linux-kernel - Re: [PATCH] mm: page_alloc: document kmemleak's non-blockable __GFP

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Date:   Thu, 25 Jul 2019 10:21:08 -0700
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     mhocko@...e.com, dvyukov@...gle.com, catalin.marinas@....com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: page_alloc: document kmemleak's non-blockable
 __GFP_NOFAIL case



On 7/24/19 7:48 PM, Andrew Morton wrote:
> On Sat, 13 Jul 2019 04:49:04 +0800 Yang Shi <yang.shi@...ux.alibaba.com> wrote:
>
>> When running ltp's oom test with kmemleak enabled, the below warning was
>> triggerred since kernel detects __GFP_NOFAIL & ~__GFP_DIRECT_RECLAIM is
>> passed in:
>>
>> ...
>>
>> The mempool_alloc_slab() clears __GFP_DIRECT_RECLAIM, kmemleak has
>> __GFP_NOFAIL set all the time due to commit
>> d9570ee3bd1d4f20ce63485f5ef05663866fe6c0 ("kmemleak: allow to coexist
>> with fault injection").
>>
>> The fault-injection would not try to fail slab or page allocation if
>> __GFP_NOFAIL is used and that commit tries to turn off fault injection
>> for kmemleak allocation.  Although __GFP_NOFAIL doesn't guarantee no
>> failure for all the cases (i.e. non-blockable allocation may fail), it
>> still makes sense to the most cases.  Kmemleak is also a debugging tool,
>> so it sounds not worth changing the behavior.
>>
>> It also meaks sense to keep the warning, so just document the special
>> case in the comment.
>>
>> ...
>>
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -4531,8 +4531,14 @@ bool gfp_pfmemalloc_allowed(gfp_t gfp_mask)
>>   	 */
>>   	if (gfp_mask & __GFP_NOFAIL) {
>>   		/*
>> -		 * All existing users of the __GFP_NOFAIL are blockable, so warn
>> -		 * of any new users that actually require GFP_NOWAIT
>> +		 * The users of the __GFP_NOFAIL are expected be blockable,
>> +		 * and this is true for the most cases except for kmemleak.
>> +		 * The kmemleak pass in __GFP_NOFAIL to skip fault injection,
>> +		 * however kmemleak may allocate object at some non-blockable
>> +		 * context to trigger this warning.
>> +		 *
>> +		 * Keep this warning since it is still useful for the most
>> +		 * normal cases.
>>   		 */
> Comment has rather a lot of typos.  I'd normally fix them but I think
> I'll duck this patch until the kmemleak situation is addressed, so we
> can add a kmemleakless long-term comment, if desired.

Actually, this has been replaced by reverting the problematic commit. 
And, the patch has been in -mm tree. Please see: 
revert-kmemleak-allow-to-coexist-with-fault-injection.patch

I think we would like to have this merged in 5.3-rc1 or rc2?