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[70.53.122.15]) by smtp.gmail.com with ESMTPSA id x126sm12083779qkb.101.2020.09.22.11.34.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 22 Sep 2020 11:34:40 -0700 (PDT) Date: Tue, 22 Sep 2020 14:34:38 -0400 From: Peter Xu To: Oleg Nesterov 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: <20200922183438.GL19098@xz-x1> 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> <20200922165216.GF11679@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200922165216.GF11679@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Sep 22, 2020 at 06:52:17PM +0200, Oleg Nesterov wrote: > 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. Yes, I think you're right - I'll need to reset that. > > > 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. Thanks. For simplicity, I'll keep the code majorly as is. But I'm still open to change if e.g. someone else still perfers the other way. > 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. Yes. Logically we should check both, but now as I'm written it as: if (unlikely(data.cow_new_page)) { WARN_ON_ONCE(copy_ret != COPY_MM_BREAK_COW); ... } I think it's even safer because it's actually checking both, but also warn if only cow_new_page is set, which should never happen anyways. Or I can also do it in inverted order if you think better: if (unlikely(copy_ret == COPY_MM_BREAK_COW)) { WARN_ON_ONCE(!data.cow_new_page); ... } > > > > 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. I think you're right, again. :) Thanks, -- Peter Xu