Re: [PATCH v7] iomap: make inline data support more flexible

[Gao Xiang]

On Mon, Jul 26, 2021 at 12:16:39AM +0200, Andreas Gruenbacher wrote:
> Here's a fixed and cleaned up version that passes fstests on gfs2.

(cont.
https://lore.kernel.org/r/YP4fk75mr%2FmIotDy@B-P7TQMD6M-0146.local)

Would you mind listing what it fixed on gfs2 compared with v7?
IOWs, I wonder which case failed with v7 on gfs2 so I could recheck
this.

> 
> I see no reason why the combination of tail packing + writing should
> cause any issues, so in my opinion, the check that disables that
> combination in iomap_write_begin_inline should still be removed.
> 
> It turns out that returning the number of bytes copied from
> iomap_read_inline_data is a bit irritating: the function is really used
> for filling the page, but that's not always the "progress" we're looking
> for.  In the iomap_readpage case, we actually need to advance by an
> antire page, but in the iomap_file_buffered_write case, we need to
> advance by the length parameter of iomap_write_actor or less.  So I've
> changed that back.
> 
> I've also renamed iomap_inline_buf to iomap_inline_data and I've turned
> iomap_inline_data_size_valid into iomap_within_inline_data, which seems
> more useful to me.
> 
> Thanks,
> Andreas
> 
> --
> 
> Subject: [PATCH] iomap: Support tail packing
> 
> The existing inline data support only works for cases where the entire
> file is stored as inline data.  For larger files, EROFS stores the
> initial blocks separately and then can pack a small tail adjacent to the
> inode.  Generalise inline data to allow for tail packing.  Tails may not
> cross a page boundary in memory.
> 
> We currently have no filesystems that support tail packing and writing,
> so that case is currently disabled (see iomap_write_begin_inline).  I'm
> not aware of any reason why this code path shouldn't work, however.
> 
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Darrick J. Wong <djwong@xxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Andreas Gruenbacher <andreas.gruenbacher@xxxxxxxxx>
> Tested-by: Huang Jianan <huangjianan@xxxxxxxx> # erofs
> Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>
> ---
>  fs/iomap/buffered-io.c | 34 +++++++++++++++++++++++-----------
>  fs/iomap/direct-io.c   | 11 ++++++-----
>  include/linux/iomap.h  | 22 +++++++++++++++++++++-
>  3 files changed, 50 insertions(+), 17 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 87ccb3438bec..334bf98fdd4a 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -205,25 +205,29 @@ struct iomap_readpage_ctx {
>  	struct readahead_control *rac;
>  };
>  
> -static void
> -iomap_read_inline_data(struct inode *inode, struct page *page,
> +static int iomap_read_inline_data(struct inode *inode, struct page *page,
>  		struct iomap *iomap)
>  {
> -	size_t size = i_size_read(inode);
> +	size_t size = i_size_read(inode) - iomap->offset;
>  	void *addr;
>  
>  	if (PageUptodate(page))
> -		return;
> +		return 0;
>  
> -	BUG_ON(page_has_private(page));
> -	BUG_ON(page->index);
> -	BUG_ON(size > PAGE_SIZE - offset_in_page(iomap->inline_data));
> +	/* inline and tail-packed data must start page aligned in the file */
> +	if (WARN_ON_ONCE(offset_in_page(iomap->offset)))
> +		return -EIO;
> +	if (WARN_ON_ONCE(size > PAGE_SIZE - offset_in_page(iomap->inline_data)))
> +		return -EIO;
> +	if (WARN_ON_ONCE(page_has_private(page)))
> +		return -EIO;
>  
>  	addr = kmap_atomic(page);
>  	memcpy(addr, iomap->inline_data, size);
>  	memset(addr + size, 0, PAGE_SIZE - size);
>  	kunmap_atomic(addr);
>  	SetPageUptodate(page);
> +	return 0;
>  }
>  
>  static inline bool iomap_block_needs_zeroing(struct inode *inode,
> @@ -247,7 +251,6 @@ iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
>  	sector_t sector;
>  
>  	if (iomap->type == IOMAP_INLINE) {
> -		WARN_ON_ONCE(pos);
>  		iomap_read_inline_data(inode, page, iomap);
>  		return PAGE_SIZE;
>  	}
> @@ -589,6 +592,15 @@ __iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, int flags,
>  	return 0;
>  }
>  
> +static int iomap_write_begin_inline(struct inode *inode,
> +		struct page *page, struct iomap *srcmap)
> +{
> +	/* needs more work for the tailpacking case, disable for now */
> +	if (WARN_ON_ONCE(srcmap->offset != 0))
> +		return -EIO;
> +	return iomap_read_inline_data(inode, page, srcmap);
> +}
> +
>  static int
>  iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
>  		struct page **pagep, struct iomap *iomap, struct iomap *srcmap)
> @@ -618,7 +630,7 @@ iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
>  	}
>  
>  	if (srcmap->type == IOMAP_INLINE)
> -		iomap_read_inline_data(inode, page, srcmap);
> +		status = iomap_write_begin_inline(inode, page, srcmap);
>  	else if (iomap->flags & IOMAP_F_BUFFER_HEAD)
>  		status = __block_write_begin_int(page, pos, len, NULL, srcmap);
>  	else
> @@ -671,11 +683,11 @@ static size_t iomap_write_end_inline(struct inode *inode, struct page *page,
>  	void *addr;
>  
>  	WARN_ON_ONCE(!PageUptodate(page));
> -	BUG_ON(pos + copied > PAGE_SIZE - offset_in_page(iomap->inline_data));
> +	BUG_ON(!iomap_within_inline_data(iomap, pos + copied - 1));
>  
>  	flush_dcache_page(page);
>  	addr = kmap_atomic(page);
> -	memcpy(iomap->inline_data + pos, addr + pos, copied);
> +	memcpy(iomap_inline_data(iomap, pos), addr + pos, copied);
>  	kunmap_atomic(addr);
>  
>  	mark_inode_dirty(inode);
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index 9398b8c31323..c9424e58f613 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -380,21 +380,22 @@ iomap_dio_inline_actor(struct inode *inode, loff_t pos, loff_t length,
>  	struct iov_iter *iter = dio->submit.iter;
>  	size_t copied;
>  
> -	BUG_ON(pos + length > PAGE_SIZE - offset_in_page(iomap->inline_data));
> +	if (WARN_ON_ONCE(!iomap_within_inline_data(iomap, pos + length - 1)))
> +		return -EIO;

