[uml-devel] GFP_ATOMIC/GFP_KERNEL broken for use by block devices!!

[uml-devel] GFP_ATOMIC/GFP_KERNEL broken for use by block devices!!

[Blaisorblade]

Jeff, remember I asked you not to merge the UBD rewrite for now? Well, I found 
one more (and bigger) reason for it, though I don't understand it fully.

Quoting from Arjan Van der Ven, "How not to write kernel driver":

In the write path of a filesystem, using
GFP_KERNEL or GFP_ATOMIC will lead to deadlocks
eventually. [...] GFP_NOFS is there for a reason, as is GFP_NOIO for block 
device drivers.

Though there's no explaination of this, I can guess that GFP_KERNEL could 
cause the kernel to enter the same (or another, but that shouldn't matter) 
block device to free some memory. Not sure what's the problem with GFP_ATOMIC 
though.

However, GFP_NO{FS,IO} both imply !ATOMIC (they include __GFP_WAIT), so we 
cannot use any of them for our purposes.

Which means that we _must_ ask for help from somebody else.

Anyhow, before merging the patches I'd _really_ like if they were reviewed by 
somebody as Jens Axboe or the such, afterwards merged to -mm, and only after 
that merged into mainline.

Please, send some feedback on this email as soon as you read - think to 
replacing UDP with TCP, ok ;-) ?
-- 
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
http://www.user-mode-linux.org/~blaisorblade


	

	
		
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[uml-devel] Re: GFP_ATOMIC/GFP_KERNEL broken for use by block devices!!

[Jeff Dike]

On Sat, Oct 22, 2005 at 06:38:16PM +0200, Blaisorblade wrote:
> In the write path of a ???lesystem, using
> GFP_KERNEL or GFP_ATOMIC will lead to deadlocks
> eventually. [...] GFP_NOFS is there for a reason, as is GFP_NOIO for block 
> device drivers.

I know about this, and this is another reason I'm not sending it in.  I
don't really understand it either.  If GFP_ATOMIC can block, then I don't
know what good it is.

> Which means that we _must_ ask for help from somebody else.

Yup.

				Jeff


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[uml-devel] Re: GFP_ATOMIC/GFP_KERNEL broken for use by block devices!!

[Blaisorblade]

On Tuesday 25 October 2005 01:03, Jeff Dike wrote:
> On Sat, Oct 22, 2005 at 06:38:16PM +0200, Blaisorblade wrote:
> > In the write path of a ???lesystem, using
> > GFP_KERNEL or GFP_ATOMIC will lead to deadlocks
> > eventually. [...] GFP_NOFS is there for a reason, as is GFP_NOIO for
> > block device drivers.
>
> I know about this, and this is another reason I'm not sending it in.
I didn't know and you didn't mention, so I thought I'd better add about this.
> I 
> don't really understand it either.

> If GFP_ATOMIC can block, then I don't 
> know what good it is.
I _don't_ think it can block. I understand why GFP_KERNEL is broken there, but 
not why GFP_ATOMIC can't be used. The only pointer I found so far is that 
__GFP_HIGH allows the kernel to use up more memory without doing reclaim (see 
mm/page_alloc.c, calls to zone_watermark_ok).
> > Which means that we _must_ ask for help from somebody else.
>
> Yup.
>
> 				Jeff

-- 
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
http://www.user-mode-linux.org/~blaisorblade

	

	
		
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