Widespread misuse of GFP_ATOMIC

Widespread misuse of GFP_ATOMIC

[coypu]

Hello linux janitors,

from kmalloc man page:

GFP_ATOMIC - Allocation will not sleep. May use emergency pools. For
example, use this inside interrupt handlers.
GFP_NOWAIT - Allocation will not sleep.

grepping linux kernel tree:

[0:/usr/linux-src> ]$ grep -R GFP_ATOMIC . |wc -l
4452
[0:/usr/linux-src> ]$ grep -R GFP_NOWAIT . |wc -l
220

(alternative flags and some accounting:
total k.*alloc calls:   34320
GFP_KERNEL appearances: 23819
GDP_ATOMIC appearances: 4452
GFP_NOWAIT appearances: 220

unaccounted: 5829)

GFP_ATOMIC is meant for interrupt handlers and emergency pool.
Widespread misuse renders the emergency pool useless.

I believe the vast majority of such uses can be converted to GFP_NOWAIT.
Even valid uses of GFP_ATOMIC are likely rendered useless by this
widespread misuse.

You may wish to replace such calls.

Cheers, coypu

Re: Widespread misuse of GFP_ATOMIC

[Julia Lawall]

On Sun, 10 Apr 2016, coypu@SDF.ORG wrote:

> Hello linux janitors,
> 
> from kmalloc man page:
> 
> GFP_ATOMIC - Allocation will not sleep. May use emergency pools. For
> example, use this inside interrupt handlers.
> GFP_NOWAIT - Allocation will not sleep.
> 
> grepping linux kernel tree:
> 
> [0:/usr/linux-src> ]$ grep -R GFP_ATOMIC . |wc -l
> 4452
> [0:/usr/linux-src> ]$ grep -R GFP_NOWAIT . |wc -l
> 220
> 
> (alternative flags and some accounting:
> total k.*alloc calls:   34320
> GFP_KERNEL appearances: 23819
> GDP_ATOMIC appearances: 4452
> GFP_NOWAIT appearances: 220
> 
> unaccounted: 5829)
> 
> GFP_ATOMIC is meant for interrupt handlers and emergency pool.
> Widespread misuse renders the emergency pool useless.
> 
> I believe the vast majority of such uses can be converted to GFP_NOWAIT.
> Even valid uses of GFP_ATOMIC are likely rendered useless by this
> widespread misuse.
> 
> You may wish to replace such calls.

How does one know which need the emergency pool?

julia

Re: Widespread misuse of GFP_ATOMIC

[Dan Carpenter]

GFP_NOWAIT is when you have a somewhat painless backup plan in case the
allocation fails.  If you're worried that there are too many
GFP_ATOMICs, then it's better to re-arrange the code so the allocation
can sleep.

regards,
dan carpenter