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=-8.8 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_NEOMUTT 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 4CF0AC48BD4 for ; Tue, 25 Jun 2019 10:34:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 12BAD21655 for ; Tue, 25 Jun 2019 10:34:10 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=shutemov-name.20150623.gappssmtp.com header.i=@shutemov-name.20150623.gappssmtp.com header.b="jj94sRMw" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728895AbfFYKeI (ORCPT ); Tue, 25 Jun 2019 06:34:08 -0400 Received: from mail-ed1-f67.google.com ([209.85.208.67]:45823 "EHLO mail-ed1-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726414AbfFYKeI (ORCPT ); Tue, 25 Jun 2019 06:34:08 -0400 Received: by mail-ed1-f67.google.com with SMTP id a14so26371172edv.12 for ; Tue, 25 Jun 2019 03:34:06 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=shutemov-name.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=dyBqPZUv3bZgPsIAy4Om+lWx9RTnalxJxxYyGRhb+b8=; b=jj94sRMwxHfsAUNS77tobLmrGsmyHnW5hmCw63c/XJdcfJqexoIOOF1SPKsDIzlKvr gMc/BFnMfUq/q7qu4iMMNuV8IfU789WADipq1XSLPxVdfkKaDamkqc+MC8hvJ5S+fbGc XSPi2Kn84De96ySR3FL/EUPkoCILydnABHKQf5NtqzXdmKkM7G3XTxThRlHOmU9Hb390 mhyGnb5ooPP/kpAwaow3xUyHJz+ULrH0/DljWQ3nf+wV3kJEFIkSOeLBHOBXv3yxkf7x n1TBMuh63pdl8/auIcnf22aAZXeipV13XS9+StRnXIlbnZhQI0Ab92aVOxqZHvy6EiGA sayA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=dyBqPZUv3bZgPsIAy4Om+lWx9RTnalxJxxYyGRhb+b8=; b=ZSV9mvXFV9Ho/2IiehAnC8Fzc0twaf5TsOy21UkGHEUf62SGgM2mBgwGk+Fbawg/uW g0+XhSRYybfYujvZL8+h2xSgiWVYIZ6P+ixyWRhK/LNxSduo4oNgn0PLiNcloJlgupZs nAM0c2O7+/4f1loGcS+3dvbd8s40oBwbXCJp4qyY/U1vEkrT/67GKVFzPbblcQ5wzT7h 5cJnlNT9CPdVsZ2orcsOWTjKYzOBiB7CUglKWy++y7CsSYUas9Q7KbulBWtwN1TfnxM1 lB6s1URUSPyCQ8iuQDIC5/sYyQfVbw0YBUd1a11F6+BchzvO2XhJ/+nO0xL81Tky+nhc ysMA== X-Gm-Message-State: APjAAAWKvkiL7jO0Kb33ZuMxnXP6qe8XZIWWtYuAJ9EJz5D490gSL/xr F4YPG3a5ARn0hq/sg9hzIRMDjQ== X-Google-Smtp-Source: APXvYqxmHgIO7ovcuIBSsktbpczJIULLinPwuuA04U070XZir6Fm6cu4yfD0Lej+nMiAzHFRA7CaiQ== X-Received: by 2002:a50:eb8b:: with SMTP id y11mr104789190edr.154.1561458845918; Tue, 25 Jun 2019 03:34:05 -0700 (PDT) Received: from box.localdomain ([86.57.175.117]) by smtp.gmail.com with ESMTPSA id b21sm1179003ejb.12.2019.06.25.03.34.04 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 25 Jun 2019 03:34:05 -0700 (PDT) Received: by box.localdomain (Postfix, from userid 1000) id D143D1043B9; Tue, 25 Jun 2019 13:34:04 +0300 (+03) Date: Tue, 25 Jun 2019 13:34:04 +0300 From: "Kirill A. Shutemov" To: Song Liu Cc: LKML , Linux-MM , "matthew.wilcox@oracle.com" , "kirill.shutemov@linux.intel.com" , "peterz@infradead.org" , "oleg@redhat.com" , "rostedt@goodmis.org" , Kernel Team , "william.kucharski@oracle.com" Subject: Re: [PATCH v6 5/6] khugepaged: enable collapse pmd for pte-mapped THP Message-ID: <20190625103404.3ypizksnpopcgwdk@box> References: <20190623054829.4018117-1-songliubraving@fb.com> <20190623054829.4018117-6-songliubraving@fb.com> <20190624131934.m6gbktixyykw65ws@box> <24FB1072-E355-4F9D-855F-337C855C9AF9@fb.