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

On Mon, 20 Dec 2021, Mark Hemment wrote:

On Thu, 16 Dec 2021 at 23:57, NeilBrown [off-list ref] wrote:

quoted

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.

...

quoted

diff --git a/fs/inode.c b/fs/inode.c
index 6b80a51129d5..bae65ccecdb1 100644
--- a/fs/inode.c
+++ b/fs/inode.c

@@ -1008,6 +1008,9 @@ EXPORT_SYMBOL(lockdep_annotate_inode_mutex_key);
 void unlock_new_inode(struct inode *inode)
 {
        lockdep_annotate_inode_mutex_key(inode);
+       if (inode->i_mapping->a_ops &&
+           inode->i_mapping->a_ops->direct_IO)
+               set_bit(AS_CAN_DIO, &inode->i_mapping->flags);
        spin_lock(&inode->i_lock);
        WARN_ON(!(inode->i_state & I_NEW));
        inode->i_state &= ~I_NEW & ~I_CREATING;

Does d_instantiate_new() also need to set AS_CAN_DIO?

Yes it does - thanks for catching that.  I'll update my patch.

Thanks,
NeilBrown