Re: [patch 6/10] mm: be sure to trim blocks

[Dmitriy Monakhov]

Nick Piggin <npiggin@xxxxxxx> writes:

> If prepare_write fails with AOP_TRUNCATED_PAGE, or if commit_write fails, then
> we may have failed the write operation despite prepare_write having
> instantiated blocks past i_size. Fix this, and consolidate the trimming into
> one place.
>
> Signed-off-by: Nick Piggin <npiggin@xxxxxxx>
>
> Index: linux-2.6/mm/filemap.c
> ===================================================================
> --- linux-2.6.orig/mm/filemap.c
> +++ linux-2.6/mm/filemap.c
> @@ -1911,22 +1911,9 @@ generic_file_buffered_write(struct kiocb
>  		}
>  
>  		status = a_ops->prepare_write(file, page, offset, offset+bytes);
> -		if (unlikely(status)) {
> -			loff_t isize = i_size_read(inode);
> +		if (unlikely(status))
> +			goto fs_write_aop_error;
May be it's stupid question but still..
Why we treat non zero prepare_write() return code as error, it may be positive.
Positive error code may be used as fine grained 'bytes' limiter in case of 
blksize < pgsize as follows:

                status = a_ops->prepare_write(file, page, offset, offset+bytes);
		if (unlikely(status)) {
                        if (status > 0) {
                                bytes = min(bytes, status);
                                status = 0;
                        } else {
                	        goto fs_write_aop_error;
                        }
                }
---
This is useful because fs may want to reduce 'bytes' by number of reasons,
for example make it blksize bound. 
Example : filesystem has 1k blksize and only two free blocks. And we try 
write 4k bytes.
Currently  write(fd, buff, 4096) will return -ENOSPC
But after this fix write(fd, buff, 4096) will return as mutch as it can (2048).
>  
> -			if (status != AOP_TRUNCATED_PAGE)
> -				unlock_page(page);
> -			page_cache_release(page);
> -			if (status == AOP_TRUNCATED_PAGE)
> -				continue;
> -			/*
> -			 * prepare_write() may have instantiated a few blocks
> -			 * outside i_size.  Trim these off again.
> -			 */
> -			if (pos + bytes > isize)
> -				vmtruncate(inode, isize);
> -			break;
> -		}
>  		if (likely(nr_segs == 1))
>  			copied = filemap_copy_from_user(page, offset,
>  							buf, bytes);
> @@ -1935,10 +1922,9 @@ generic_file_buffered_write(struct kiocb
>  						cur_iov, iov_offset, bytes);
>  		flush_dcache_page(page);
>  		status = a_ops->commit_write(file, page, offset, offset+bytes);
> -		if (status == AOP_TRUNCATED_PAGE) {
> -			page_cache_release(page);
> -			continue;
> -		}
> +		if (unlikely(status))
> +			goto fs_write_aop_error;
> +
>  		if (likely(copied > 0)) {
>  			if (!status)
>  				status = copied;
> @@ -1969,6 +1955,25 @@ generic_file_buffered_write(struct kiocb
>  			break;
>  		balance_dirty_pages_ratelimited(mapping);
>  		cond_resched();
> +		continue;
> +
> +fs_write_aop_error:
> +		if (status != AOP_TRUNCATED_PAGE)
> +			unlock_page(page);
> +		page_cache_release(page);
> +
> +		/*
> +		 * prepare_write() may have instantiated a few blocks
> +		 * outside i_size.  Trim these off again. Don't need
> +		 * i_size_read because we hold i_mutex.
> +		 */
> +		if (pos + bytes > inode->i_size)
> +			vmtruncate(inode, inode->i_size);
> +		if (status == AOP_TRUNCATED_PAGE)
> +			continue;
> +		else
> +			break;
> +
>  	} while (count);
>  	*ppos = pos;
>  
> -
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