Re: Decompression success/failure dependent on PAGE_SIZE?

[Nikolay Borisov <n.borisov.lkml@gmail.com>]

On 29.08.2017 19:43, Marek Behún wrote:
> Hello,
> 
> so I've been studying the linux btrfs code and have come across this:
> 
> in inode.c function uncompress_inline the max_size size variable is set
> to min(max_size, PAGE_SIZE) and only max_size of output data are
> decompressed.
> 
> The code for compression (in lzo.c for example) uses PAGE_SIZEd chunks
> to compress an inline extent.
> 
> If I understand it correctly, then if the filesystem is created and used
> on a computer with PAGE_SIZE for example 16KB, and an extent of size
> 16KB is compressed to (for example 9KB) and stored as inline extent,
> and then the filesystem is mounted on a computer with PAGE_SIZE = 4KB,
> reading the extent will result in a failure or incomplete read.
> 
> Is this a bug, or is this behaviour a feature?

Please have a look in btrfs_check_super_valid(), which is being called
from open_ctree, which in turn is called when a filesystem is mounted.

Currently btrfs supports a single blocksize value - PAGE_SIZE, also it's
forbidden to mount a filesystem on systems with different page_sizes.
When the sub-page blocksizes patch is merged this might be have to be
taken into consideration but certainly not at present times.

> 
> Thank you.
> 
> Marek
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>