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SubjectRe: Memory Problem in 2.4.9 ?


On Wed, 22 Aug 2001, Daniel Phillips wrote:

> On August 22, 2001 06:47 am, Tommy Wu wrote:
> > I've tried the patch in the kernel list. Got the result as following...
> > This message for command:
> > dd if=/dev/zero of=test.dmp bs=1000k count=2500
> > on a PIII 1G SMP box with 1G RAM (HIGHMEM enabled)
> > kernel 2.4.9 with XFS filesystem patch.
> >
> > Aug 22 11:51:04 standby kernel: __alloc_pages: 0-order allocation failed
> > (gfp=0x30/1).
> > Aug 22 11:51:11 standby last message repeated 111 times
>
> OK, this is a straight-up design bug. Although this can also happen with
> normal memory, it's much more likely to happen with highmem because of heavy
> demand for bounce buffers while a process is in PF_MEMALLOC state. You can
> just turn off highmem and these messages will go away, or become so rare that
> you are unlikely to ever see one.
>
> Now lets chase the real problem. The gfp=0x30 tells us the requestor is
> willing to wait (0x10) and that it is not allowed to do any io (0x40) or call
> ->writepage (0x80). (By process of elimination, it's a bounce buffer.)
> Furthermore, this is a recursive memory request (/1) so __alloc_pages won't
> call page_launder because that could hit another allocation request resulting
> in a fatal infinite recursion (note to self: why couldn't we call
> page_launder here, with NOIO?).
>
> There are probably dirty pages in flight and __alloc_pages is allowed to wait
> for them, but it doesn't - it trys reclaim_page once (in
> __alloc_pages_limit), falls the rest of the way through __alloc_pages and
> gives up with NULL. This is clearly a bad thing because whoever wanted the
> page needs it to do writeout. Memory users are supposed to be able to
> tolerate alloc failure, but in a case like this, there isn't much choice
> other than to spin.
>
> So what could we do better here? Well, obviously when there are writeout
> pages in flight, __alloc_pages should wait and not give up. Secondly, we
> should be sure that when writeout does complete, the newly freeable page is
> given to a PF_MEMALLOC waiter in preference to a normal user. We don't have
> mechanisms in place for doing either of those things right now, although some
> preliminary design ideas have been discussed. This gets way outside the
> bound of what we should be doing in 2.4, we will need such things as
> reservations (which Ben has done some work on) and orderly prioritization of
> requests in __alloc_pages, with explicit blocking for low priority requests.
>
> What can we do right now? We could always just comment out the alloc failed
> message. The result will be a lot of busy waiting on dirty page writeout
> which will work but it will keep us from focussing on the question: how did
> we get so short of bounce buffers? Well, maybe we are submitting too much IO
> without intelligent throttling (/me waves at Ben). That sounds like the
> place to attack first.

We can just wait on the writeout of lowmem buffers at page_launder()
(which will not cause IO buffering since we are doing lowmem IO, duh), and
then we are done.

Take a look at the patch I posted before (__GFP_NOBOUNCE).

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