Re: [RFC PATCH] btrfs: don't call btrfs_sync_file from iomap context

[Matthew Wilcox <willy@infradead.org>]

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.