Re: Lazy block allocation and block_prepare_write?

[Alex Tomas <alex@clusterfs.com>]

>>>>> Nikita Danilov (ND) writes:

 >>> > In order to do the correct accounting, we need to mark a page
 >>> > to indicate if we reserved a block or not. One way to do this,
 >>> > to use page->private to indicate this. But then, all the generic
 >>> 
 >>> I believe one can use PG_mappedtodisk bit in page->flags for this
 >>> purpose. There was old Andrew Morton's patch that introduced new bit
 >>> (PG_delalloc?) for this purpose.
 >> 
 >> That would be good. But I don't feel like asking for a bit in page
 >> if there is a way to get around it.

 ND> Clarification: PG_mappedtodisk is already here, it seems you can reuse
 ND> this already existing bit to implement delayed allocation support.

I think we need another one, because mappedtodisk != reserved. we could use
mappedtodisk, but this means in ->commit_write() we'd need to check that one
more time (first time in ->prepare_write())

thanks, Alex