Re: [PATCH v9 06/10] iomap: fix iomap_dio_zero() for fs bs > system page size

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, Jul 04, 2024 at 11:23:16AM +0000, Pankaj Raghav (Samsung) wrote:
> From: Pankaj Raghav <p.raghav@samsung.com>
> 
> iomap_dio_zero() will pad a fs block with zeroes if the direct IO size
> < fs block size. iomap_dio_zero() has an implicit assumption that fs block
> size < page_size. This is true for most filesystems at the moment.
> 
> If the block size > page size, this will send the contents of the page
> next to zero page(as len > PAGE_SIZE) to the underlying block device,
> causing FS corruption.
> 
> iomap is a generic infrastructure and it should not make any assumptions
> about the fs block size and the page size of the system.
> 
> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> ---
>  fs/iomap/buffered-io.c |  4 ++--
>  fs/iomap/direct-io.c   | 45 ++++++++++++++++++++++++++++++++++++------
>  2 files changed, 41 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index f420c53d86acc..d745f718bcde8 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -2007,10 +2007,10 @@ iomap_writepages(struct address_space *mapping, struct writeback_control *wbc,
>  }
>  EXPORT_SYMBOL_GPL(iomap_writepages);
>  
> -static int __init iomap_init(void)
> +static int __init iomap_buffered_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_buffered_init);
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index f3b43d223a46e..c02b266bba525 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -11,6 +11,7 @@
>  #include <linux/iomap.h>
>  #include <linux/backing-dev.h>
>  #include <linux/uio.h>
> +#include <linux/set_memory.h>
>  #include <linux/task_io_accounting_ops.h>
>  #include "trace.h"
>  
> @@ -27,6 +28,13 @@
>  #define IOMAP_DIO_WRITE		(1U << 30)
>  #define IOMAP_DIO_DIRTY		(1U << 31)
>  
> +/*
> + * Used for sub block zeroing in iomap_dio_zero()
> + */
> +#define IOMAP_ZERO_PAGE_SIZE (SZ_64K)
> +#define IOMAP_ZERO_PAGE_ORDER (get_order(IOMAP_ZERO_PAGE_SIZE))
> +static struct page *zero_page;
> +
>  struct iomap_dio {
>  	struct kiocb		*iocb;
>  	const struct iomap_dio_ops *dops;
> @@ -232,13 +240,20 @@ void iomap_dio_bio_end_io(struct bio *bio)
>  }
>  EXPORT_SYMBOL_GPL(iomap_dio_bio_end_io);
>  
> -static void iomap_dio_zero(const struct iomap_iter *iter, struct iomap_dio *dio,
> +static int iomap_dio_zero(const struct iomap_iter *iter, struct iomap_dio *dio,
>  		loff_t pos, unsigned len)
>  {
>  	struct inode *inode = file_inode(dio->iocb->ki_filp);
> -	struct page *page = ZERO_PAGE(0);
>  	struct bio *bio;
>  
> +	if (!len)
> +		return 0;
> +	/*
> +	 * Max block size supported is 64k
> +	 */
> +	if (WARN_ON_ONCE(len > IOMAP_ZERO_PAGE_SIZE))
> +		return -EINVAL;
> +
>  	bio = iomap_dio_alloc_bio(iter, dio, 1, REQ_OP_WRITE | REQ_SYNC | REQ_IDLE);
>  	fscrypt_set_bio_crypt_ctx(bio, inode, pos >> inode->i_blkbits,
>  				  GFP_KERNEL);
> @@ -246,8 +261,9 @@ static void iomap_dio_zero(const struct iomap_iter *iter, struct iomap_dio *dio,
>  	bio->bi_private = dio;
>  	bio->bi_end_io = iomap_dio_bio_end_io;
>  
> -	__bio_add_page(bio, page, len, 0);
> +	__bio_add_page(bio, zero_page, len, 0);
>  	iomap_dio_submit_bio(iter, dio, bio, pos);
> +	return 0;
>  }
>  
>  /*
> @@ -356,8 +372,10 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
>  	if (need_zeroout) {
>  		/* zero out from the start of the block to the write offset */
>  		pad = pos & (fs_block_size - 1);
> -		if (pad)
> -			iomap_dio_zero(iter, dio, pos - pad, pad);
> +
> +		ret = iomap_dio_zero(iter, dio, pos - pad, pad);
> +		if (ret)
> +			goto out;
>  	}
>  
>  	/*
> @@ -431,7 +449,8 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
>  		/* zero out from the end of the write to the end of the block */
>  		pad = pos & (fs_block_size - 1);
>  		if (pad)
> -			iomap_dio_zero(iter, dio, pos, fs_block_size - pad);
> +			ret = iomap_dio_zero(iter, dio, pos,
> +					     fs_block_size - pad);
>  	}
>  out:
>  	/* Undo iter limitation to current extent */
> @@ -753,3 +772,17 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
>  	return iomap_dio_complete(dio);
>  }
>  EXPORT_SYMBOL_GPL(iomap_dio_rw);
> +
> +static int __init iomap_dio_init(void)
> +{
> +	zero_page = alloc_pages(GFP_KERNEL | __GFP_ZERO,
> +				IOMAP_ZERO_PAGE_ORDER);
> +
> +	if (!zero_page)
> +		return -ENOMEM;
> +
> +	set_memory_ro((unsigned long)page_address(zero_page),
> +		      1U << IOMAP_ZERO_PAGE_ORDER);
> +	return 0;
> +}
> +fs_initcall(iomap_dio_init);

This ^^^ could be refactored into a zeropage.ko module some day as a
separate patch to amortize the cost of the 64k buffer that never goes
away.

This patch looks ok though, so:
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> -- 
> 2.44.1
> 
>