Re: [PATCH 07/14] iomap: Support THPs in readpage

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, Oct 14, 2020 at 04:03:50AM +0100, Matthew Wilcox (Oracle) wrote:
> The VFS only calls readpage if readahead has encountered an error.
> Assume that any error requires the page to be split, and attempt to
> do so.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  fs/iomap/buffered-io.c | 39 +++++++++++++++++++++++++++++++++++++--
>  1 file changed, 37 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 4ea6c601a183..ca305fbaf811 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -343,15 +343,50 @@ iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
>  	return pos - orig_pos + plen;
>  }
>  
> +/*
> + * The page that was passed in has become Uptodate.  This may be due to
> + * the storage being synchronous or due to a page split finding the page
> + * is actually uptodate.  The page is still locked.
> + * Lift this into the VFS at some point.
> + */
> +#define AOP_UPDATED_PAGE       (AOP_TRUNCATED_PAGE + 1)

Er... why not lift it now?

"Because touching fs.h causes the whole kernel to be rebuilt and that's
annoying"? :D

--D

> +static int iomap_split_page(struct inode *inode, struct page *page)
> +{
> +	struct page *head = thp_head(page);
> +	bool uptodate = iomap_range_uptodate(inode, head,
> +				(page - head) * PAGE_SIZE, PAGE_SIZE);
> +
> +	iomap_page_release(head);
> +	if (split_huge_page(page) < 0) {
> +		unlock_page(page);
> +		return AOP_TRUNCATED_PAGE;
> +	}
> +	if (!uptodate)
> +		return 0;
> +	SetPageUptodate(page);
> +	return AOP_UPDATED_PAGE;
> +}
> +
>  int
>  iomap_readpage(struct page *page, const struct iomap_ops *ops)
>  {
>  	struct iomap_readpage_ctx ctx = { .cur_page = page };
> -	struct inode *inode = page->mapping->host;
> +	struct inode *inode = thp_head(page)->mapping->host;
>  	unsigned poff;
>  	loff_t ret;
>  
> -	trace_iomap_readpage(page->mapping->host, 1);
> +	trace_iomap_readpage(inode, 1);
> +
> +	if (PageTransCompound(page)) {
> +		int status = iomap_split_page(inode, page);
> +		if (status == AOP_UPDATED_PAGE) {
> +			unlock_page(page);

/me wonders why not do the unlock in iomap_split_page?

--D

> +			return 0;
> +		}
> +		if (status)
> +			return status;
> +	}
>  
>  	for (poff = 0; poff < PAGE_SIZE; poff += ret) {
>  		ret = iomap_apply(inode, page_offset(page) + poff,
> -- 
> 2.28.0
>