Re: [PATCH] mm: avoid livelock on !__GFP_FS allocations

[Colin Cross <ccross@android.com>]

On Tue, Oct 25, 2011 at 12:40 AM, Pekka Enberg <penberg@cs.helsinki.fi> wrote:
> On Tue, Oct 25, 2011 at 9:39 AM, Colin Cross <ccross@android.com> wrote:
>> Under the following conditions, __alloc_pages_slowpath can loop
>> forever:
>> gfp_mask & __GFP_WAIT is true
>> gfp_mask & __GFP_FS is false
>> reclaim and compaction make no progress
>> order <= PAGE_ALLOC_COSTLY_ORDER
>>
>> These conditions happen very often during suspend and resume,
>> when pm_restrict_gfp_mask() effectively converts all GFP_KERNEL
>> allocations into __GFP_WAIT.
>
> Why does it do that? Why don't we fix the gfp mask instead?
It disables __GFP_IO and __GFP_FS because the IO drivers may be suspended.

>> The oom killer is not run because gfp_mask & __GFP_FS is false,
>> but should_alloc_retry will always return true when order is less
>> than PAGE_ALLOC_COSTLY_ORDER.
>>
>> Fix __alloc_pages_slowpath to skip retrying when oom killer is
>> not allowed by the GFP flags, the same way it would skip if the
>> oom killer was allowed but disabled.
>>
>> Signed-off-by: Colin Cross <ccross@android.com>
>> ---
>>
>> An alternative patch would add a did_some_progress argument to
>> __alloc_pages_may_oom, and remove the checks in
>> __alloc_pages_slowpath that require knowledge of when
>> __alloc_pages_may_oom chooses to run out_of_memory. If
>> did_some_progress was still zero, it would goto nopage whether
>> or not __alloc_pages_may_oom was actually called.
>>
>>  mm/page_alloc.c |    4 ++++
>>  1 files changed, 4 insertions(+), 0 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index fef8dc3..dcd99b3 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -2193,6 +2193,10 @@ rebalance:
>>                        }
>>
>>                        goto restart;
>> +               } else {
>> +                       /* If we aren't going to try the OOM killer, give up */
>> +                       if (!(gfp_mask & __GFP_NOFAIL))
>> +                               goto nopage;
>>                }
>>        }
>
> I don't quite understand how __GFP_WAIT is involved here. Which path
> is causing the infinite loop?
GFP_KERNEL is __GFP_WAIT | __GFP_IO | __GFP_FS.  Once driver suspend
has started, gfp_allowed_mask is ~(__GFP_IO | GFP_FS), so any call to
__alloc_pages_nodemask(GFP_KERNEL, ...) gets masked to effectively
__alloc_pages_nodemask(__GFP_WAIT, ...).

The loop is in __alloc_pages_slowpath, from the rebalance label to
should_alloc_retry.  Under the conditions I listed in the commit
message, there is no path to the nopage label, because all the
relevant "goto nopage" lines that would normally allow a GFP_KERNEL
allocation to fail are inside a check for __GFP_FS.

Modifying the gfp_allowed_mask would not completely fix the issue, a
GFP_NOIO allocation can meet the conditions outside of suspend.
gfp_allowed_mask just makes the issue more likely, by converting
GFP_KERNEL into GFP_NOIO.

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