A question on block_prepare_write()

A question on block_prepare_write()

[Namhyung Kim]

Hello,

I see block_prepare_write() has local variable wait[2] to keep track of
buffer_heads which are not up-to-date. But I'm wondering how it could be
guaranteed there will be no more than 2 such buffer_heads? Is there any
restriction on the usage of the function? Using MAX_BUF_PER_PAGE instead
of the magic number 2 is just useless? I couldn't find any comments or
documentation on this.

Any of your comments would be greatly appreciated. TIA. :-)

-- 
Regards,
Namhyung Kim

Re: A question on block_prepare_write()

[Andrew Morton]

On Sat, 23 Oct 2010 22:44:42 +0900 Namhyung Kim <namhyung@gmail.com> wrote:

> Hello,
> 
> I see block_prepare_write() has local variable wait[2] to keep track of
> buffer_heads which are not up-to-date. But I'm wondering how it could be
> guaranteed there will be no more than 2 such buffer_heads? Is there any
> restriction on the usage of the function? Using MAX_BUF_PER_PAGE instead
> of the magic number 2 is just useless? I couldn't find any comments or
> documentation on this.
> 
> Any of your comments would be greatly appreciated. TIA. :-)
> 

block_prepare_write() may need to preread any buffer_head which are
being only partially modified by the write().

Buffers which aren't being modified at all don't need to be preread. 
Buffers which are being fully modified don't need to be preread
(because all their data is being overwritten).

page:                      |-----------------------|
buffer_heads:              |-----|-----|-----|-----|
area we're writing to:               |---------|

There can only be a maximum of two partially-modified buffers in the page.

Re: A question on block_prepare_write()

[Namhyung Kim]

2010-10-23 (토), 10:40 -0700, Andrew Morton:
> On Sat, 23 Oct 2010 22:44:42 +0900 Namhyung Kim <namhyung@gmail.com> wrote:
> 
> > Hello,
> > 
> > I see block_prepare_write() has local variable wait[2] to keep track of
> > buffer_heads which are not up-to-date. But I'm wondering how it could be
> > guaranteed there will be no more than 2 such buffer_heads? Is there any
> > restriction on the usage of the function? Using MAX_BUF_PER_PAGE instead
> > of the magic number 2 is just useless? I couldn't find any comments or
> > documentation on this.
> > 
> > Any of your comments would be greatly appreciated. TIA. :-)
> > 
> 
> block_prepare_write() may need to preread any buffer_head which are
> being only partially modified by the write().
> 
> Buffers which aren't being modified at all don't need to be preread. 
> Buffers which are being fully modified don't need to be preread
> (because all their data is being overwritten).
> 
> page:                      |-----------------------|
> buffer_heads:              |-----|-----|-----|-----|
> area we're writing to:               |---------|
> 
> There can only be a maximum of two partially-modified buffers in the page.

I see. It considers both edges of the to-be-written data. Thanks for
your clear and kindly explanation. :-)


-- 
Regards,
Namhyung Kim


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