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Date: Tue, 25 Aug 2020 09:59:18 +1000
From: Dave Chinner <david@fromorbit.com>
To: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Subject: Re: [PATCH 5/9] iomap: Support arbitrarily many blocks per page
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On Mon, Aug 24, 2020 at 03:55:06PM +0100, Matthew Wilcox (Oracle) wrote:
> Size the uptodate array dynamically to support larger pages in the
> page cache.  With a 64kB page, we're only saving 8 bytes per page today,
> but with a 2MB maximum page size, we'd have to allocate more than 4kB
> per page.  Add a few debugging assertions.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  fs/iomap/buffered-io.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index dbf9572dabe9..844e95cacea8 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -22,18 +22,19 @@
>  #include "../internal.h"
>  
>  /*
> - * Structure allocated for each page when block size < PAGE_SIZE to track
> + * Structure allocated for each page when block size < page size to track
>   * sub-page uptodate status and I/O completions.
>   */
>  struct iomap_page {
>  	atomic_t		read_count;
>  	atomic_t		write_count;
>  	spinlock_t		uptodate_lock;
> -	DECLARE_BITMAP(uptodate, PAGE_SIZE / 512);
> +	unsigned long		uptodate[];
>  };
>  
>  static inline struct iomap_page *to_iomap_page(struct page *page)
>  {
> +	VM_BUG_ON_PGFLAGS(PageTail(page), page);
>  	if (page_has_private(page))
>  		return (struct iomap_page *)page_private(page);
>  	return NULL;

Just to confirm: this vm bug check is to needed becuse we only
attach the iomap_page to the head page of a compound page?

Assuming that I've understood the above correctly:

Reviewed-by: Dave Chinner <dchinner@redhat.com>
-- 
Dave Chinner
david@fromorbit.com
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