Re: [PATCH 08/18] MM: Add AS_CAN_DIO mapping flag

On Fri, Dec 17, 2021 at 10:48:23AM +1100, NeilBrown wrote:

Currently various places test if direct IO is possible on a file by
checking for the existence of the direct_IO address space operation.
This is a poor choice, as the direct_IO operation may not be used - it is
only used if the generic_file_*_iter functions are called for direct IO
and some filesystems - particularly NFS - don't do this.

Instead, introduce a new mapping flag: AS_CAN_DIO and change the various
places to check this (avoiding a pointer dereference).
unlock_new_inode() will set this flag if ->direct_IO is present, so
filesystems do not need to be changed.

NFS *is* changed, to set the flag explicitly and discard the direct_IO
entry in the address_space_operations for files.

For other can flags related to file operations we usuall stash them into
file->f_mode.  Any reason to treat this different?