File and device pages and buffers

File and device pages and buffers

[Martin Jambor]

There is one thing about cache pages and buffers that puzzles me and
that I have not found in any documenatation and that is not quite
obvious from the source.

What is the relationship between:
a) pages and their buffers associated with the device (tybically
bh->bdev->bd_inode and its mapping) and

b) pages and their buffers associated with files on that device.

I am almost certain they are not the same but at the same time the
relationship seems to be quite important, otherwise we wouldn't have
such dark functions as unmap_underlying_metadata (why on earth is it
called this way? - Does metadata mean, in true kernel naming confusion
spirit, device pages/blocks?).

Can anyone tell me  what the relationship exactly is and what one has
to be aware of when manipulating either of these?

Thank you very much in advance,

Martin Jambor

Re: File and device pages and buffers

[Martin Jambor]

Can I try to bring your attention once more to the following issue:

On 4/18/05, Martin Jambor <jamborm@gmail.com> wrote:
> There is one thing about cache pages and buffers that puzzles me and
> that I have not found in any documenatation and that is not quite
> obvious from the source.
> 
> What is the relationship between:
> a) pages and their buffers associated with the device (typically
> bh->bdev->bd_inode and its mapping) and
> 
> b) pages and their buffers associated with files on that device.
> 
> Can anyone tell me  what the relationship exactly is and what one has
> to be aware of when manipulating either of these?

My conclusion from reading the source is that after mapping a file
buffer (buffer belonging to a page that is a part of a file on a
device) to a particular block on a device, one should call
unmap_underlying_metadata to make sure any writing using a device
buffer will not overwrite the file. And that one must not use device
buffers to write to places which have been mapped to files.

Is that correct?

TIA

Martin