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <24FB1072-E355-4F9D-855F-337C855C9AF9@fb.com> User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jun 24, 2019 at 02:25:42PM +0000, Song Liu wrote: > > > > On Jun 24, 2019, at 6:19 AM, Kirill A. Shutemov wrote: > > > > On Sat, Jun 22, 2019 at 10:48:28PM -0700, Song Liu wrote: > >> khugepaged needs exclusive mmap_sem to access page table. When it fails > >> to lock mmap_sem, the page will fault in as pte-mapped THP. As the page > >> is already a THP, khugepaged will not handle this pmd again. > >> > >> This patch enables the khugepaged to retry retract_page_tables(). > >> > >> A new flag AS_COLLAPSE_PMD is introduced to show the address_space may > >> contain pte-mapped THPs. When khugepaged fails to trylock the mmap_sem, > >> it sets AS_COLLAPSE_PMD. Then, at a later time, khugepaged will retry > >> compound pages in this address_space. > >> > >> Since collapse may happen at an later time, some pages may already fault > >> in. To handle these pages properly, it is necessary to prepare the pmd > >> before collapsing. prepare_pmd_for_collapse() is introduced to prepare > >> the pmd by removing rmap, adjusting refcount and mm_counter. > >> > >> prepare_pmd_for_collapse() also double checks whether all ptes in this > >> pmd are mapping to the same THP. This is necessary because some subpage > >> of the THP may be replaced, for example by uprobe. In such cases, it > >> is not possible to collapse the pmd, so we fall back. > >> > >> Signed-off-by: Song Liu > >> --- > >> include/linux/pagemap.h | 1 + > >> mm/khugepaged.c | 69 +++++++++++++++++++++++++++++++++++------ > >> 2 files changed, 60 insertions(+), 10 deletions(-) > >> > >> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h > >> index 9ec3544baee2..eac881de2a46 100644 > >> --- a/include/linux/pagemap.h > >> +++ b/include/linux/pagemap.h > >> @@ -29,6 +29,7 @@ enum mapping_flags { > >> AS_EXITING = 4, /* final truncate in progress */ > >> /* writeback related tags are not used */ > >> AS_NO_WRITEBACK_TAGS = 5, > >> + AS_COLLAPSE_PMD = 6, /* try collapse pmd for THP */ > >> }; > >> > >> /** > >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c > >> index a4f90a1b06f5..9b980327fd9b 100644 > >> --- a/mm/khugepaged.c > >> +++ b/mm/khugepaged.c > >> @@ -1254,7 +1254,47 @@ static void collect_mm_slot(struct mm_slot *mm_slot) > >> } > >> > >> #if defined(CONFIG_SHMEM) && defined(CONFIG_TRANSPARENT_HUGE_PAGECACHE) > >> -static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff) > >> + > >> +/* return whether the pmd is ready for collapse */ > >> +bool prepare_pmd_for_collapse(struct vm_area_struct *vma, pgoff_t pgoff, > >> + struct page *hpage, pmd_t *pmd) > >> +{ > >> + unsigned long haddr = page_address_in_vma(hpage, vma); > >> + unsigned long addr; > >> + int i, count = 0; > >> + > >> + /* step 1: check all mapped PTEs are to this huge page */ > >> + for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) { > >> + pte_t *pte = pte_offset_map(pmd, addr); > >> + > >> + if (pte_none(*pte)) > >> + continue; > >> + > >> + if (hpage + i != vm_normal_page(vma, addr, *pte)) > >> + return false; > >> + count++; > >> + } > >> + > >> + /* step 2: adjust rmap */ > >> + for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) { > >> + pte_t *pte = pte_offset_map(pmd, addr); > >> + struct page *page; > >> + > >> + if (pte_none(*pte)) > >> + continue; > >> + page = vm_normal_page(vma, addr, *pte); > >> + page_remove_rmap(page, false); > >> + } > >> + > >> + /* step 3: set proper refcount and mm_counters. */ > >> + page_ref_sub(hpage, count); > >> + add_mm_counter(vma->vm_mm, mm_counter_file(hpage), -count); > >> + return true; > >> +} > >> + > >> +extern pid_t sysctl_dump_pt_pid; > >> +static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff, > >> + struct page *hpage) > >> { > >> struct vm_area_struct *vma; > >> unsigned long addr; > >> @@ -1273,21 +1313,21 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff) > >> pmd = mm_find_pmd(vma->vm_mm, addr); > >> if (!pmd) > >> continue; > >> - /* > >> - * We need exclusive mmap_sem to retract page table. > >> - * If trylock fails we would end up with pte-mapped THP after > >> - * re-fault. Not ideal, but it's more important to not disturb > >> - * the system too much. > >> - */ > >> if (down_write_trylock(&vma->vm_mm->mmap_sem)) { > >> spinlock_t *ptl = pmd_lock(vma->vm_mm, pmd); > >> - /* assume page table is clear */ > >> + > >> + if (!prepare_pmd_for_collapse(vma, pgoff, hpage, pmd)) { > >> + spin_unlock(ptl); > >> + up_write(&vma->vm_mm->mmap_sem); > >> + continue; > >> + } > >> _pmd = pmdp_collapse_flush(vma, addr, pmd); > >> spin_unlock(ptl); > >> up_write(&vma->vm_mm->mmap_sem); > >> mm_dec_nr_ptes(vma->vm_mm); > >> pte_free(vma->vm_mm, pmd_pgtable(_pmd)); > >> - } > >> + } else > >> + set_bit(AS_COLLAPSE_PMD, &mapping->flags); > >> } > >> i_mmap_unlock_write(mapping); > >> } > >> @@ -1561,7 +1601,7 @@ static void collapse_file(struct mm_struct *mm, > >> /* > >> * Remove pte page tables, so we can re-fault the page as huge. > >> */ > >> - retract_page_tables(mapping, start); > >> + retract_page_tables(mapping, start, new_page); > >> *hpage = NULL; > >> > >> khugepaged_pages_collapsed++; > >> @@ -1622,6 +1662,7 @@ static void khugepaged_scan_file(struct mm_struct *mm, > >> int present, swap; > >> int node = NUMA_NO_NODE; > >> int result = SCAN_SUCCEED; > >> + bool collapse_pmd = false; > >> > >> present = 0; > >> swap = 0; > >> @@ -1640,6 +1681,14 @@ static void khugepaged_scan_file(struct mm_struct *mm, > >> } > >> > >> if (PageTransCompound(page)) { > >> + if (collapse_pmd || > >> + test_and_clear_bit(AS_COLLAPSE_PMD, > >> + &mapping->flags)) { > > > > Who said it's the only PMD range that's subject to collapse? The bit has > > to be per-PMD, not per-mapping. > > I didn't assume this is the only PMD range that subject to collapse. > So once we found AS_COLLAPSE_PMD, it will continue scan the whole mapping: > retract_page_tables(), then continue. I still don't get it. Assume we have two ranges that subject to collapse. khugepaged_scan_file() sees and clears AS_COLLAPSE_PMD. Tries to collapse the first range, fails and set the bit again. khugepaged_scan_file() sees the second range, clears the bit, but this time collapse is successful: the bit is still not set, but it should be. > > I beleive we can store the bit in struct page of PTE page table, clearing > > it if we've mapped anyting that doesn't belong to there from fault path. > > > > And in general this calls for more substantial re-design for khugepaged: > > we might want to split if into two different kernel threads. One works on > > collapsing small pages into compound and the other changes virtual address > > space to map the page as PMD. > > I had almost same idea of splitting into two threads. Or ask one thread to > scan two lists: one for pages to collapse, the other for page tables to > map as PMD. However, that does need substantial re-design. > > On the other hand, this patch is much simpler and does the work fine. It is > not optimal, as it scans the whole mapping for opportunity in just one PMD > range. But the overhead is not in any hot path, so it just works. The extra > double check in prepare_pmd_for_collapse() makes sure it will not collapse > wrong pages into pmd mapped. > > Overall, I believe this is an accurate solution for the problem. We can > further improve it. But I really hope to do the improvements after these > two patchsets get in. I don't think AS_COLLAPSE_PMD adds much value. I don't see it relieably serving the purpose. -- Kirill A. Shutemov