Messages in this thread |  | | Date | Thu, 27 Sep 2001 16:19:06 +0200 (MEST) | From | Bernd Harries <> | Subject | Re: __get_free_pages(): is the MEM really mine? |
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Ingo Molnar wrote:
> well - what did you expect to happen? A freed page is going to be reused > for other purposes. A big 2MB allocation can be reused in part, once > memory usage grows.
With my knowledge, I expected exactly that.
> So you should not expect the device to be able to DMA > into a page that got freed, unpunished.
I am not. The DMA ioctl() finishes before the close() -> free happens after the hexdump and the DMA. The buffer is allocated in open. The fact that I get the same buffer again next time shows that the free is sucessful and effective, right?
Sep 27 11:43:28 pcma73 kernel: rsc_open() minor=$1B Sep 27 11:43:28 pcma73 kernel: DMA blk 0 at KV:$CE800000 BUS:$0E800000 Sep 27 11:43:28 pcma73 kernel: DMA blk 1 at KV:$CE600000 BUS:$0E600000 contig <
Sep 27 11:43:28 pcma73 kernel: Collected DMA Buffer1 at KS:$0000CE600000
Sep 27 11:43:28 pcma73 kernel: rsc_ioctl() Sep 27 11:43:28 pcma73 kernel: RSC_IOC_DMA_OUT
Sep 27 11:43:28 pcma73 kernel: rsc_close()
Sep 27 11:43:28 pcma73 kernel: Free DMA blk 0 at KS:$CE800000 Sep 27 11:43:28 pcma73 kernel: Free DMA blk 1 at KS:$CE600000
> Perhaps i'm misunderstanding the problem.
My problem is, I'm out of ideas. All I can think of is describe as much as possible the relevant things that I do and the things that occur. Maybe someone more experienced recognizes a principal flaw in the concept.
> Plus, if you allocate a 2MB > physically continuous chunk then the likelyhood is high that there were > fragmented pages skipped during the initial search for a 2MB block - so > you still have a fair likelyhood to reallocate it after some time, if > memory usage is light. But this likelyhood nears zero once RAM usage gets > near 100%.
And I can rely on the fact that all the 2 MB are contig memory without holes, right? It's completely mine, isn't it? Or is it perhaps illegal to let the mem usage pump? Should I better allocate the mem in init_module() instead of rsc_open()? Probably page tables are more likely to get corrupted than they would be if I allocate only once. Or do I have to use a spin_lock somewhere in the nopage method?
>From my tests I'm ready the believe the 1st page really _is_ mine but now I'm not so sure all the (1 << 9) pages really are.
If I don't access the pages, just allocate them and free them after some time, I never saw any instabilities. But it seems that as soon as I access pages above the 1st in the buffer, something gets corrupted. So maybe today it's only legal to allocate 1 page at a time and I have to do that (1<<10) times...
Or maybe some of the VM trouble I read about recntly would also cover my problems?
Thanks,
-- Bernd Harries
bha@gmx.de http://bharries.freeyellow.com bharries@web.de Tel. +49 421 809 7343 priv. | MSB First! harries@stn-atlas.de +49 421 457 3966 offi. | Linux-m68k bernd@linux-m68k.org +49 172 139 6054 handy | Medusa T40
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