On Tue, Jul 27, 2021 at 04:30:16PM +0800, Chao Yu wrote: > On 2021/7/27 15:38, Eric Biggers wrote: > > That's somewhat helpful, but I've been doing some more investigation and now I'm > > even more confused. How can f2fs support non-overwrite DIO writes at all > > (meaning DIO writes in LFS mode as well as DIO writes to holes in non-LFS mode), > > given that it has no support for unwritten extents? AFAICS, as-is users can > > I'm trying to pick up DAX support patch created by Qiuyang from huawei, and it > looks it faces the same issue, so it tries to fix this by calling sb_issue_zeroout() > in f2fs_map_blocks() before it returns. I really hope you don't, because zeroing the region before memcpy'ing it is absurd. I don't know if f2fs can do that (xfs can't really) without pinning resources during a potentially lengthy memcpy operation, but you /could/ allocate the space in ->iomap_begin, attach some record of that to iomap->private, and only commit the mapping update in ->iomap_end. --D > > easily leak uninitialized disk contents on f2fs by issuing a DIO write that > > won't complete fully (or might not complete fully), then reading back the blocks > > that got allocated but not written to. > > > > I think that f2fs will have to take the ext2 approach of not allowing > > non-overwrite DIO writes at all... > Yes, > > Another option is to enhance f2fs metadata's scalability which needs to update layout > of dnode block or SSA block, after that we can record the status of unwritten data block > there... it's a big change though... > > Thanks, > > > > > - Eric > >
