm/hugetlbpage.rst > > index f7b1c7462991..fb8f649e5635 100644 > > --- a/Documentation/admin-guide/mm/hugetlbpage.rst > > +++ b/Documentation/admin-guide/mm/hugetlbpage.rst > > @@ -60,6 +60,10 @@ HugePages_Surp > > the pool above the value in ``/proc/sys/vm/nr_hugepages``. The > > maximum number of surplus huge pages is controlled by > > ``/proc/sys/vm/nr_overcommit_hugepages``. > > + Note: When the feature of freeing unused vmemmap pages associated > > + with each hugetlb page is enabled, the number of the surplus huge > > Small wording change: > > with each hugetlb page is enabled, the number of surplus huge Thanks. I will update this. > > > + pages may be temporarily larger than the maximum number of surplus > > + huge pages when the system is under memory pressure. > > Hugepagesize > > is the default hugepage size (in Kb). > > Hugetlb > > @@ -80,6 +84,10 @@ returned to the huge page pool when freed by a task. A user with root > > privileges can dynamically allocate more or free some persistent huge pages > > by increasing or decreasing the value of ``nr_hugepages``. > > > > +Note: When the feature of freeing unused vmemmap pages associated with each > > +hugetlb page is enabled, we can failed to free the huge pages triggered by > > Small wording change: > > hugetlb page is enabled, we can fail to free the huge pages triggered by Thanks. I will update this. > > > +the user when ths system is under memory pressure. Please try again later. > > + > > Pages that are used as huge pages are reserved inside the kernel and cannot > > be used for other purposes. Huge pages cannot be swapped out under > > memory pressure. > > diff --git a/include/linux/mm.h b/include/linux/mm.h > > index d7dddf334779..33c5911afe18 100644 > > --- a/include/linux/mm.h > > +++ b/include/linux/mm.h > > @@ -2981,6 +2981,8 @@ static inline void print_vma_addr(char *prefix, unsigned long rip) > > > > void vmemmap_remap_free(unsigned long start, unsigned long end, > > unsigned long reuse); > > +int vmemmap_remap_alloc(unsigned long start, unsigned long end, > > + unsigned long reuse, gfp_t gfp_mask); > > > > void *sparse_buffer_alloc(unsigned long size); > > struct page * __populate_section_memmap(unsigned long pfn, > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > > index 4cfca27c6d32..bcf856974c48 100644 > > --- a/mm/hugetlb.c > > +++ b/mm/hugetlb.c > > @@ -1305,37 +1305,68 @@ static inline void destroy_compound_gigantic_page(struct page *page, > > unsigned int order) { } > > #endif > > > > -static void update_and_free_page(struct hstate *h, struct page *page) > > +static int update_and_free_page(struct hstate *h, struct page *page) > > + __releases(&hugetlb_lock) __acquires(&hugetlb_lock) > > { > > int i; > > + int nid = page_to_nid(page); > > > > if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported()) > > - return; > > + return 0; > > > > h->nr_huge_pages--; > > - h->nr_huge_pages_node[page_to_nid(page)]--; > > + h->nr_huge_pages_node[nid]--; > > + VM_BUG_ON_PAGE(hugetlb_cgroup_from_page(page), page); > > + VM_BUG_ON_PAGE(hugetlb_cgroup_from_page_rsvd(page), page); > > + set_compound_page_dtor(page, NULL_COMPOUND_DTOR); > > + set_page_refcounted(page); > > I think you added the set_page_refcounted() because the huge page will > appear as just a compound page without a reference after dropping the > hugetlb lock? Right. > It might be better to set the reference before modifying > the destructor. Otherwise, page scanning code could find the non-hugetlb > compound page with no reference. I could not find any code where this > would be a problem, but I think it would be safer to set the reference > first. Make sense to me. It is better to set the refcount first. > > > + spin_unlock(&hugetlb_lock); > > I really like the way this code is structured. It is much simpler than > previous versions with retries or workqueue. There is nothing wrong with > always dropping the lock here. However, I wonder if we should think about > optimizing for the case where this feature is not enabled and we are not > freeing a 1G huge page. I suspect this will be the most common case for > some time, and there is no need to drop the lock in this case. > > Please do not change the code based on my comment. I just wanted to bring > this up for thought. At least make sense to me. It may take a long time to free a 1G huge page. Dropping the lock may be a good choice. But I also want to listen to Oscar and Michal's opinion on this. > > Is it as simple as checking? > if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h)) > spin_unlock(&hugetlb_lock); > > /* before return */ > if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h)) > spin_lock(&hugetlb_lock); > > > + > > + if (alloc_huge_page_vmemmap(h, page)) { > > + int