linux-kernel - Re: upcoming kerneloops.org item: get_page_from

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Message-ID: <20090625040556.GA15782@wotan.suse.de>
Date:	Thu, 25 Jun 2009 06:05:56 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	penberg@...helsinki.fi, arjan@...radead.org,
	linux-kernel@...r.kernel.org, cl@...ux-foundation.org
Subject: Re: upcoming kerneloops.org item: get_page_from_freelist

On Wed, Jun 24, 2009 at 03:07:14PM -0700, Andrew Morton wrote:
> On Wed, 24 Jun 2009 13:40:11 -0700 (PDT)
> Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> > On Wed, 24 Jun 2009, Linus Torvalds wrote:
> > > On Wed, 24 Jun 2009, Andrew Morton wrote:
> > actually makes tons of sense. 
> 
> I suspect that warning will trigger.
> 
> alloc_pages
> -> ...
>   -> pageout
>     -> ...
>       -> get_request
>         -> blk_alloc_request
>           -> elv_set_request
>             -> cfq_set_request
>               -> cfq_get_queue
>                 -> cfq_find_alloc_queue
>                   -> kmem_cache_alloc_node(__GFP_NOFAIL)
>                     -> Jens
> 
> How much this can happen in practice I don't know, but it looks bad.
> 
> > There are other cases where __GFP_NOFAIL doesn't make sense too, and that 
> > could be warned about. The __GFP_NORETRY thing was already mentioned. 
> > Similarly, !__GFP_WAIT doesn't work with __GFP_NOFAIL - because the nofail 
> > obviously relies on being able to do something about the failure case.
> > 
> > We might want to also have rules like "in order to have NOFAIL, you need 
> > to allow IO and FS accesses". 
> 
> Sure, that's sane.
> 
> fs/jbd/journal.c:       new_bh = alloc_buffer_head(GFP_NOFS|__GFP_NOFAIL);
> 
> But that isn't :(

Unfortunately there's a lot of "hidden" ones in fs/buffer.c.
alloc_page_buffers with arg3=1, which is used quite a lot in
create_empty_buffers.

Also getblk. Sad.

fsblock FTW :)

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