linux-kernel - Re: [PATCH 2/2] mm: Make swap_readpage() for SWP_FS_OPS use ->direct

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Date:   Thu, 12 Aug 2021 14:00:14 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     trond.myklebust@...marydata.com, darrick.wong@...cle.com,
        hch@....de, jlayton@...nel.org, sfrench@...ba.org,
        torvalds@...ux-foundation.org, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: Make swap_readpage() for SWP_FS_OPS use
 ->direct_IO() not ->readpage()

On Thu, Aug 12, 2021 at 12:57:58PM +0100, David Howells wrote:

I'm not quite sure why we need the refcount.

> +	refcount_set(&ki->ki_refcnt, 2);
> +	init_sync_kiocb(&ki->iocb, swap_file);
> +	ki->page = page;
> +	ki->iocb.ki_flags = IOCB_DIRECT | IOCB_SWAP;
> +	ki->iocb.ki_pos	= page_file_offset(page);
> +	ki->iocb.ki_filp = get_file(swap_file);
> +	if (!synchronous)
> +		ki->iocb.ki_complete = swapfile_read_complete;
> +
> +	iov_iter_bvec(&to, READ, &bv, 1, PAGE_SIZE);
> +	ret = swap_file->f_mapping->a_ops->direct_IO(&ki->iocb, &to);

After submitting the IO here ...

> +	if (ret != -EIOCBQUEUED)
> +		swapfile_read_complete(&ki->iocb, ret, 0);

We only touch the 'ki' here ... if the caller didn't call read_complete

> +	swapfile_put_kiocb(ki);

Except for here, which is only touched in order to put the refcount.

So why can't swapfile_read_complete() do the work of freeing the ki?