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Tue, 22 Sep 2020 16:52:25 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.40.192.146]) by smtp.corp.redhat.com (Postfix) with SMTP id 7058119D61; Tue, 22 Sep 2020 16:52:18 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Tue, 22 Sep 2020 18:52:25 +0200 (CEST) Date: Tue, 22 Sep 2020 18:52:17 +0200 From: Oleg Nesterov To: Peter Xu Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Linus Torvalds , Michal Hocko , Kirill Shutemov , Jann Horn , Kirill Tkhai , Hugh Dickins , Leon Romanovsky , Jan Kara , John Hubbard , Christoph Hellwig , Andrew Morton , Jason Gunthorpe , Andrea Arcangeli Subject: Re: [PATCH 4/5] mm: Do early cow for pinned pages during fork() for ptes Message-ID: <20200922165216.GF11679@redhat.com> References: <20200921211744.24758-1-peterx@redhat.com> <20200921212028.25184-1-peterx@redhat.com> <20200922114839.GC11679@redhat.com> <20200922124013.GD11679@redhat.com> <20200922155842.GG19098@xz-x1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200922155842.GG19098@xz-x1> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 09/22, Peter Xu wrote: > > On Tue, Sep 22, 2020 at 02:40:14PM +0200, Oleg Nesterov wrote: > > On 09/22, Oleg Nesterov wrote: > > > > > > On 09/21, Peter Xu wrote: > > > > > > > > @@ -859,6 +989,25 @@ static int copy_pte_range(struct mm_struct *dst_mm, struct mm_struct *src_mm, > > > > spin_needbreak(src_ptl) || spin_needbreak(dst_ptl)) > > > > break; > > > > } > > > > + > > > > + if (unlikely(data.cow_new_page)) { > > > > + /* > > > > + * If cow_new_page set, we must be at the 2nd round of > > > > + * a previous COPY_MM_BREAK_COW. Try to arm the new > > > > + * page now. Note that in all cases page_break_cow() > > > > + * will properly release the objects in copy_mm_data. > > > > + */ > > > > + WARN_ON_ONCE(copy_ret != COPY_MM_BREAK_COW); > > > > + if (pte_install_copied_page(dst_mm, new, src_pte, > > > > + dst_pte, addr, rss, > > > > + &data)) { > > > > + /* We installed the pte successfully; move on */ > > > > + progress++; > > > > + continue; > > > > > > I'm afraid I misread this patch too ;) > > > > > > But it seems to me in this case the main loop can really "leak" > > > COPY_MM_BREAK_COW. Suppose the the next 31 pte's are pte_none() and > > > need_resched() is true. > > > > > > No? > > I still think it's a no... > > Note that now we'll reset "progress" every time before the do loop, so we'll > never reach need_resched() (since progress<32) before pte_install_copied_page() > when needed. Yes. But copy_ret is still COPY_MM_BREAK_COW after pte_install_copied_page(). Now suppose that the next 31 pte's are pte_none(), progress will be incremented every time. > I explicitly put the pte_install_copied_page() into the loop just... ... > > progress = 0; > > + if (unlikely(copy_ret == COPY_MM_BREAK_COW)) { > > + /* > > + * Note that in all cases pte_install_copied_page() > > + * will properly release the objects in copy_mm_data. > > + */ > > + copy_ret = COPY_MM_DONE; > > + if (pte_install_copied_page(dst_mm, new, src_pte, > > + dst_pte, addr, rss, > > + &data)) { > > + /* We installed the pte successfully; move on */ > > + progress++; > > + goto next; > > ... to avoid jumps like this because I think it's really tricky. :) To me it looks better before the main loop because we know that data.cow_new_page != NULL is only possible at the 1st iterattion after restart ;) But I agree, this is subjective, please ignore. However, I still think it is better to rely on the copy_ret == COPY_MM_BREAK_COW check rather than data.cow_new_page != NULL. > > case COPY_MM_SWAP_CONT: > > if (add_swap_count_continuation(data.entry, GFP_KERNEL) < 0) > > return -ENOMEM; > > - break; > > + copy_ret = COPY_MM_DONE; > > Kind of a continuation of the discussion from previous patch - I think we'd > better reset copy_ret not only for this case, but move it after the switch > (just in case there'll be new ones). The new BREAK_COW uses goto so it's quite > special. > > > + goto again; > > I feel like this could go wrong without the "addr != end" check later, when > this is the last pte to check. How? We know that copy_one_pte() failed and returned COPY_MM_SWAP_CONT before addr = end. And this matters "case COPY_MM_BREAK_COW" below which does "goto again" without the "addr != end" check. Oleg.