From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pd0-f170.google.com (mail-pd0-f170.google.com [209.85.192.170]) by kanga.kvack.org (Postfix) with ESMTP id EFA366B0037 for ; Thu, 17 Oct 2013 07:23:17 -0400 (EDT) Received: by mail-pd0-f170.google.com with SMTP id x10so2571745pdj.1 for ; Thu, 17 Oct 2013 04:23:17 -0700 (PDT) Message-ID: <525FC89E.6040208@suse.cz> Date: Thu, 17 Oct 2013 13:23:10 +0200 From: Vlastimil Babka MIME-Version: 1.0 Subject: Re: [PATCH 2/2] vmsplice: Add limited zero copy to vmsplice References: <1381177293-27125-1-git-send-email-rcj@linux.vnet.ibm.com> <1381177293-27125-3-git-send-email-rcj@linux.vnet.ibm.com> In-Reply-To: <1381177293-27125-3-git-send-email-rcj@linux.vnet.ibm.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Robert C Jennings , linux-kernel@vger.kernel.org Cc: linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, Alexander Viro , Rik van Riel , Andrea Arcangeli , Dave Hansen , Matt Helsley , Anthony Liguori , Michael Roth , Lei Li , Leonardo Garcia On 10/07/2013 10:21 PM, Robert C Jennings wrote: > From: Matt Helsley > > It is sometimes useful to move anonymous pages over a pipe rather than > save/swap them. Check the SPLICE_F_GIFT and SPLICE_F_MOVE flags to see > if userspace would like to move such pages. This differs from plain > SPLICE_F_GIFT in that the memory written to the pipe will no longer > have the same contents as the original -- it effectively faults in new, > empty anonymous pages. > > On the read side the page written to the pipe will be copied unless > SPLICE_F_MOVE is used. Otherwise copying will be performed and the page > will be reclaimed. Note that so long as there is a mapping to the page > copies will be done instead because rmap will have upped the map count for > each anonymous mapping; this can happen do to fork(), for example. This > is necessary because moving the page will usually change the anonymous > page's nonlinear index and that can only be done if it's unmapped. You might want to update comments of vmsplice_to_user() and SYSCALL_DEFINE4(vmsplice) as they both explain how it's done only via copying. Vlastimil > Signed-off-by: Matt Helsley > Signed-off-by: Robert C Jennings > --- > fs/splice.c | 63 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 63 insertions(+) > > diff --git a/fs/splice.c b/fs/splice.c > index a62d61e..9d2ed128 100644 > --- a/fs/splice.c > +++ b/fs/splice.c > @@ -32,6 +32,10 @@ > #include > #include > #include > +#include > +#include > +#include > +#include > #include "internal.h" > > /* > @@ -1562,6 +1566,65 @@ static int pipe_to_user(struct pipe_inode_info *pipe, struct pipe_buffer *buf, > char *src; > int ret; > > + if (!buf->offset && (buf->len == PAGE_SIZE) && > + (buf->flags & PIPE_BUF_FLAG_GIFT) && (sd->flags & SPLICE_F_MOVE)) { > + struct page *page = buf->page; > + struct mm_struct *mm; > + struct vm_area_struct *vma; > + spinlock_t *ptl; > + pte_t *ptep, pte; > + unsigned long useraddr; > + > + if (!PageAnon(page)) > + goto copy; > + if (PageCompound(page)) > + goto copy; > + if (PageHuge(page) || PageTransHuge(page)) > + goto copy; > + if (page_mapped(page)) > + goto copy; > + useraddr = (unsigned long)sd->u.userptr; > + mm = current->mm; > + > + ret = -EAGAIN; > + down_read(&mm->mmap_sem); > + vma = find_vma_intersection(mm, useraddr, useraddr + PAGE_SIZE); > + if (IS_ERR_OR_NULL(vma)) > + goto up_copy; > + if (!vma->anon_vma) { > + ret = anon_vma_prepare(vma); > + if (ret) > + goto up_copy; > + } > + zap_page_range(vma, useraddr, PAGE_SIZE, NULL); > + ret = lock_page_killable(page); > + if (ret) > + goto up_copy; > + ptep = get_locked_pte(mm, useraddr, &ptl); > + if (!ptep) > + goto unlock_up_copy; > + pte = *ptep; > + if (pte_present(pte)) > + goto unlock_up_copy; > + get_page(page); > + page_add_anon_rmap(page, vma, useraddr); > + pte = mk_pte(page, vma->vm_page_prot); > + set_pte_at(mm, useraddr, ptep, pte); > + update_mmu_cache(vma, useraddr, ptep); > + pte_unmap_unlock(ptep, ptl); > + ret = 0; > +unlock_up_copy: > + unlock_page(page); > +up_copy: > + up_read(&mm->mmap_sem); > + if (!ret) { > + ret = sd->len; > + goto out; > + } > + /* else ret < 0 and we should fallback to copying */ > + VM_BUG_ON(ret > 0); > + } > +copy: > /* > * See if we can use the atomic maps, by prefaulting in the > * pages and doing an atomic copy > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org