zeroed; > > + > > + spin_lock(&hugetlb_lock); > > + INIT_LIST_HEAD(&page->lru); > > + set_compound_page_dtor(page, HUGETLB_PAGE_DTOR); > > + h->nr_huge_pages++; > > + h->nr_huge_pages_node[nid]++; > > + > > + /* > > + * If we cannot allocate vmemmap pages, just refuse to free the > > + * page and put the page back on the hugetlb free list and treat > > + * as a surplus page. > > + */ > > + h->surplus_huge_pages++; > > + h->surplus_huge_pages_node[nid]++; > > + > > + /* > > + * This page is now managed by the hugetlb allocator and has > > + * no users -- drop the last reference. > > + */ > > + zeroed = put_page_testzero(page); > > + VM_BUG_ON_PAGE(!zeroed, page); > > + arch_clear_hugepage_flags(page); > > + enqueue_huge_page(h, page); > > + > > + return -ENOMEM; > > + } > > + > > for (i = 0; i < pages_per_huge_page(h); i++) { > > page[i].flags &= ~(1 << PG_locked | 1 << PG_error | > > 1 << PG_referenced | 1 << PG_dirty | > > 1 << PG_active | 1 << PG_private | > > 1 << PG_writeback); > > } > > - VM_BUG_ON_PAGE(hugetlb_cgroup_from_page(page), page); > > - VM_BUG_ON_PAGE(hugetlb_cgroup_from_page_rsvd(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); > > } else { > > __free_pages(page, huge_page_order(h)); > > } > > + > > + spin_lock(&hugetlb_lock); > > + > > + return 0; > > } > > > > struct hstate *size_to_hstate(unsigned long size) > > @@ -1403,9 +1434,9 @@ static void __free_huge_page(struct page *page) > > } else if (h->surplus_huge_pages_node[nid]) { > > /* remove the page from active list */ > > list_del(&page->lru); > > - update_and_free_page(h, page); > > h->surplus_huge_pages--; > > h->surplus_huge_pages_node[nid]--; > > + update_and_free_page(h, page); > > } else { > > arch_clear_hugepage_flags(page); > > enqueue_huge_page(h, page); > > @@ -1693,6 +1724,7 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed, > > struct page *page = > > list_entry(h->hugepage_freelists[node].next, > > struct page, lru); > > + ClearHPageFreed(page); > > Quick question. Is this change directly related to the vmemmap changes, > or is it a cleanup that you noticed? Just a cleanup. Maybe there should be a separate patch for this. > > > list_del(&page->lru); > > h->free_huge_pages--; > > h->free_huge_pages_node[node]--; > > @@ -1700,8 +1732,7 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed, > > h->surplus_huge_pages--; > > h->surplus_huge_pages_node[node]--; > > } > > - update_and_free_page(h, page); > > - ret = 1; > > + ret = !update_and_free_page(h, page); > > break; > > } > > } > > @@ -1714,10 +1745,14 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed, > > * nothing for in-use hugepages and non-hugepages. > > * This function returns values like below: > > * > > - * -EBUSY: failed to dissolved free hugepages or the hugepage is in-use > > - * (allocated or reserved.) > > - * 0: successfully dissolved free hugepages or the page is not a > > - * hugepage (considered as already dissolved) > > + * -ENOMEM: failed to allocate vmemmap pages to free the freed hugepages > > + * when the system is under memory pressure and the feature of > > + * freeing unused vmemmap pages associated with each hugetlb page > > + * is enabled. > > + * -EBUSY: failed to dissolved free hugepages or the hugepage is in-use > > + * (allocated or reserved.) > > + * 0: successfully dissolved free hugepages or the page is not a > > + * hugepage (considered as already dissolved) > > */ > > int dissolve_free_huge_page(struct page *page) > > { > > @@ -1768,12 +1803,14 @@ int dissolve_free_huge_page(struct page *page) > > SetPageHWPoison(page); > > ClearPageHWPoison(head); > > } > > + ClearHPageFreed(page); > > list_del(&head->lru); > > h->free_huge_pages--; > > h->free_huge_pages_node[nid]--; > > h->max_huge_pages--; > > - update_and_free_page(h, head); > > - rc = 0; > > + rc = update_and_free_page(h, head); > > + if (rc) > > + h->max_huge_pages++; > > Since update_and_free_page failed, the number of surplus pages was > incremented. Surplus pages are the number of pages greater than > max_huge_pages. Since we are incrementing max_huge_pages, we should > decrement (undo) the addition to surplus_huge_pages and > surplus_huge_pages_node[nid]. So, I think we want > h->surplus_huge_pages--; > h->surplus_huge_pages_node[nid]--; > here as well. You are right. Thanks for reminding me of this. > > > } > > out: > > spin_unlock(&hugetlb_lock); > > In previous version of this patch series, we discussed and refined the > vmemmap manipulation routines below. They still look good to me. > > In general, I like the approach taken in this patch. Hopefully, others > will comment and we can move the series forward. > -- > Mike Kravetz > > > diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c > > index 0209b736e0b4..29a3380f3b20 100644 > > --- a/mm/hugetlb_vmemmap.c > > +++ b/mm/hugetlb_vmemmap.c > > @@ -198,6 +198,28 @@ static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h) > > return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT; > > } > > > > +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head) > > +{ > > + unsigned long vmemmap_addr = (unsigned long)head; > > + unsigned long vmemmap_end, vmemmap_reuse; > > + > > + if (!free_vmemmap_pages_per_hpage(h)) > > + return 0; > > + > > + vmemmap_addr += RESERVE_VMEMMAP_SIZE; > > + vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h); > > + vmemmap_reuse = vmemmap_addr - PAGE_SIZE; > > + /* > > + * The pages which the vmemmap virtual address range [@vmemmap_addr, > > + * @vmemmap_end) are mapped to are freed to the buddy allocator, and > > + * the range is mapped to the page which @vmemmap_reuse is mapped to. > > + * When a HugeTLB page is freed to the buddy allocator, previously > > + * discarded vmemmap pages must be allocated and remapping. > > + */ > > + return vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse, > > + GFP_KERNEL | __GFP_NORETRY | __GFP_THISNODE); > > +} > > + > > void free_huge_page_vmemmap(struct hstate *h, struct page *head) > > { > > unsigned long vmemmap_addr = (unsigned long)head; > > diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h > > index 6923f03534d5..e5547d53b9f5 100644 > > --- a/mm/hugetlb_vmemmap.h > > +++ b/mm/hugetlb_vmemmap.h > > @@ -11,8 +11,14 @@ > > #include > > > > #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP > > +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head); > > void free_huge_page_vmemmap(struct hstate *h, struct page *head); > > #else > > +static inline int alloc_huge_page_vmemmap(struct hstate *h, struct page *head) > > +{ > > + return 0; > > +} > > + > > static inline void free_huge_page_vmemmap(struct hstate *h, struct page *head) > > { > > } > > diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c > > index d3076a7a3783..60fc6cd6cd23 100644 > > --- a/mm/sparse-vmemmap.c > > +++ b/mm/sparse-vmemmap.c > > @@ -40,7 +40,8 @@ > > * @remap_pte: called for each lowest-level entry (PTE). > > * @reuse_page: the page which is reused for the tail vmemmap pages. > > * @reuse_addr: the virtual address of the @reuse_page page. > > - * @vmemmap_pages: the list head of the vmemmap pages that can be freed. > > + * @vmemmap_pages: the list head of the vmemmap pages that can be freed > > + * or is mapped from. > > */ > > struct vmemmap_remap_walk { > > void (*remap_pte)(pte_t *pte, unsigned long addr, > > @@ -237,6 +238,78 @@ void vmemmap_remap_free(unsigned long start, unsigned long end, > > free_vmemmap_page_list(&vmemmap_pages); > > } > > > > +static void vmemmap_restore_pte(pte_t *pte, unsigned long addr, > > + struct vmemmap_remap_walk *walk) > > +{ > > + pgprot_t pgprot = PAGE_KERNEL; > > + struct page *page; > > + void *to; > > + > > + BUG_ON(pte_page(*pte) != walk->reuse_page); > > + > > + page = list_first_entry(walk->vmemmap_pages, struct page, lru); > > + list_del(&page->lru); > > + to = page_to_virt(page); > > + copy_page(to, (void *)walk->reuse_addr); > > + > > + set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot)); > > +} > > + > > +static int alloc_vmemmap_page_list(unsigned long start, unsigned long end, > > + gfp_t gfp_mask, struct list_head *list) > > +{ > > + unsigned long nr_pages = (end - start) >> PAGE_SHIFT; > > + int nid = page_to_nid((struct page *)start); > > + struct page *page, *next; > > + > > + while (nr_pages--) { > > + page = alloc_pages_node(nid, gfp_mask, 0); > > + if (!page) > > + goto out; > > + list_add_tail(&page->lru, list); > > + } > > + > > + return 0; > > +out: > > + list_for_each_entry_safe(page, next, list, lru) > > + __free_pages(page, 0); > > + return -ENOMEM; > > +} > > + > > +/** > > + * vmemmap_remap_alloc - remap the vmemmap virtual address range [@start, end) > > + * to the page which is from the @vmemmap_pages > > + * respectively. > > + * @start: start address of the vmemmap virtual address range that we want > > + * to remap. > > + * @end: end address of the vmemmap virtual address range that we want to > > + * remap. > > + * @reuse: reuse address. > > + * @gpf_mask: GFP flag for allocating vmemmap pages. > > + */ > > +int vmemmap_remap_alloc(unsigned long start, unsigned long end, > > + unsigned long reuse, gfp_t gfp_mask) > > +{ > > + LIST_HEAD(vmemmap_pages); > > + struct vmemmap_remap_walk walk = { > > + .remap_pte = vmemmap_restore_pte, > > + .reuse_addr = reuse, > > + .vmemmap_pages = &vmemmap_pages, > > + }; > > + > > + /* See the comment in the vmemmap_remap_free(). */ > > + BUG_ON(start - reuse != PAGE_SIZE); > > + > > + might_sleep_if(gfpflags_allow_blocking(gfp_mask)); > > + > > + if (alloc_vmemmap_page_list(start, end, gfp_mask, &vmemmap_pages)) > > + return -ENOMEM; > > + > > + vmemmap_remap_range(reuse, end, &walk); > > + > > + return 0; > > +} > > + > > /* > > * Allocate a block of memory to be used to back the virtual memory map > > * or to back the page tables that are used to create the mapping. > >