linux-kernel - Re: [PATCH 08/18] MM: Add AS_CAN

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Date:   Sun, 19 Dec 2021 13:38:39 +0000
From:   Mark Hemment <markhemm@...glemail.com>
To:     NeilBrown <neilb@...e.de>
Cc:     Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Chuck Lever <chuck.lever@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        Christoph Hellwig <hch@...radead.org>,
        David Howells <dhowells@...hat.com>, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/18] MM: Add AS_CAN_DIO mapping flag

On Thu, 16 Dec 2021 at 23:57, NeilBrown <neilb@...e.de> 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.
...
> 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?

Mark