Re: [PATCH 18/29] fsdax: decouple zeroing from the iomap buffered I/O code

Dan Williams
2021-11-23T21:46:49Z

On Tue, Nov 9, 2021 at 12:33 AM Christoph Hellwig <hch@lst.de> wrote:
>
> Unshare the DAX and iomap buffered I/O page zeroing code. This code
> previously did a IS_DAX check deep inside the iomap code, which in
> fact was the only DAX check in the code. Instead move these checks
> into the callers. Most callers already have DAX special casing anyway
> and XFS will need it for reflink support as well.

Looks ok, a tangential question below about iomap_truncate_page(), but
you can add:

Reviewed-by: Dan Williams <dan.j.williams@intel.com>

>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> fs/dax.c | 77 ++++++++++++++++++++++++++++++++++--------
> fs/ext2/inode.c | 6 ++--
> fs/ext4/inode.c | 4 +--
> fs/iomap/buffered-io.c | 35 +++++++------------
> fs/xfs/xfs_iomap.c | 6 ++++
> include/linux/dax.h | 6 +++-
> 6 files changed, 91 insertions(+), 43 deletions(-)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index dc9ebeff850ab..5b52b878124ac 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -1135,24 +1135,73 @@ static int dax_memzero(struct dax_device *dax_dev, pgoff_t pgoff,
> return rc;
> }
>
> -s64 dax_iomap_zero(loff_t pos, u64 length, struct iomap *iomap)
> +static loff_t dax_zero_iter(struct iomap_iter *iter, bool *did_zero)
> {
> - pgoff_t pgoff = dax_iomap_pgoff(iomap, pos);
> - long rc, id;
> - unsigned offset = offset_in_page(pos);
> - unsigned size = min_t(u64, PAGE_SIZE - offset, length);
> + const struct iomap *iomap = &iter->iomap;
> + const struct iomap *srcmap = iomap_iter_srcmap(iter);
> + loff_t pos = iter->pos;
> + loff_t length = iomap_length(iter);
> + loff_t written = 0;
> +
> + /* already zeroed? we're done. */
> + if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN)
> + return length;
> +
> + do {
> + unsigned offset = offset_in_page(pos);
> + unsigned size = min_t(u64, PAGE_SIZE - offset, length);
> + pgoff_t pgoff = dax_iomap_pgoff(iomap, pos);
> + long rc;
> + int id;
>
> - id = dax_read_lock();
> - if (IS_ALIGNED(pos, PAGE_SIZE) && size == PAGE_SIZE)
> - rc = dax_zero_page_range(iomap->dax_dev, pgoff, 1);
> - else
> - rc = dax_memzero(iomap->dax_dev, pgoff, offset, size);
> - dax_read_unlock(id);
> + id = dax_read_lock();
> + if (IS_ALIGNED(pos, PAGE_SIZE) && size == PAGE_SIZE)
> + rc = dax_zero_page_range(iomap->dax_dev, pgoff, 1);
> + else
> + rc = dax_memzero(iomap->dax_dev, pgoff, offset, size);
> + dax_read_unlock(id);
>
> - if (rc < 0)
> - return rc;
> - return size;
> + if (rc < 0)
> + return rc;
> + pos += size;
> + length -= size;
> + written += size;
> + if (did_zero)
> + *did_zero = true;
> + } while (length > 0);
> +
> + return written;
> +}
> +
> +int dax_zero_range(struct inode *inode, loff_t pos, loff_t len, bool *did_zero,
> + const struct iomap_ops *ops)
> +{
> + struct iomap_iter iter = {
> + .inode = inode,
> + .pos = pos,
> + .len = len,
> + .flags = IOMAP_ZERO,
> + };
> + int ret;
> +
> + while ((ret = iomap_iter(&iter, ops)) > 0)
> + iter.processed = dax_zero_iter(&iter, did_zero);
> + return ret;
> +}
> +EXPORT_SYMBOL_GPL(dax_zero_range);
> +
> +int dax_truncate_page(struct inode *inode, loff_t pos, bool *did_zero,
> + const struct iomap_ops *ops)
> +{
> + unsigned int blocksize = i_blocksize(inode);
> + unsigned int off = pos & (blocksize - 1);
> +
> + /* Block boundary? Nothing to do */
> + if (!off)
> + return 0;

It took me a moment to figure out why this was correct. I see it was
also copied from iomap_truncate_page(). It makes sense for DAX where
blocksize >= PAGE_SIZE so it's always the case that the amount of
capacity to zero relative to a page is from @pos to the end of the
block. Is there something else that protects the blocksize < PAGE_SIZE
case outside of DAX?

Nothing to change for this patch, just a question I had while reviewing.