I also wonder what is wrong with the previous patch:

+	if (WARN_ON_ONCE(!iomap_inline_data_size_valid(iomap)))
+		return -EIO;

+/*
+ * iomap->inline_data is a potentially kmapped page, ensure it never crosses a
+ * page boundary.
+ */
+static inline bool iomap_inline_data_size_valid(const struct iomap *iomap)
+{
+	return iomap->length <= PAGE_SIZE - offset_in_page(iomap->inline_data);
+}

In principle, the relationship of iomap->offset, pos, length and
iomap->length is:

"	iomap->offset <= pos < pos + length <= iomap->offset +
iomap->length	"

pos and pos + length are also impacted by what user requests rather
than the original extent itself reported by fs.

Here we need to make sure the whole extent in the page, so I think
it'd be better to check with iomap->length rather than some pos,
length related stuffs.

>  
>  	if (dio->flags & IOMAP_DIO_WRITE) {
> -		loff_t size = inode->i_size;
> +		loff_t size = iomap->offset + iomap->length;

and here, since it's the last extent and due to the current limitation
in practice,
iomap->offset + iomap->length == inode->i_size,

yet I wonder why this part uses iomap->length to calculate instead of
using i_size as in iomap_read_inline_data().

My thought is "here it handles the i_size pointer and append write",
so I think "loff_t size = inode->i_size" makes more sense here.

>  
>  		if (pos > size)
> -			memset(iomap->inline_data + size, 0, pos - size);
> -		copied = copy_from_iter(iomap->inline_data + pos, length, iter);
> +			memset(iomap_inline_data(iomap, size), 0, pos - size);
> +		copied = copy_from_iter(iomap_inline_data(iomap, pos), length, iter);

iomap_inline_buf() was suggested by Darrick. From my point of view,
I think it's better since it's a part of iomap->inline_data due to
pos involved.

Thanks,
Gao Xiang