On Thu, Aug 30, 2018 at 01:51:56PM -0500, Eric Sandeen wrote: > On 8/30/18 1:28 PM, Darrick J. Wong wrote: > > On Thu, Aug 30, 2018 at 02:02:05PM -0400, Brian Foster wrote: > >> On Thu, Aug 30, 2018 at 11:35:46AM -0500, Eric Sandeen wrote: > >>> On 8/30/18 11:36 AM, Christoph Hellwig wrote: > >>>> On Thu, Aug 30, 2018 at 11:31:40AM -0500, Eric Sandeen wrote: > >>>>> That's no reason to uniquely disallow it for reflinked files, though; > >>>>> the problem is universal. It's true for fiemap as well. So I'm not sure > >>>>> that's an argument against the patch? > >>>> > >>>> fiemap at least tells you an extent is shared, bmap does not. > >>> > >>> yes, so bmap is clearly the wrong interface to use if you want to > >>> write directly to a file's blocks. But if you know enough to check > >>> the fiemap shared flag, you know enough to not use fibmap for that purpose... > >>> > >> > >> FWIW, this patch seems reasonable to me. To Christoph's point, I don't > >> think either interface really grants license to write to the underlying > >> blocks, so either way it's technically being abused for this purpose. > >> Unless there's a clear way to return an error for a particular type of > >> file, I think it's reasonable behavior for fibmap to expose the data it > >> supports (i.e., block maps) and drop the data it doesn't (reflink > >> state). > > > > But shared block status isn't something that can be dropped lightly. If > > you write to a shared block without realizing it, you'll corrupt every > > other file that shares the block. > > But there is no circumstance under which it is safe to write to a mapped > block no matter how you mapped it, tbh. <sigh> That's what all the break_layouts() code in XFS provides. It's a mechanism for applications to prevent the block layout from changing unexpected until they - the layout lease owner - give up their exclusive access to the file layout. Seriously, this has been talked about so much in the past year or two in the context of DAX, RDMA, get_user_pages() races in direct IO, etc. it pains me to see this discussion rehashing it all over again. We want applications to do what they need to do safely. FIBMAP is unsafe and, worse, it's unfixable. We need to get apps to move away from it to something is actualayl safe. Adding a file lease interface to block 3rd party changes to the file layout until the app releases the lease is a safe way of allowing userspace apps to use FIEMAP to map and identify file extents they can write directly to if they need to. IOWs, we need to get the FL_LAYOUT flag out into the external file lease interface (IIRC Dan Williams posted patches for this a while back) and get these "FIBMAP + write()" apps to use "FL_LAYOUT, fsync(), FIEMAP, write(), ~FL_LAYOUT". We need to make FIBMAP go away by providing a safer, more robust solution to the problem people are trying to solve. -Dave. -- Dave Chinner david@xxxxxxxxxxxxx
