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SubjectRe: [PATCH 08/18] MM: Add AS_CAN_DIO mapping flag
On Tue, 21 Dec 2021, Christoph Hellwig wrote:
> 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?

f_mode is a much better place to put the flag! Thanks for that
suggestion and all the other feedback. I'll post a new series early
next week.


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