On 27 April 2012 20:36, David Rientjes [off-list ref] wrote:

On Fri, 27 Apr 2012, Minchan Kim wrote:

quoted

Now there are several places to use __vmalloc with GFP_ATOMIC,
GFP_NOIO, GFP_NOFS but unfortunately __vmalloc calls map_vm_area
which calls alloc_pages with GFP_KERNEL to allocate page tables.
It means it's possible to happen deadlock.
I don't know why it doesn't have reported until now.

Firstly, I tried passing gfp_t to lower functions to support __vmalloc
with such flags but other mm guys don't want and decided that
all of caller should be fixed.

http://marc.info/?l=linux-kernel&m=133517143616544&w=2

To begin with, let's listen other's opinion whether they can fix it
by other approach without calling __vmalloc with such flags.

So this patch adds warning to detect and to be fixed hopely.
I Cced related maintainers.
If I miss someone, please Cced them.

side-note:
  I added WARN_ON instead of WARN_ONCE to detect all of callers
  and each WARN_ON for each flag to detect to use any flag easily.
  After we fix all of caller or reduce such caller, we can merge
  a warning with WARN_ONCE.

I disagree with this approach since it's going to violently spam an
innocent kernel user's log with no ratelimiting and for a situation that
actually may not be problematic.

With WARN_ON_ONCE, it should be good.

Passing any of these bits (the difference between GFP_KERNEL and
GFP_ATOMIC) only means anything when we're going to do reclaim.  And I'm
suspecting we would have seen problems with this already since
pte_alloc_kernel() does __GFP_REPEAT on most architectures meaning that it
will loop infinitely in the page allocator until at least one page is
freed (since its an order-0 allocation) which would hardly ever happen if
__GFP_FS or __GFP_IO actually meant something in this context.

In other words, we would already have seen these deadlocks and it would
have been diagnosed as a vmalloc(GFP_ATOMIC) problem.  Where are those bug
reports?

That's not sound logic to disprove a bug.

I think simply most callers are permissive and don't mask out flags.
But for example a filesystem holding an fs lock and then doing
vmalloc(GFP_NOFS) can certainly deadlock.

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