The address returned by the kmalloc is a virtual address which is already mapped in the kernel page tables The address has a direct mapping to the physical memory and the address will be in between PAGE_OFFSET ( 3GB normally) and high_memory (The virutail address that corresponds to the end of physical memory, remember the kernel maps all the physical RAM above PAGE_OFFSET). Kmalloc() uses the slab allocater to ge a contiguous physical memory (good for DMA, more efficient to avoid internal and external fragmentation) area of any size but less than 131072 (I think). kmem_cache_alloc() is a similar function but used to allocate memory for well known kernel structures from the slab allocator (For eg: tast_struct). In both cases the allocated memory will be physically contiguous and the virtual address also will be contiguous. Also there is NO manipulation of kernel page tables as the physical memory is already mapped in the page tables. Vmalloc does not guarantee that the physical memory is contiguous, but the virtual address will be contiguous. Return the virtual address above high_memory and vmalloc requires the manipulation ( It updates the page directory/page table to add the new mapping) of kernel page tables to map the physical memory frames allocated (From the buddy system in multiple of 4K) above the high_memory and return the virtual address. Aboo -----Original Message----- From: kernelnewbies-bounce@xxxxxxxxxxxx [mailto:kernelnewbies-bounce@xxxxxxxxxxxx] On Behalf Of Mandeep Sandhu Sent: Monday, October 11, 2004 3:20 PM To: arm@xxxxxxxxxxxxxxx Cc: 'matrix reloaded'; 'kernelnewbies'; 'manish regmi' Subject: RE: memory used by kernel? On Fri, 2004-10-08 at 21:09, Anand wrote: > hey this link sld help u better http://kerneltrap.org/node/view/2450 > , > More over in the kernel space kmalloc() does the Physical Memory > allocation, but the address returned by kmalloc is also a virtual address right?? it's only that kmalloc gives a continuous chunk of mem. as compared to vmalloc thanx, -mandeep > and vmalloc does the Virtual Memory allocation. > > Rgds, > Anand > -----Original Message----- > From: kernelnewbies-bounce@xxxxxxxxxxxx > [mailto:kernelnewbies-bounce@xxxxxxxxxxxx]On Behalf Of matrix > reloaded > Sent: Friday, October 08, 2004 3:46 PM > To: Mandeep Sandhu > Cc: kernelnewbies; manish regmi > Subject: Re: memory used by kernel? > > > > > > > On Fri, 08 Oct 2004 Mandeep Sandhu wrote : > > >then is there amax limit upto which the kernel can allocate > mem > >for itself + what is the significance of the 3G/1G split?? > >Is'nt this 1G mapping the limit upto which all kernel space > >addresses can exist??? can the kernel eat into user-space mem > >as well? > > This 1G thing is virtual address space division, but you are > right in the sense that > this may be the limit for kernel memory usage, as for physical > memory you need to > have virtual address space also in protected mode on x86. > > Thanks. > Sumit Sharma > IBM, Bangalore. > > > > -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/ -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
