Re: Alloc and lock down large amounts of memory

From: Gilad Ben-Yossef (
Date: Tue Aug 20 2002 - 00:38:31 EST

On Mon, 2002-08-19 at 15:06, Bhavana Nagendra wrote:

> Here's the scenario: I need to be able to alloc virtual memory, create the
> page table
> and temporarily pin down (lock) the pages in memory in order to do the DMA.
> This
> memory should be paged in and out as needed. This memory should also have
> visibility across processes.

OK. *AFAIK* most drivers that use DMA request the memory allocation with
GFP_DMA so that the allocators (whatever they chose) will give them
DMAbale memory from the DMA memory zone. Memory from that zone is fixed
(not pagable). You can then map that memory into proccesses address
space according to demand (and the Linux device drivers second addition
has a good example of doing that).

Now, if I understood you correctly you want to allocate 256M of memroy
and perform DMA into it? if so, you *cannot* use memory allocated using
vmalloc because the memory it supplies is virtual, that is it not
contigous in physical memory but might be scattered all over the place
and I doubt that whatever device you're DMAing from can handle that.

Also, I think you don't want this memory to be swapable, because if it
is swapped out and then in and might very well end up on a completly
different address in physical memory from where it were and again, I
don't think the device that does DMA will be able to handle that - all
the ones I know require physical addresses (well actualy bus addresses
but for the sake of argument let's ignore that for a second).

In short, I don't think you need what you think want... :-)

> Does the VM_RESERVED flag lock down the memory so that it doesn't get paged out
> during DMA?

AFAIK the VM_RESERVED flag will cause kswapd to ignore the page
completly - no paging in or out at all.

> >
> >
> > > Can 256M be allocated using vmalloc, if so is it swappable?
> >
> > It can be alloacted via vmalloc and AFAIK it is not swappable by
> > default. This doesn't sound like a very good idea though.
> Is there a good way to allocate large sums of memory in Linux? Doesn't have to
> be
> vmalloc but I don't think kmalloc, get_free_pages will work for this purpose. I
> looked
> into get_free_pages, but the largest order is 9 which results in 512 pages.

> Does the memory allocated by vmalloc has visibility across processes?

See my previous answer regarding why you don't vmalloced memory at

> I didn't mean shared memory. If several processes open a given device,
> under normal conditions the data structure stays till the last close at which
> time a
> release is done. This depends on the usage or minor number count. Can there
> be a case where the device exits before the processes close? In which case
> the processes will be left hanging. How is the close handled if the driver is
> killed?

Very simple - you can't unload the device until the ref count says all
the users (proccesses in thuis case) have closed it.


Gilad Ben-Yossef <>

"Money talks, bullshit walks and GNU awks." -- Shachar "Sun" Shemesh, debt collector for the GNU/Yakuza

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