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

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Message-Id: <20190724194835.59947a6b4df3c2ae7816470d@linux-foundation.org>
Date:   Wed, 24 Jul 2019 19:48:35 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
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 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.