Re: can a kmalloc be both GFP_ATOMIC and GFP_KERNEL at the same time?

On Mon, 30 Apr 2007, Jan Engelhardt wrote:


On Apr 30 2007 04:46, Robert P. J. Day wrote:

i'd always assumed that the type flags of GFP_ATOMIC and GFP_KERNEL
were mutually exclusive when it came to calling kmalloc(), at least
based on everything i'd read. so i'm not sure how to interpret the
following:

drivers/scsi/aic7xxx_old.c: aic_dev = kmalloc(sizeof(struct aic_dev_data), GFP_ATOMIC | GFP_KERNEL);
drivers/message/i2o/device.c: resblk = kmalloc(buflen + 8, GFP_KERNEL | GFP_ATOMIC);

clarification?

GFP_ATOMIC implies that the memory comes from the zones which
GFP_KERNEL also uses. So the above usage of GFP_KERNEL is redundant
and should be removed.

hang on ... based on an email i just got, is that reference to
GFP_KERNEL "redundant" or "conflicting"? big difference there. and
is the proper fix to remove "GFP_KERNEL" in both cases?

include/linux/gfp.h:
#define GFP_ATOMIC (__GFP_HIGH)
#define GFP_KERNEL (__GFP_WAIT | __GFP_IO | __GFP_FS)

So combining GFP_ATOMIC with GFP_KERNEL gives you
"allow io, allow fs, allow waiting, and use emergency pools when it's getting
tight"
which to me looks like a valid, but probably unwanted combination.

... snip ...

at this point, maybe i'll just leave this in the hands of those who
know far more about it than i do. but, while we're here, are there
any *other* combinations that wouldn't make any sense? might as well
check for those in my cleanup script as well.

rday

p.s. as a suggestion that borders on overkill, one could always add a
configuration debugging option that, when set, compiles code into
kmalloc() that does a sanity check on its type flag arguments.

--
========================================================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA

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