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Subject: Re: [LKP] [mm] cc87317726f: WARNING: CPU: 0 PID: 1 at drivers/iommu/io-pgtable-arm.c:413 __arm_lpae_unmap+0x341/0x380()
From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
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Message-Id: <201503210041.HJB73900.FVQFOFSLHOOMtJ@I-love.SAKURA.ne.jp>
Date: Sat, 21 Mar 2015 00:41:19 +0900
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To: mhocko@suse.cz
Cc: ying.huang@intel.com, hannes@cmpxchg.org, torvalds@linux-foundation.org, rientjes@google.com, akpm@linux-foundation.org, david@fromorbit.com, linux-kernel@vger.kernel.org, lkp@01.org, linux-mm@kvack.org

Michal Hocko wrote:
> On Fri 20-03-15 23:02:09, Tetsuo Handa wrote:
> > Michal Hocko wrote:
> > > On Fri 20-03-15 22:34:21, Tetsuo Handa wrote:
> > > > Huang Ying wrote:
> > > > > > > BTW: the test is run on 32 bit system.
> > > > > > 
> > > > > > That sounds like the cause of your problem. The system might be out of
> > > > > > address space available for the kernel (only 1GB if x86_32). You should
> > > > > > try running tests on 64 bit systems.
> > > > > 
> > > > > We run test on 32 bit and 64 bit systems.  Try to catch problems on both
> > > > > platforms.  I think we still need to support 32 bit systems?
> > > > 
> > > > Yes, testing on both platforms is good. But please read
> > > > http://lwn.net/Articles/627419/ , http://lwn.net/Articles/635354/ and
> > > > http://lwn.net/Articles/636017/ . Then please add __GFP_NORETRY to memory
> > > > allocations in btrfs code if it is appropriate.
> > > 
> > > I guess you meant __GFP_NOFAIL?
> > > 
> > No. btrfs's selftest (which is not using __GFP_NOFAIL) is already looping
> > forever. If we want to avoid btrfs's selftest from looping forever, btrfs
> > needs __GFP_NORETRY than __GFP_NOFAIL (until we establish a way to safely
> > allow small allocations to fail).
> 
> Sigh. If the code is using GFP_NOFS allocation (which seem to be the
> case because it worked with the 9879de7373fc) and the proper fix for
> this IMO is to simply not retry endlessly for these allocations.

We can avoid looping forever by passing __GFP_NORETRY (from the caller side)
or by using sysctl_nr_alloc_retry == 1 (from the callee side). But

> We
> have to sort some other issues before we can make NOFS allocations fail
> but let's not pile more workarounds on top in the meantime. But if btrfs
> people really think __GFP_NORETRY then I do not really care much.

https://lkml.org/lkml/2015/3/19/221 suggests that changing each caller to
use either __GFP_NOFAIL or __GFP_NORETRY is the safer way to allow small
allocations to fail than using sysctl_nr_alloc_retry, for we don't want to
add __GFP_NOFAIL to allocations by page fault.

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