From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Mon, 9 Oct 2017 14:08:24 +1100 From: Dave Chinner To: Dan Williams Cc: linux-nvdimm@lists.01.org, linux-xfs@vger.kernel.org, Jan Kara , "Darrick J. Wong" , linux-rdma@vger.kernel.org, linux-api@vger.kernel.org, Christoph Hellwig , "J. Bruce Fields" , linux-mm@kvack.org, Jeff Moyer , linux-fsdevel@vger.kernel.org, Jeff Layton , Ross Zwisler Subject: Re: [PATCH v7 03/12] fs: introduce i_mapdcount Message-ID: <20171009030824.GG3666@dastard> References: <150732931273.22363.8436792888326501071.stgit@dwillia2-desk3.amr.corp.intel.com> <150732933283.22363.570426117546397495.stgit@dwillia2-desk3.amr.corp.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <150732933283.22363.570426117546397495.stgit@dwillia2-desk3.amr.corp.intel.com> Sender: owner-linux-mm@kvack.org List-ID: On Fri, Oct 06, 2017 at 03:35:32PM -0700, Dan Williams wrote: > When ->iomap_begin() sees this count being non-zero and determines that > the block map of the file needs to be modified to satisfy the I/O > request it will instead return an error. This is needed for MAP_DIRECT > where, due to locking constraints, we can't rely on xfs_break_layouts() > to protect against allocating write-faults either from the process that > setup the MAP_DIRECT mapping nor other processes that have the file > mapped. xfs_break_layouts() requires XFS_IOLOCK which is problematic to > mix with the XFS_MMAPLOCK in the fault path. > > Cc: Jan Kara > Cc: Jeff Moyer > Cc: Christoph Hellwig > Cc: Dave Chinner > Cc: "Darrick J. Wong" > Cc: Ross Zwisler > Cc: Jeff Layton > Cc: "J. Bruce Fields" > Signed-off-by: Dan Williams > --- > fs/xfs/xfs_iomap.c | 9 +++++++++ > include/linux/fs.h | 31 +++++++++++++++++++++++++++++++ > 2 files changed, 40 insertions(+) > > diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c > index a1909bc064e9..6816f8ebbdcf 100644 > --- a/fs/xfs/xfs_iomap.c > +++ b/fs/xfs/xfs_iomap.c > @@ -1053,6 +1053,15 @@ xfs_file_iomap_begin( > goto out_unlock; > } > /* > + * If a file has MAP_DIRECT mappings disable block map > + * updates. This should only effect mmap write faults as > + * other paths are protected by an FL_LAYOUT lease. > + */ > + if (i_mapdcount_read(inode)) { > + error = -ETXTBSY; > + goto out_unlock; > + } That looks really fragile. For one, it's going to miss modifications to reflinked files altogether. Ignoring that, however, I don't want to have to care one bit about the internals of the MAP_DIRECT implementation in the filesystem code. Hide it behind something with an obvious name that returns the appropriate error and the filesystem code becomes self documenting: if ((flags & IOMAP_WRITE) && imap_needs_alloc(inode, &imap, nimaps)) { ..... error = iomap_can_allocate(inode); if (error) goto out_unlock; Then you can put all the MAP_DIRECT stuff and the comments explaining what is does inside the generic function that determines if we are allowed to allocate on that inode or not. > + /* > * We cap the maximum length we map here to MAX_WRITEBACK_PAGES > * pages to keep the chunks of work done where somewhat symmetric > * with the work writeback does. This is a completely arbitrary > diff --git a/include/linux/fs.h b/include/linux/fs.h > index c2b9bf3dc4e9..f83871b188ff 100644 > --- a/include/linux/fs.h > +++ b/include/linux/fs.h > @@ -642,6 +642,9 @@ struct inode { > atomic_t i_count; > atomic_t i_dio_count; > atomic_t i_writecount; > +#ifdef CONFIG_FS_DAX > + atomic_t i_mapdcount; /* count of MAP_DIRECT vmas */ > +#endif Is there any way to avoid growing the struct inode for this? Cheers, Dave. -- Dave Chinner david@fromorbit.com -- 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