Return-Path: linux-nfs-owner@vger.kernel.org Received: from cantor2.suse.de ([195.135.220.15]:38816 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751554AbaDPESr (ORCPT ); Wed, 16 Apr 2014 00:18:47 -0400 From: NeilBrown To: linux-mm@kvack.org, linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org Date: Wed, 16 Apr 2014 14:03:36 +1000 Subject: [PATCH 08/19] Set PF_FSTRANS while write_cache_pages calls ->writepage Cc: xfs@oss.sgi.com Message-ID: <20140416040336.10604.34673.stgit@notabene.brown> In-Reply-To: <20140416033623.10604.69237.stgit@notabene.brown> References: <20140416033623.10604.69237.stgit@notabene.brown> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Sender: linux-nfs-owner@vger.kernel.org List-ID: It is normally safe for direct reclaim to enter filesystems even when a page is locked - as can happen if ->writepage allocates memory with GFP_KERNEL (which xfs does). However if a localhost NFS mount is present, then a flush-* thread might hold a page locked and then in direct reclaim, ask nfs to commit an inode (nfs_release_page). When nfsd performs the fsync it might try to lock the same page, which leads to a deadlock. A ->writepage should not allocate much memory, or do so very often, so it is safe to set PF_FSTRANS, and this removes the possible deadlock. This was not detected by lockdep as it doesn't monitor the page lock. It was found as a real deadlock in testing. Signed-off-by: NeilBrown --- mm/page-writeback.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/mm/page-writeback.c b/mm/page-writeback.c index 7106cb1aca8e..572e70b9a3f7 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -1909,6 +1909,7 @@ retry: for (i = 0; i < nr_pages; i++) { struct page *page = pvec.pages[i]; + unsigned int pflags; /* * At this point, the page may be truncated or @@ -1960,8 +1961,10 @@ continue_unlock: if (!clear_page_dirty_for_io(page)) goto continue_unlock; + current_set_flags_nested(&pflags, PF_FSTRANS); trace_wbc_writepage(wbc, mapping->backing_dev_info); ret = (*writepage)(page, wbc, data); + current_restore_flags_nested(&pflags, PF_FSTRANS); if (unlikely(ret)) { if (ret == AOP_WRITEPAGE_ACTIVATE) { unlock_page(page);