Re: [RFC] mm: kmemleak: replace __GFP_NOFAIL to GFP_NOWAIT in gfp_kmemleak_mask

[Dmitry Vyukov]

On Fri, Apr 20, 2018 at 7:50 PM, Catalin Marinas
<catalin.marinas@xxxxxxx> wrote:
> On Sat, Apr 21, 2018 at 12:58:33AM +0800, Chunyu Hu wrote:
>> __GFP_NORETRY and  __GFP_NOFAIL are combined in gfp_kmemleak_mask now.
>> But it's a wrong combination. As __GFP_NOFAIL is blockable, but
>> __GFP_NORETY is not blockable, make it self-contradiction.
>>
>> __GFP_NOFAIL means 'The VM implementation _must_ retry infinitely'. But
>> it's not the real intention, as kmemleak allow alloc failure happen in
>> memory pressure, in that case kmemleak just disables itself.
>
> Good point. The __GFP_NOFAIL flag was added by commit d9570ee3bd1d
> ("kmemleak: allow to coexist with fault injection") to keep kmemleak
> usable under fault injection.
>
>> commit 9a67f6488eca ("mm: consolidate GFP_NOFAIL checks in the allocator
>> slowpath") documented that what user wants here should use GFP_NOWAIT, and
>> the WARN in __alloc_pages_slowpath caught this weird usage.
>>
>>  <snip>
>>  WARNING: CPU: 3 PID: 64 at mm/page_alloc.c:4261 __alloc_pages_slowpath+0x1cc3/0x2780
> [...]
>> Replace the __GFP_NOFAIL with GFP_NOWAIT in gfp_kmemleak_mask, __GFP_NORETRY
>> and GFP_NOWAIT are in the gfp_kmemleak_mask. So kmemleak object allocaion
>> is no blockable and no reclaim, making kmemleak less disruptive to user
>> processes in pressure.
>
> It doesn't solve the fault injection problem for kmemleak (unless we
> change __should_failslab() somehow, not sure yet). An option would be to
> replace __GFP_NORETRY with __GFP_NOFAIL in kmemleak when fault injection
> is enabled.
>
> BTW, does the combination of NOWAIT and NORETRY make kmemleak
> allocations more likely to fail?
>
> Cc'ing Dmitry as well.

Yes, it would be bad if there allocations fail due to fault injection.
These are both debugging tools and ideally should not interfere.

>> Signed-off-by: Chunyu Hu <chuhu@xxxxxxxxxx>
>> CC: Michal Hocko <mhocko@xxxxxxxx>
>> ---
>>  mm/kmemleak.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/kmemleak.c b/mm/kmemleak.c
>> index 9a085d5..4ea07e4 100644
>> --- a/mm/kmemleak.c
>> +++ b/mm/kmemleak.c
>> @@ -126,7 +126,7 @@
>>  /* GFP bitmask for kmemleak internal allocations */
>>  #define gfp_kmemleak_mask(gfp)       (((gfp) & (GFP_KERNEL | GFP_ATOMIC)) | \
>>                                __GFP_NORETRY | __GFP_NOMEMALLOC | \
>> -                              __GFP_NOWARN | __GFP_NOFAIL)
>> +                              __GFP_NOWARN | GFP_NOWAIT)
>>
>>  /* scanning area inside a memory block */
>>  struct kmemleak_scan_area {
>> --
>> 1.8.3.1