On Wed, Sep 02, 2020 at 09:58:30AM +1000, Dave Chinner wrote: > Put simply: converting a filesystem to use iomap is not a "change > the filesystem interfacing code and it will work" modification. We > ask that filesystems are modified to conform to the iomap IO > exclusion model; adding special cases for every potential > locking and mapping quirk every different filesystem has is part of > what turned the old direct IO code into an unmaintainable nightmare. > > > That's fine, but this is kind of a bad way to find > > out. We really shouldn't have generic helper's that have different generic > > locking rules based on which file system uses them. > > We certainly can change the rules for new infrastructure. Indeed, we > had to change the rules to support DAX. The whole point of the > iomap infrastructure was that it enabled us to use code that already > worked for DAX (the XFS code) in multiple filesystems. And as people > have realised that the DIO via iomap is much faster than the old DIO > code and is a much more efficient way of doing large buffered IO, > other filesystems have started to use it. > > However.... > > > Because then we end up > > with situations like this, where suddenly we're having to come up with some > > weird solution because the generic thing only works for a subset of file > > systems. Thanks, > > .... we've always said "you need to change the filesystem code to > use iomap". This is simply a reflection on the fact that iomap has > different rules and constraints to the old code and so it's not a > direct plug in replacement. There are no short cuts here... Can you point me (and I suspect Josef!) towards the documentation of the locking model? I was hoping to find Documentation/filesystems/iomap.rst but all the 'iomap' strings in Documentation/ refer to pci_iomap and similar, except for this in the DAX documentation: : - implementing ->read_iter and ->write_iter operations which use dax_iomap_rw() : when inode has S_DAX flag set : - implementing an mmap file operation for DAX files which sets the : VM_MIXEDMAP and VM_HUGEPAGE flags on the VMA, and setting the vm_ops to : include handlers for fault, pmd_fault, page_mkwrite, pfn_mkwrite. These : handlers should probably call dax_iomap_fault() passing the appropriate : fault size and iomap operations. : - calling iomap_zero_range() passing appropriate iomap operations instead of : block_truncate_page() for DAX files : - ensuring that there is sufficient locking between reads, writes, : truncates and page faults : : The iomap handlers for allocating blocks must make sure that allocated blocks : are zeroed out and converted to written extents before being returned to avoid : exposure of uninitialized data through mmap. which doesn't bear on this situation.
