Re: [PATCH 9/9] iomap: Change calling convention for zeroing

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Mon, Aug 24, 2020 at 03:55:10PM +0100, Matthew Wilcox (Oracle) wrote:
> Pass the full length to iomap_zero() and dax_iomap_zero(), and have
> them return how many bytes they actually handled.  This is preparatory
> work for handling THP, although it looks like DAX could actually take
> advantage of it if there's a larger contiguous area.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  fs/dax.c               | 13 ++++++-------
>  fs/iomap/buffered-io.c | 33 +++++++++++++++------------------
>  include/linux/dax.h    |  3 +--
>  3 files changed, 22 insertions(+), 27 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index 95341af1a966..f2b912cb034e 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -1037,18 +1037,18 @@ static vm_fault_t dax_load_hole(struct xa_state *xas,
>  	return ret;
>  }
>  
> -int dax_iomap_zero(loff_t pos, unsigned offset, unsigned size,
> -		   struct iomap *iomap)
> +loff_t dax_iomap_zero(loff_t pos, u64 length, struct iomap *iomap)

Sorry for my ultra-slow response to this.  The u64 length seems ok to me
(or uint64_t, I don't care all /that/ much), but using loff_t as a
return type bothers me because I see that and think that this function
is returning a new file offset, e.g. (pos + number of bytes zeroed).

So please, let's use s64 or something that isn't so misleading.

FWIW, Linus also[0] doesn't[1] like using loff_t for the number of bytes
copied.

--D

[0] https://lore.kernel.org/linux-fsdevel/CAHk-=wgcPAfOSigMf0xwaGfVjw413XN3UPATwYWHrss+QuivhQ@mail.gmail.com/
[1] https://lore.kernel.org/linux-fsdevel/CAHk-=wgvROUnrEVADVR_zTHY8NmYo-_jVjV37O1MdDm2de+Lmw@mail.gmail.com/

>  {
>  	sector_t sector = iomap_sector(iomap, pos & PAGE_MASK);
>  	pgoff_t pgoff;
>  	long rc, id;
>  	void *kaddr;
>  	bool page_aligned = false;
> -
> +	unsigned offset = offset_in_page(pos);
> +	unsigned size = min_t(u64, PAGE_SIZE - offset, length);
>  
>  	if (IS_ALIGNED(sector << SECTOR_SHIFT, PAGE_SIZE) &&
> -	    IS_ALIGNED(size, PAGE_SIZE))
> +	    (size == PAGE_SIZE))
>  		page_aligned = true;
>  
>  	rc = bdev_dax_pgoff(iomap->bdev, sector, PAGE_SIZE, &pgoff);
> @@ -1058,8 +1058,7 @@ int dax_iomap_zero(loff_t pos, unsigned offset, unsigned size,
>  	id = dax_read_lock();
>  
>  	if (page_aligned)
> -		rc = dax_zero_page_range(iomap->dax_dev, pgoff,
> -					 size >> PAGE_SHIFT);
> +		rc = dax_zero_page_range(iomap->dax_dev, pgoff, 1);
>  	else
>  		rc = dax_direct_access(iomap->dax_dev, pgoff, 1, &kaddr, NULL);
>  	if (rc < 0) {
> @@ -1072,7 +1071,7 @@ int dax_iomap_zero(loff_t pos, unsigned offset, unsigned size,
>  		dax_flush(iomap->dax_dev, kaddr + offset, size);
>  	}
>  	dax_read_unlock(id);
> -	return 0;
> +	return size;
>  }
>  
>  static loff_t
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 7f618ab4b11e..2dba054095e8 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -901,11 +901,13 @@ iomap_file_unshare(struct inode *inode, loff_t pos, loff_t len,
>  }
>  EXPORT_SYMBOL_GPL(iomap_file_unshare);
>  
> -static int iomap_zero(struct inode *inode, loff_t pos, unsigned offset,
> -		unsigned bytes, struct iomap *iomap, struct iomap *srcmap)
> +static loff_t iomap_zero(struct inode *inode, loff_t pos, u64 length,
> +		struct iomap *iomap, struct iomap *srcmap)
>  {
>  	struct page *page;
>  	int status;
> +	unsigned offset = offset_in_page(pos);
> +	unsigned bytes = min_t(u64, PAGE_SIZE - offset, length);
>  
>  	status = iomap_write_begin(inode, pos, bytes, 0, &page, iomap, srcmap);
>  	if (status)
> @@ -917,38 +919,33 @@ static int iomap_zero(struct inode *inode, loff_t pos, unsigned offset,
>  	return iomap_write_end(inode, pos, bytes, bytes, page, iomap, srcmap);
>  }
>  
> -static loff_t
> -iomap_zero_range_actor(struct inode *inode, loff_t pos, loff_t count,
> -		void *data, struct iomap *iomap, struct iomap *srcmap)
> +static loff_t iomap_zero_range_actor(struct inode *inode, loff_t pos,
> +		loff_t length, void *data, struct iomap *iomap,
> +		struct iomap *srcmap)
>  {
>  	bool *did_zero = data;
>  	loff_t written = 0;
> -	int status;
>  
>  	/* already zeroed?  we're done. */
>  	if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN)
> -		return count;
> +		return length;
>  
>  	do {
> -		unsigned offset, bytes;
> -
> -		offset = offset_in_page(pos);
> -		bytes = min_t(loff_t, PAGE_SIZE - offset, count);
> +		loff_t bytes;
>  
>  		if (IS_DAX(inode))
> -			status = dax_iomap_zero(pos, offset, bytes, iomap);
> +			bytes = dax_iomap_zero(pos, length, iomap);
>  		else
> -			status = iomap_zero(inode, pos, offset, bytes, iomap,
> -					srcmap);
> -		if (status < 0)
> -			return status;
> +			bytes = iomap_zero(inode, pos, length, iomap, srcmap);
> +		if (bytes < 0)
> +			return bytes;
>  
>  		pos += bytes;
> -		count -= bytes;
> +		length -= bytes;
>  		written += bytes;
>  		if (did_zero)
>  			*did_zero = true;
> -	} while (count > 0);
> +	} while (length > 0);
>  
>  	return written;
>  }
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index 6904d4e0b2e0..80f17946f940 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -214,8 +214,7 @@ 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);
> -int dax_iomap_zero(loff_t pos, unsigned offset, unsigned size,
> -			struct iomap *iomap);
> +loff_t 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);
> -- 
> 2.28.0
>