Re: [PATCH v3 2/9] mm: Add 'supports' field to the address_space_operations to list features

[Matthew Wilcox]

On Fri, Sep 24, 2021 at 06:18:14PM +0100, David Howells wrote:
> Rather than depending on .direct_IO to point to something to indicate that
> direct I/O is supported, add a 'supports' bitmask that we can test, since
> we only need one bit.

Why would you add mapping->aops->supports instead of using one of the free
bits in mapping->flags?  enum mapping_flags in pagemap.h.

It could also be a per-fs flag, or per-sb flag, but it's fewer
dereferences at check time if it's in mapping->flags.