Re: [PATCH 02/11] persistent_ram: Fix buffer size clamping during writes

Anton Vorontsov
2012-05-13T20:40:08Z

On Sun, May 13, 2012 at 07:56:01PM +0300, Dan Carpenter wrote:
> On Fri, May 11, 2012 at 05:17:17PM -0700, Anton Vorontsov wrote:
> > This is a longstanding bug, almost unnoticeable when calling
> > persistent_ram_write() for small buffers.
> >
> > But when called for large data buffers, the write routine behaves
> > incorrectly, as the size may never update: instead of clamping
> > the size to the maximum buffer size, buffer_size_add_clamp() returns
> > an error (which is never checked by the write routine, btw).
> >
> > To fix this, we now use buffer_size_add() that actually clamps the
> > size to the max value.
> >
> > Also remove buffer_size_add_clamp(), it is no longer needed.
> >
>
> Say if you did notice it, what would that look like? It's just that
> something gets lost instead of written to the screen right?

Yep. Suppose the ring buffer size is 4096 bytes, when somebody tries to
write a data in a 2000 bytes chunk, the first write will succeed (buffer
size will be 2000), but the second now 3000-bytes write will left the
size equal to 2000, instead of clamping it to 4096.

When we had a large buffer but a small writes (e.g. ram_console usage
scenario), this is almost unnoticeable. But when we started using large
writes the bug showed up.

Thanks,

--
Anton Vorontsov
Email: cbouatmailru@gmail.com