Re: Lazy block allocation and block_prepare_write?

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

Alex Tomas wrote:
>>>>>>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())

Yep. We need one more to indicate the we reserved a block for this page.

Other option I was thinking on how to avoid is, by "reserving" a block
when a mapped page changes from read -> write. Andrew's -mm tree has
patch to give us a notification when it happens.

Thanks,
Badari