> + return dax_zero_range(inode, pos, blocksize - off, did_zero, ops);
> }
> +EXPORT_SYMBOL_GPL(dax_truncate_page);
>
> static loff_t dax_iomap_iter(const struct iomap_iter *iomi,
> struct iov_iter *iter)
> diff --git a/fs/ext2/inode.c b/fs/ext2/inode.c
> index 333fa62661d56..ae9993018a015 100644
> --- a/fs/ext2/inode.c
> +++ b/fs/ext2/inode.c
> @@ -1297,9 +1297,9 @@ static int ext2_setsize(struct inode *inode, loff_t newsize)
> inode_dio_wait(inode);
>
> if (IS_DAX(inode)) {
> - error = iomap_zero_range(inode, newsize,
> - PAGE_ALIGN(newsize) - newsize, NULL,
> - &ext2_iomap_ops);
> + error = dax_zero_range(inode, newsize,
> + PAGE_ALIGN(newsize) - newsize, NULL,
> + &ext2_iomap_ops);
> } else if (test_opt(inode->i_sb, NOBH))
> error = nobh_truncate_page(inode->i_mapping,
> newsize, ext2_get_block);
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 0f06305167d5a..8c443b753b815 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -3783,8 +3783,8 @@ static int ext4_block_zero_page_range(handle_t *handle,
> length = max;
>
> if (IS_DAX(inode)) {
> - return iomap_zero_range(inode, from, length, NULL,
> - &ext4_iomap_ops);
> + return dax_zero_range(inode, from, length, NULL,
> + &ext4_iomap_ops);
> }
> return __ext4_block_zero_page_range(handle, mapping, from, length);
> }
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 1753c26c8e76e..b1511255b4df8 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -870,26 +870,8 @@ iomap_file_unshare(struct inode *inode, loff_t pos, loff_t len,
> }
> EXPORT_SYMBOL_GPL(iomap_file_unshare);
>
> -static s64 __iomap_zero_iter(struct iomap_iter *iter, loff_t pos, u64 length)
> -{
> - struct page *page;
> - int status;
> - unsigned offset = offset_in_page(pos);
> - unsigned bytes = min_t(u64, PAGE_SIZE - offset, length);
> -
> - status = iomap_write_begin(iter, pos, bytes, &page);
> - if (status)
> - return status;
> -
> - zero_user(page, offset, bytes);
> - mark_page_accessed(page);
> -
> - return iomap_write_end(iter, pos, bytes, bytes, page);
> -}
> -
> static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
> {
> - struct iomap *iomap = &iter->iomap;
> const struct iomap *srcmap = iomap_iter_srcmap(iter);
> loff_t pos = iter->pos;
> loff_t length = iomap_length(iter);
> @@ -900,12 +882,19 @@ static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
> return length;
>
> do {
> - s64 bytes;
> + unsigned offset = offset_in_page(pos);
> + size_t bytes = min_t(u64, PAGE_SIZE - offset, length);
> + struct page *page;
> + int status;
>
> - if (IS_DAX(iter->inode))
> - bytes = dax_iomap_zero(pos, length, iomap);
> - else
> - bytes = __iomap_zero_iter(iter, pos, length);
> + status = iomap_write_begin(iter, pos, bytes, &page);
> + if (status)
> + return status;
> +
> + zero_user(page, offset, bytes);
> + mark_page_accessed(page);
> +
> + bytes = iomap_write_end(iter, pos, bytes, bytes, page);
> if (bytes < 0)
> return bytes;
>
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index d6d71ae9f2ae4..604000b6243ec 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -1321,6 +1321,9 @@ xfs_zero_range(
> {
> struct inode *inode = VFS_I(ip);
>
> + if (IS_DAX(inode))
> + return dax_zero_range(inode, pos, len, did_zero,
> + &xfs_buffered_write_iomap_ops);
> return iomap_zero_range(inode, pos, len, did_zero,
> &xfs_buffered_write_iomap_ops);
> }
> @@ -1333,6 +1336,9 @@ xfs_truncate_page(
> {
> struct inode *inode = VFS_I(ip);
>
> + if (IS_DAX(inode))
> + return dax_truncate_page(inode, pos, did_zero,
> + &xfs_buffered_write_iomap_ops);
> return iomap_truncate_page(inode, pos, did_zero,
> &xfs_buffered_write_iomap_ops);
> }
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index 324363b798ecd..a5cc2f1aa840e 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -14,6 +14,7 @@ typedef unsigned long dax_entry_t;
> struct dax_device;
> struct gendisk;
> struct iomap_ops;
> +struct iomap_iter;
> struct iomap;
>
> struct dax_operations {
> @@ -124,6 +125,10 @@ struct page *dax_layout_busy_page(struct address_space *mapping);
> struct page *dax_layout_busy_page_range(struct address_space *mapping, loff_t start, loff_t end);
> dax_entry_t dax_lock_page(struct page *page);
> void dax_unlock_page(struct page *page, dax_entry_t cookie);
> +int dax_zero_range(struct inode *inode, loff_t pos, loff_t len, bool *did_zero,
> + const struct iomap_ops *ops);
> +int dax_truncate_page(struct inode *inode, loff_t pos, bool *did_zero,
> + const struct iomap_ops *ops);
> #else
> static inline int dax_add_host(struct dax_device *dax_dev, struct gendisk *disk)
> {
> @@ -204,7 +209,6 @@ vm_fault_t dax_finish_sync_fault(struct vm_fault *vmf,
> int dax_delete_mapping_entry(struct address_space *mapping, pgoff_t index);
> int dax_invalidate_mapping_entry_sync(struct address_space *mapping,
> pgoff_t index);
> -s64 dax_iomap_zero(loff_t pos, u64 length, struct iomap *iomap);
> static inline bool dax_mapping(struct address_space *mapping)
> {
> return mapping->host && IS_DAX(mapping->host);
> --
> 2.30.2
>