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Tue, 22 Sep 2020 12:40:24 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.40.192.146]) by smtp.corp.redhat.com (Postfix) with SMTP id EF5DB5DD99; Tue, 22 Sep 2020 12:40:15 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Tue, 22 Sep 2020 14:40:24 +0200 (CEST) Date: Tue, 22 Sep 2020 14:40:14 +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: <20200922124013.GD11679@redhat.com> References: <20200921211744.24758-1-peterx@redhat.com> <20200921212028.25184-1-peterx@redhat.com> <20200922114839.GC11679@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200922114839.GC11679@redhat.com> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 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, 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? If yes, perhaps we can simplify the copy_ret/cow_new_page logic and make it more explicit? Something like below, on top of this patch... Oleg. --- x/mm/memory.c +++ x/mm/memory.c @@ -704,17 +704,6 @@ }; }; -static inline void page_release_cow(struct copy_mm_data *data) -{ - /* The old page should only be released in page_duplicate() */ - WARN_ON_ONCE(data->cow_old_page); - - if (data->cow_new_page) { - put_page(data->cow_new_page); - data->cow_new_page = NULL; - } -} - /* * Duplicate the page for this PTE. Returns zero if page copied (so we need to * retry on the same PTE again to arm the copied page very soon), or negative @@ -925,7 +914,7 @@ if (!pte_same(*src_pte, data->cow_oldpte)) { /* PTE has changed under us. Release the page and retry */ - page_release_cow(data); + put_page(data->cow_new_page); return false; } @@ -936,12 +925,6 @@ set_pte_at(dst_mm, addr, dst_pte, entry); rss[mm_counter(new_page)]++; - /* - * Manually clear the new page pointer since we've moved ownership to - * the newly armed PTE. - */ - data->cow_new_page = NULL; - return true; } @@ -958,16 +941,12 @@ struct copy_mm_data data; again: - /* We don't reset this for COPY_MM_BREAK_COW */ - memset(&data, 0, sizeof(data)); - -again_break_cow: init_rss_vec(rss); dst_pte = pte_alloc_map_lock(dst_mm, dst_pmd, addr, &dst_ptl); if (!dst_pte) { - /* Guarantee that the new page is released if there is */ - page_release_cow(&data); + if (unlikely(copy_ret == COPY_MM_BREAK_COW)) + put_page(data.cow_new_page); return -ENOMEM; } src_pte = pte_offset_map(src_pmd, addr); @@ -978,6 +957,22 @@ arch_enter_lazy_mmu_mode(); 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; + } + /* PTE changed. Retry this pte (falls through) */ + } + do { /* * We are holding two locks at this point - either of them @@ -990,24 +985,6 @@ 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; - } - /* PTE changed. Retry this pte (falls through) */ - } - if (pte_none(*src_pte)) { progress++; continue; @@ -1017,6 +994,7 @@ if (copy_ret != COPY_MM_DONE) break; progress += 8; +next: } while (dst_pte++, src_pte++, addr += PAGE_SIZE, addr != end); arch_leave_lazy_mmu_mode(); @@ -1030,13 +1008,14 @@ case COPY_MM_SWAP_CONT: if (add_swap_count_continuation(data.entry, GFP_KERNEL) < 0) return -ENOMEM; - break; + copy_ret = COPY_MM_DONE; + goto again; case COPY_MM_BREAK_COW: /* Do accounting onto parent mm directly */ ret = page_duplicate(src_mm, vma, addr, &data); if (ret) return ret; - goto again_break_cow; + goto again; case COPY_MM_DONE: /* This means we're all good. */ break;