Re: [PATCH v8 06/10] iomap: fix iomap_dio_zero() for fs bs > system page size

[Christoph Hellwig <hch@lst.de>]

On Tue, Jun 25, 2024 at 11:44:16AM +0000, Pankaj Raghav (Samsung) wrote:
> -static int __init iomap_init(void)
> +static int __init iomap_pagecache_init(void)
>  {
>  	return bioset_init(&iomap_ioend_bioset, 4 * (PAGE_SIZE / SECTOR_SIZE),
>  			   offsetof(struct iomap_ioend, io_bio),
>  			   BIOSET_NEED_BVECS);
>  }
> -fs_initcall(iomap_init);
> +fs_initcall(iomap_pagecache_init);

s/iomap_pagecache_init/iomap_buffered_init/

We don't use pagecache naming anywhere else in the file.

> +/*
> + * Used for sub block zeroing in iomap_dio_zero()
> + */
> +#define ZERO_PAGE_64K_SIZE (65536)

just use SZ_64K

> +#define ZERO_PAGE_64K_ORDER (get_order(ZERO_PAGE_64K_SIZE))

No really point in having this.

> +static struct page *zero_page_64k;

This should be a folio.  Encoding the size in the name is also really
weird and just creates churn when we have to increase it.


> +	/*
> +	 * Max block size supported is 64k
> +	 */
> +	WARN_ON_ONCE(len > ZERO_PAGE_64K_SIZE);


A WARN_ON without actually erroring out here is highly dangerous. 

> +
>  	bio = iomap_dio_alloc_bio(iter, dio, 1, REQ_OP_WRITE | REQ_SYNC | REQ_IDLE);

Overly long line here.

> +
> +static int __init iomap_dio_init(void)
> +{
> +	zero_page_64k = alloc_pages(GFP_KERNEL | __GFP_ZERO,
> +				    ZERO_PAGE_64K_ORDER);

> +
> +	if (!zero_page_64k)
> +		return -ENOMEM;
> +
> +	set_memory_ro((unsigned long)page_address(zero_page_64k),
> +		      1U << ZERO_PAGE_64K_ORDER);

What's the point of the set_memory_ro here?  Yes, we won't write to
it, but it's hardly an attack vector and fragments the direct map.