From: Ross Zwisler Subject: Re: [PATCH v4 0/2] ext4: fix DAX dma vs truncate/hole-punch Date: Wed, 25 Jul 2018 16:28:39 -0600 Message-ID: <20180725222839.GA28304@linux.intel.com> References: <20180710191031.17919-1-ross.zwisler@linux.intel.com> <20180711081741.lmr44sp4cmt3f6um@quack2.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: linux-nvdimm-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org, "Darrick J. Wong" , Dave Chinner , Lukas Czerner , linux-ext4-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Christoph Hellwig To: Jan Kara Return-path: Content-Disposition: inline In-Reply-To: <20180711081741.lmr44sp4cmt3f6um-4I4JzKEfoa/jFM9bn6wA6Q@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: linux-nvdimm-bounces-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org Sender: "Linux-nvdimm" List-Id: linux-ext4.vger.kernel.org On Wed, Jul 11, 2018 at 10:17:41AM +0200, Jan Kara wrote: > On Tue 10-07-18 13:10:29, Ross Zwisler wrote: > > Changes since v3: > > * Added an ext4_break_layouts() call to ext4_insert_range() to ensure > > that the {ext4,xfs}_break_layouts() calls have the same meaning. > > (Dave, Darrick and Jan) > > How about the occasional WARN_ON_ONCE you mention below. Were you able to > hunt them down? The root cause of this issue is that while the ei->i_mmap_sem provides synchronization between ext4_break_layouts() and page faults, it doesn't provide synchronize us with the direct I/O path. This exact same issue exists in XFS AFAICT, with the synchronization tool there being the XFS_MMAPLOCK. This allows the direct I/O path to do I/O and raise & lower page->_refcount while we're executing a truncate/hole punch. This leads to us trying to free a page with an elevated refcount. Here's one instance of the race: CPU 0 CPU 1 ----- ----- ext4_punch_hole() ext4_break_layouts() # all pages have refcount=1 ext4_direct_IO() ... lots of layers ... follow_page_pte() get_page() # elevates refcount truncate_pagecache_range() ... a few layers ... dax_disassociate_entry() # sees elevated refcount, WARN_ON_ONCE() A similar race occurs when the refcount is being dropped while we're running ext4_break_layouts(), and this is the one that my test was actually hitting: CPU 0 CPU 1 ----- ----- ext4_direct_IO() ... lots of layers ... follow_page_pte() get_page() # elevates refcount of page X ext4_punch_hole() ext4_break_layouts() # two pages, X & Y, have refcount == 2 __wait_var_event() # called for page X __put_devmap_managed_page() # drops refcount of X to 1 # __wait_var_events() checks X's refcount in "if (condition)", and breaks. # We never actually called ext4_wait_dax_page(), so 'retry' in # ext4_break_layouts() is still false. Exit do/while loop in # ext4_break_layouts, never attempting to wait on page Y which still has an # elevated refcount of 2. truncate_pagecache_range() ... a few layers ... dax_disassociate_entry() # sees elevated refcount for Y, WARN_ON_ONCE() This second race can be fixed with the patch at the end of this function, which I think should go in, unless there is a benfit to the current retry scheme which relies on the 'retry' variable in {ext4,xfs}_break_layouts()? With this patch applied I've been able to run my unit test through thousands of iterations, where it used to failed consistently within 10 or so. Even so, I wonder if the real solution is to add synchronization between the direct I/O path and {ext4,xfs}_break_layouts()? Other ideas on how this should be handled? --- >8 --- >From a4519b0f40362f0a63ae96acaf986092aff0f0d3 Mon Sep 17 00:00:00 2001 From: Ross Zwisler Date: Wed, 25 Jul 2018 16:16:05 -0600 Subject: [PATCH] ext4: Close race between direct IO and ext4_break_layouts() If the refcount of a page is lowered between the time that it is returned by dax_busy_page() and when the refcount is again checked in ext4_break_layouts() => ___wait_var_event(), the waiting function ext4_wait_dax_page() will never be called. This means that ext4_break_layouts() will still have 'retry' set to false, so we'll stop looping and never check the refcount of other pages in this inode. Instead, always continue looping as long as dax_layout_busy_page() gives us a page which it found with an elevated refcount. Note that this works around the race exposed by my unit test, but I think that there is another race that needs to be addressed, probably with additional synchronization added between direct I/O and {ext4,xfs}_break_layouts(). Signed-off-by: Ross Zwisler --- fs/ext4/inode.c | 9 +++------ 1 file changed, 3 insertions(+), 6 deletions(-) diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c index 27e9643bc13b..fedb88104bbf 100644 --- a/fs/ext4/inode.c +++ b/fs/ext4/inode.c @@ -4193,9 +4193,8 @@ int ext4_update_disksize_before_punch(struct inode *inode, loff_t offset, return 0; } -static void ext4_wait_dax_page(struct ext4_inode_info *ei, bool *did_unlock) +static void ext4_wait_dax_page(struct ext4_inode_info *ei) { - *did_unlock = true; up_write(&ei->i_mmap_sem); schedule(); down_write(&ei->i_mmap_sem); @@ -4205,14 +4204,12 @@ int ext4_break_layouts(struct inode *inode) { struct ext4_inode_info *ei = EXT4_I(inode); struct page *page; - bool retry; int error; if (WARN_ON_ONCE(!rwsem_is_locked(&ei->i_mmap_sem))) return -EINVAL; do { - retry = false; page = dax_layout_busy_page(inode->i_mapping); if (!page) return 0; @@ -4220,8 +4217,8 @@ int ext4_break_layouts(struct inode *inode) error = ___wait_var_event(&page->_refcount, atomic_read(&page->_refcount) == 1, TASK_INTERRUPTIBLE, 0, 0, - ext4_wait_dax_page(ei, &retry)); - } while (error == 0 && retry); + ext4_wait_dax_page(ei)); + } while (error == 0); return error; } -- 2.14.4