From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg1-f198.google.com (mail-pg1-f198.google.com [209.85.215.198]) by kanga.kvack.org (Postfix) with ESMTP id 301496B7336 for ; Wed, 5 Sep 2018 08:55:29 -0400 (EDT) Received: by mail-pg1-f198.google.com with SMTP id f32-v6so3618367pgm.14 for ; Wed, 05 Sep 2018 05:55:29 -0700 (PDT) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id u83-v6sor413247pfj.72.2018.09.05.05.55.27 for (Google Transport Security); Wed, 05 Sep 2018 05:55:27 -0700 (PDT) Date: Wed, 5 Sep 2018 15:55:22 +0300 From: "Kirill A. Shutemov" Subject: Re: [PATCH] mm: hugepage: mark splitted page dirty when needed Message-ID: <20180905125522.x2puwfn5sr2zo3go@kshutemo-mobl1> References: <20180904075510.22338-1-peterx@redhat.com> <20180904080115.o2zj4mlo7yzjdqfl@kshutemo-mobl1> <20180905073037.GA23021@xz-x1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180905073037.GA23021@xz-x1> Sender: owner-linux-mm@kvack.org List-ID: To: Peter Xu Cc: Zi Yan , linux-kernel@vger.kernel.org, Andrea Arcangeli , Andrew Morton , Michal Hocko , Huang Ying , Dan Williams , Naoya Horiguchi , =?utf-8?B?SsOpcsO0bWU=?= Glisse , "Aneesh Kumar K.V" , Konstantin Khlebnikov , Souptick Joarder , linux-mm@kvack.org On Wed, Sep 05, 2018 at 03:30:37PM +0800, Peter Xu wrote: > On Tue, Sep 04, 2018 at 10:00:28AM -0400, Zi Yan wrote: > > On 4 Sep 2018, at 4:01, Kirill A. Shutemov wrote: > > > > > On Tue, Sep 04, 2018 at 03:55:10PM +0800, Peter Xu wrote: > > >> When splitting a huge page, we should set all small pages as dirty if > > >> the original huge page has the dirty bit set before. Otherwise we'll > > >> lose the original dirty bit. > > > > > > We don't lose it. It got transfered to struct page flag: > > > > > > if (pmd_dirty(old_pmd)) > > > SetPageDirty(page); > > > > > > > Plus, when split_huge_page_to_list() splits a THP, its subroutine __split_huge_page() > > propagates the dirty bit in the head page flag to all subpages in __split_huge_page_tail(). > > Hi, Kirill, Zi, > > Thanks for your responses! > > Though in my test the huge page seems to be splitted not by > split_huge_page_to_list() but by explicit calls to > change_protection(). The stack looks like this (again, this is a > customized kernel, and I added an explicit dump_stack() there): > > kernel: dump_stack+0x5c/0x7b > kernel: __split_huge_pmd+0x192/0xdc0 > kernel: ? update_load_avg+0x8b/0x550 > kernel: ? update_load_avg+0x8b/0x550 > kernel: ? account_entity_enqueue+0xc5/0xf0 > kernel: ? enqueue_entity+0x112/0x650 > kernel: change_protection+0x3a2/0xab0 > kernel: mwriteprotect_range+0xdd/0x110 > kernel: userfaultfd_ioctl+0x50b/0x1210 > kernel: ? do_futex+0x2cf/0xb20 > kernel: ? tty_write+0x1d2/0x2f0 > kernel: ? do_vfs_ioctl+0x9f/0x610 > kernel: do_vfs_ioctl+0x9f/0x610 > kernel: ? __x64_sys_futex+0x88/0x180 > kernel: ksys_ioctl+0x70/0x80 > kernel: __x64_sys_ioctl+0x16/0x20 > kernel: do_syscall_64+0x55/0x150 > kernel: entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > At the very time the userspace is sending an UFFDIO_WRITEPROTECT ioctl > to kernel space, which is handled by mwriteprotect_range(). In case > you'd like to refer to the kernel, it's basically this one from > Andrea's (with very trivial changes): > > https://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git userfault > > So... do we have two paths to split the huge pages separately? We have two entiries that can be split: page table enties and underlying compound page. split_huge_pmd() (and variants of it) split the PMD entry into a PTE page table. It doens't touch underlying compound page. The page still can be mapped in other place as huge. split_huge_page() (and ivariants of it) split compound page into a number of 4k (or whatever PAGE_SIZE is). The operation requires splitting all PMD, but not other way around. > > Another (possibly very naive) question is: could any of you hint me > how the page dirty bit is finally applied to the PTEs? These two > dirty flags confused me for a few days already (the SetPageDirty() one > which sets the page dirty flag, and the pte_mkdirty() which sets that > onto the real PTEs). Dirty bit from page table entries transferes to sturct page flug and used for decision making in reclaim path. -- Kirill A. Shutemov