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Tue, 22 Sep 2020 15:48:52 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.40.192.146]) by smtp.corp.redhat.com (Postfix) with SMTP id 4DA6981C41; Tue, 22 Sep 2020 15:48:47 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Tue, 22 Sep 2020 17:48:52 +0200 (CEST) Date: Tue, 22 Sep 2020 17:48:46 +0200 From: Oleg Nesterov To: Peter Xu Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Jason Gunthorpe , Andrew Morton , Jan Kara , Michal Hocko , Kirill Tkhai , Kirill Shutemov , Hugh Dickins , Christoph Hellwig , Andrea Arcangeli , John Hubbard , Leon Romanovsky , Linus Torvalds , Jann Horn Subject: Re: [PATCH 3/5] mm: Rework return value for copy_one_pte() Message-ID: <20200922154845.GE11679@redhat.com> References: <20200921211744.24758-1-peterx@redhat.com> <20200921211744.24758-4-peterx@redhat.com> <20200922100840.GA11679@redhat.com> <20200922101815.GB11679@redhat.com> <20200922153612.GF19098@xz-x1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200922153612.GF19098@xz-x1> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 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 12:18:16PM +0200, Oleg Nesterov wrote: > > On 09/22, Oleg Nesterov wrote: > > > > > > On 09/21, Peter Xu wrote: > > > > > > > > @@ -866,13 +877,18 @@ static int copy_pte_range(struct mm_struct *dst_mm, struct mm_struct *src_mm, > > > > pte_unmap_unlock(orig_dst_pte, dst_ptl); > > > > cond_resched(); > > > > > > > > - if (entry.val) { > > > > - if (add_swap_count_continuation(entry, GFP_KERNEL) < 0) > > > > + switch (copy_ret) { > > > > + case COPY_MM_SWAP_CONT: > > > > + if (add_swap_count_continuation(data.entry, GFP_KERNEL) < 0) > > > > return -ENOMEM; > > > > - progress = 0; > > > > + break; > > > > > > Note that you didn't clear copy_ret, it is still COPY_MM_SWAP_CONT, > > > > > > > + default: > > > > + break; > > > > } > > > > + > > > > if (addr != end) > > > > goto again; > > > > > > After that the main loop can stop again because of need_resched(), and > > > in this case add_swap_count_continuation(data.entry) will be called again? > > > > No, this is not possible, copy_one_pte() should be called at least once, > > progress = 0 before restart. Sorry for noise. > > Oh wait, I think you're right... when we get a COPY_MM_SWAP_CONT, goto "again", > then if there're 32 pte_none() ptes _plus_ an need_resched(), then we might > reach again at the same add_swap_count_continuation() with the same swp entry. Yes, please see my reply to 4/5 ;) > However since I didn't change this logic in this patch, it probably means this > bug is also in the original code before this series... I'm thinking maybe I > should prepare a standalone patch to clear the swp_entry_t and cc stable. Well, if copy_one_pte(src_pte) hits a swap entry and returns entry.val != 0, then pte_none(*src_pte) is not possible after restart? This means that copy_one_pte() will be called at least once. So _think_ that the current code is fine, but I can be easily wrong and I agree this doesn't look clean. Oleg.