| Message ID | 20200208193445.27421-6-ira.weiny@intel.com |
|---|---|
| State | Not Applicable |
| Headers | show |
| Series | Enable per-file/directory DAX operations V3 | expand |
On Sat, Feb 08, 2020 at 11:34:38AM -0800, ira.weiny@intel.com wrote: > From: Ira Weiny <ira.weiny@intel.com> > > The IS_DAX() check in io_is_direct() causes a race between changing the > DAX state and creating the iocb flags. > > Remove the check because DAX now emulates the page cache API and > therefore it does not matter if the file state is DAX or not when the > iocb flags are created. This statement is ... weird. DAX doesn't "emulate" the page cache API at all - it has it's own read/write methods that filesystems call based on the iomap infrastructure (dax_iomap_rw()). i.e. there are 3 different IO paths through the filesystems: the DAX IO path, the direct IO path, and the buffered IO path. Indeed, it seems like this works a bit by luck: Ext4 and XFS always check IS_DAX(inode) in the read/write_iter methods before checking for IOCB_DIRECT, and hence the IOCB_DIRECT flag is ignored by the filesystems. i.e. when we got rid of the O_DIRECT paths from DAX, we forgot to clean up io_is_direct() and it's only due to the ordering of checks that we went down the DAX path correctly.... That said, the code change is good, but the commit message needs a rewrite. Cheers, Dave.
On Tue, Feb 11, 2020 at 04:34:01PM +1100, Dave Chinner wrote: > On Sat, Feb 08, 2020 at 11:34:38AM -0800, ira.weiny@intel.com wrote: > > From: Ira Weiny <ira.weiny@intel.com> > > > > The IS_DAX() check in io_is_direct() causes a race between changing the > > DAX state and creating the iocb flags. > > > > Remove the check because DAX now emulates the page cache API and > > therefore it does not matter if the file state is DAX or not when the > > iocb flags are created. > > This statement is ... weird. > > DAX doesn't "emulate" the page cache API at all ah... yea emulate is a bad word here. > - it has it's own > read/write methods that filesystems call based on the iomap > infrastructure (dax_iomap_rw()). i.e. there are 3 different IO paths > through the filesystems: the DAX IO path, the direct IO path, and > the buffered IO path. > > Indeed, it seems like this works a bit by luck: Ext4 and XFS always > check IS_DAX(inode) in the read/write_iter methods before checking > for IOCB_DIRECT, and hence the IOCB_DIRECT flag is ignored by the > filesystems. i.e. when we got rid of the O_DIRECT paths from DAX, we > forgot to clean up io_is_direct() and it's only due to the ordering > of checks that we went down the DAX path correctly.... > > That said, the code change is good, but the commit message needs a > rewrite. How about? <commit msg> fs: Remove unneeded IS_DAX() check The IS_DAX() check in io_is_direct() causes a race between changing the DAX state and creating the iocb flags. Remove the check because DAX now has it's own read/write methods and file systems which support DAX check IS_DAX() prior to IOCB_DIRECT. Therefore, it does not matter if the file state is DAX when the iocb flags are created, and we can avoid the race. Reviewed-by: Jan Kara <jack@suse.cz> Signed-off-by: Ira Weiny <ira.weiny@intel.com> </commit msg> Ira > > Cheers, > > Dave. > -- > Dave Chinner > david@fromorbit.com
