Re: BUG in the block layer (partial reads not reported)

[Alan Cox <alan@lxorguk.ukuu.org.uk>]

On Sad, 2005-10-22 at 11:40 -0400, Alan Stern wrote:
> 	Handling an error somewhere in the middle of the medium, and
> 
> 	Handling an error beyond the real end of the medium.
> 
> The mm and block subsystems have no way at all to retrieve partial data
> for the first case.  Even though only one hardware sector may be bad,

The block layer can handle this at the bottom level but the caches above
it cannot. 

> failure to read an entire page means that none of the good sectors on that
> page will be accessible.  While annoying, it's understandable and I don't 
> see any simple way to accomodate such partial reads.

Agreed it is hairy with things like mmap. One way is to use raw I/O and
disable readahead.

> The second case appears to be more tractable, as you said.  In fact,
> do_generic_mapping_read() in mm/filemap.c will recheck the inode's size
> after a successful read, to avoid copying data beyond the end of the
> device.
> 
> Could part of the problem also be that the set_capacity() call, used to
> revise the device size downward when the CD driver realizes it is smaller
> than originally thought, doesn't update the inode?  Should the driver call
> bd_set_size() as well?  (In addition to completing the read successfully
> with garbage data beyond the actual EOF.)

Beats me. Perhaps Jens can enlighten us and I can improve the ide-cd
driver code further as well.

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