Re: Warning on memory offline (and possible in usual migration?)

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Wed, 23 Apr 2008 04:53:58 +0200
Nick Piggin <npiggin@suse.de> wrote:

> > BTW, can I ask a question for understanding this change ?
> > 
> > ==this check==
> >  WARN_ON_ONCE(!PagePrivate(page) && !PageUptodate(page));
> > 
> > in __set_page_dirty_nobuffers() seems to check "the page should have buffer or
> > be up-to-date when it calls this function."
> > 
> > When it comes to __set_page_dirty() (in fs/buffer.c)
> > == this check==
> >  WARN_ON_ONCE(warn && !PageUptodate(page));
> > 
> > is used and doesn't see page has buffers or not.
> > What's difference between two functions's condition for WARNING ?
> 
> Yes, __set_page_dirty_nobuffers confusingly can also be called for pages
> with buffers. In the case that the page has buffers (or any other private
> metadata), then __set_page_dirty_nobuffers does not have enough information
> to know whether the page should be uptodate before being marked dirty.
> 
> In the __set_page_dirty case in fs/buffer.c, we _do_ know that the page
> has buffers and that it would be wrong to have a situation where the
> page is !uptodate at this point.
> 
> Is that clear? Or have I explained it poorly?
> 

Hmm...does that comes from difference of the purpose of the functions ?

Is this correct ?
==
set_page_dirty_buffers() (in fs/buffer.c) makes a page and _all_ buffers on it
dirty. So, a page *must* be up-to-date when it calls set_page_dirty_buffers().
This is used for mapped pages or some callers which requires the whole
page containes valid data.

In set_page_dirty_nobuffers()case , it just makes a page to be dirty. We can't
see whether a page is really up-to-date or not when PagePrivate(page) &&
!PageUptodate(page). This is used for a page which contains some data
to be written out. (part of buffers contains data.)

==

Thank you.

-Kame

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