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

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

On Wed, 23 Apr 2008 02:48:04 +0200
Nick Piggin <npiggin@suse.de> wrote:
> KAMEZAWA Hiroyuki found a warning message in the buffer dirtying code that
> is coming from page migration caller.
> 
> WARNING: at fs/buffer.c:720 __set_page_dirty+0x330/0x360()
> Call Trace:
>  [<a000000100015220>] show_stack+0x80/0xa0
>  [<a000000100015270>] dump_stack+0x30/0x60
>  [<a000000100089ed0>] warn_on_slowpath+0x90/0xe0
>  [<a0000001001f8b10>] __set_page_dirty+0x330/0x360
>  [<a0000001001ffb90>] __set_page_dirty_buffers+0xd0/0x280
>  [<a00000010012fec0>] set_page_dirty+0xc0/0x260
>  [<a000000100195670>] migrate_page_copy+0x5d0/0x5e0
>  [<a000000100197840>] buffer_migrate_page+0x2e0/0x3c0
>  [<a000000100195eb0>] migrate_pages+0x770/0xe00
> 
> What was happening is that migrate_page_copy wants to transfer the PG_dirty
> bit from old page to new page, so what it would do is set_page_dirty(newpage).
> However set_page_dirty() is used to set the entire page dirty, wheras in
> this case, only part of the page was dirty, and it also was not uptodate.
> 
> Marking the whole page dirty with set_page_dirty would lead to corruption or
> unresolvable conditions -- a dirty && !uptodate page and dirty && !uptodate
> buffers.
> 
> Possibly we could just ClearPageDirty(oldpage); SetPageDirty(newpage);
> however in the interests of keeping the change minimal...
> 
> Signed-off-by: Nick Piggin <npiggin@suse.de>

Tested and seems to work well. thank you!

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 ?

-Kame

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