On Tue, Feb 11, 2020 at 08:38:31AM -0800, Ira Weiny wrote: > On Tue, Feb 11, 2020 at 04:34:01PM +1100, Dave Chinner wrote: > > On Sat, Feb 08, 2020 at 11:34:38AM -0800, ira.weiny@intel.com wrote: > > > From: Ira Weiny <ira.weiny@intel.com> > > > > > > The IS_DAX() check in io_is_direct() causes a race between changing the > > > DAX state and creating the iocb flags. > > > > > > Remove the check because DAX now emulates the page cache API and > > > therefore it does not matter if the file state is DAX or not when the > > > iocb flags are created. > > > > This statement is ... weird. > > > > DAX doesn't "emulate" the page cache API at all > > ah... yea emulate is a bad word here. > > > - it has it's own > > read/write methods that filesystems call based on the iomap > > infrastructure (dax_iomap_rw()). i.e. there are 3 different IO paths > > through the filesystems: the DAX IO path, the direct IO path, and > > the buffered IO path. > > > > Indeed, it seems like this works a bit by luck: Ext4 and XFS always > > check IS_DAX(inode) in the read/write_iter methods before checking > > for IOCB_DIRECT, and hence the IOCB_DIRECT flag is ignored by the > > filesystems. i.e. when we got rid of the O_DIRECT paths from DAX, we > > forgot to clean up io_is_direct() and it's only due to the ordering > > of checks that we went down the DAX path correctly.... > > > > That said, the code change is good, but the commit message needs a > > rewrite. > > How about? > > <commit msg> > fs: Remove unneeded IS_DAX() check > > The IS_DAX() check in io_is_direct() causes a race between changing the > DAX state and creating the iocb flags. This is irrelevant - the check is simply wrong and has been since ~2016 when we moved DAX to use the iomap infrastructure... Cheers, Dave.
On Wed, Feb 12, 2020 at 07:41:07AM +1100, Dave Chinner wrote: > On Tue, Feb 11, 2020 at 08:38:31AM -0800, Ira Weiny wrote: > > On Tue, Feb 11, 2020 at 04:34:01PM +1100, Dave Chinner wrote: > > > On Sat, Feb 08, 2020 at 11:34:38AM -0800, ira.weiny@intel.com wrote: > > > > From: Ira Weiny <ira.weiny@intel.com> > > > > > > > > The IS_DAX() check in io_is_direct() causes a race between changing the > > > > DAX state and creating the iocb flags. > > > > > > > > Remove the check because DAX now emulates the page cache API and > > > > therefore it does not matter if the file state is DAX or not when the > > > > iocb flags are created. > > > > > > This statement is ... weird. > > > > > > DAX doesn't "emulate" the page cache API at all > > > > ah... yea emulate is a bad word here. > > > > > - it has it's own > > > read/write methods that filesystems call based on the iomap > > > infrastructure (dax_iomap_rw()). i.e. there are 3 different IO paths > > > through the filesystems: the DAX IO path, the direct IO path, and > > > the buffered IO path. > > > > > > Indeed, it seems like this works a bit by luck: Ext4 and XFS always > > > check IS_DAX(inode) in the read/write_iter methods before checking > > > for IOCB_DIRECT, and hence the IOCB_DIRECT flag is ignored by the > > > filesystems. i.e. when we got rid of the O_DIRECT paths from DAX, we > > > forgot to clean up io_is_direct() and it's only due to the ordering > > > of checks that we went down the DAX path correctly.... > > > > > > That said, the code change is good, but the commit message needs a > > > rewrite. > > > > How about? > > > > <commit msg> > > fs: Remove unneeded IS_DAX() check > > > > The IS_DAX() check in io_is_direct() causes a race between changing the > > DAX state and creating the iocb flags. > > This is irrelevant - the check is simply wrong and has been since > ~2016 when we moved DAX to use the iomap infrastructure... Deleted. Ira > > Cheers, > > Dave. > > -- > Dave Chinner > david@fromorbit.com
diff --git a/include/linux/fs.h b/include/linux/fs.h index 3cd4fe6b845e..63d1e533a07d 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -3388,7 +3388,7 @@ extern int file_update_time(struct file *file); static inline bool io_is_direct(struct file *filp) { - return (filp->f_flags & O_DIRECT) || IS_DAX(filp->f_mapping->host); + return (filp->f_flags & O_DIRECT); } static inline bool vma_is_dax(struct vm_area_struct *vma)