Re: [PATCH 2/7] brd: convert to folios

[Matthew Wilcox <willy@infradead.org>]

On Wed, Jun 14, 2023 at 01:46:32PM +0200, Hannes Reinecke wrote:
>  /*
> - * Each block ramdisk device has a xarray brd_pages of pages that stores
> - * the pages containing the block device's contents. A brd page's ->index is
> - * its offset in PAGE_SIZE units. This is similar to, but in no way connected
> - * with, the kernel's pagecache or buffer cache (which sit above our block
> - * device).
> + * Each block ramdisk device has a xarray of folios that stores the folios
> + * containing the block device's contents. A brd folio's ->index is its offset
> + * in PAGE_SIZE units. This is similar to, but in no way connected with,
> + * the kernel's pagecache or buffer cache (which sit above our block device).

Having read my way to the end of the series, I can now circle back and
say this comment is wrong.  The folio->index is its offset in PAGE_SIZE
units if the sector size is <= PAGE_SIZE, otherwise it's the offset in
sector size units.  This is _different from_ the pagecache which uses
PAGE_SIZE units and multi-index entries in the XArray.

> @@ -144,29 +143,29 @@ static int copy_to_brd_setup(struct brd_device *brd, sector_t sector, size_t n,
>  static void copy_to_brd(struct brd_device *brd, const void *src,
>  			sector_t sector, size_t n)
>  {
> -	struct page *page;
> +	struct folio *folio;
>  	void *dst;
>  	unsigned int offset = (sector & (PAGE_SECTORS-1)) << SECTOR_SHIFT;
>  	size_t copy;
>  
>  	copy = min_t(size_t, n, PAGE_SIZE - offset);
> -	page = brd_lookup_page(brd, sector);
> -	BUG_ON(!page);
> +	folio = brd_lookup_folio(brd, sector);
> +	BUG_ON(!folio);
>  
> -	dst = kmap_atomic(page);
> -	memcpy(dst + offset, src, copy);
> -	kunmap_atomic(dst);
> +	dst = kmap_local_folio(folio, offset);
> +	memcpy(dst, src, copy);
> +	kunmap_local(dst);

This should use memcpy_to_folio(), which doesn't exist yet.
Compile-tested patch incoming shortly ...

> +	folio = brd_lookup_folio(brd, sector);
> +	if (folio) {
> +		src = kmap_local_folio(folio, offset);
> +		memcpy(dst, src, copy);
> +		kunmap_local(src);

And this will need memcpy_from_folio(), patch for that incoming too.

> @@ -226,15 +225,15 @@ static int brd_do_bvec(struct brd_device *brd, struct page *page,
>  			goto out;
>  	}
>  
> -	mem = kmap_atomic(page);
> +	mem = kmap_local_folio(folio, off);
>  	if (!op_is_write(opf)) {
> -		copy_from_brd(mem + off, brd, sector, len);
> -		flush_dcache_page(page);
> +		copy_from_brd(mem, brd, sector, len);
> +		flush_dcache_folio(folio);

Nngh.  This will need to be a more complex loop.  I don't think we can
do a simple abstraction here.  Perhaps you can base it on the two
patches you're about to see?