Hi, On Fri, 30 Dec 2005 15:39:21 +0530 Parag N(पराग़) <panemade@xxxxxxxxx> wrote: > I think you have to use kmalloc(64,GFP_ATOMIC) then u will not > get any memory allocation error. Okay, let's make a longer explanation. The code that actually does this is __alloc_pages, at http://lxr.linux.no/source/mm/page_alloc.c#L695. As you can see in http://lxr.linux.no/source/include/linux/gfp.h#L50, GFP_ATOMIC doesn't have the __GFP_WAIT flag. This means that an atomic allocations cannot sleep. In __alloc_pages(), in we reach http://lxr.linux.no/source/mm/page_alloc.c#L770 for an atomic allocation, we fail and return NULL. Otherwise, if it's not an atomic allocation (__GFP_WAIT is set), then we can try harder to free memory, which will possibly require to sleep. However, the thing is even more complicated because GFP_ATOMIC is __GFP_HIGH, which autorizes __alloc_pages() to allocate memory from an emergency pool (see http://lxr.linux.no/source/mm/page_alloc.c#L664 with gfp_high = TRUE). On the opposite, GFP_KERNEL doesn't have __GFP_HIGH (but has __GFP_WAIT), so such allocations can sleep, but won't go allocate inside the emergency pool. See http://lxr.linux.no/source/include/linux/gfp.h#L29 for an explanation of each __GFP_* flag. For more information, read http://lwn.net/images/pdf/LDD3/ch08.pdf, which is the 8th chapter of Linux Device Drivers. It explains all that stuff very well. Sincerly, Thomas -- PETAZZONI Thomas - thomas.petazzoni@xxxxxxxx http://{thomas,sos,kos}.enix.org - Jabber: thomas.petazzoni@xxxxxxxxx http://{agenda,livret}dulibre.org - http://www.toulibre.org Fingerprint : 0BE1 4CF3 CEA4 AC9D CC6E 1624 F653 CB30 98D3 F7A7
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