Re: Is nobh code still useful?

[Badari Pulavarty <pbadari@us.ibm.com>]

Jan Kara wrote:
>   Hi,
>
>   during my page_mkwrite() work, I've looked at who uses nobh_ versions of
> various functions in fs/buffer.c. It seems only ext2 and jfs use them. ext3
> uses them only from writepage() (which means we needn't attach buffers to a
> page when it was written via mmap in writeback mode) and ext4 tries to use
> them but in fact it's nop because it always attaches buffers to the page
> earlier. So it's not really widely used, there's quite some code to support
> it (including one page flag), and it also slightly complicates my
> page_mkwrite() fixes.
>   So I wanted to ask does somebody actually remember what it is good for?
> Buffer heads obviously consume some memory so was that the reason? OTOH we
> have to map the page whenever we write to it or send it to disk via
> writepage().
>
>   
Originally it was supported on ext2. I added support nobh support for 
ext3. At that time, the main
issue/complaint was that, these bufferheads consume memory from 
ZONE_NORMAL causing
memory pressure on 32-bit (i386) configurations.

Thanks,
Badari