linux-kernel - Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers

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Date:   Tue, 17 Jan 2017 21:59:13 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Michal Hocko <mhocko@...nel.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Rientjes <rientjes@...gle.com>,
        Mel Gorman <mgorman@...e.de>,
        Johannes Weiner <hannes@...xchg.org>,
        Al Viro <viro@...iv.linux.org.uk>, <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Anatoly Stepanov <astepanov@...udlinux.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers


On 01/16/2017 11:51 PM, Michal Hocko wrote:
> On Mon 16-01-17 13:57:43, John Hubbard wrote:
>>
>>
>> On 01/16/2017 01:48 PM, Michal Hocko wrote:
>>> On Mon 16-01-17 13:15:08, John Hubbard wrote:
>>>>
>>>>
>>>> On 01/16/2017 11:40 AM, Michal Hocko wrote:
>>>>> On Mon 16-01-17 11:09:37, John Hubbard wrote:
>>>>>>
>>>>>>
>>>>>> On 01/16/2017 12:47 AM, Michal Hocko wrote:
>>>>>>> On Sun 15-01-17 20:34:13, John Hubbard wrote:
>>>>> [...]
>>>>>>>> Is that "Reclaim modifiers" line still true, or is it a leftover from an
>>>>>>>> earlier approach? I am having trouble reconciling it with rest of the
>>>>>>>> patchset, because:
>>>>>>>>
>>>>>>>> a) the flags argument below is effectively passed on to either kmalloc_node
>>>>>>>> (possibly adding, but not removing flags), or to __vmalloc_node_flags.
>>>>>>>
>>>>>>> The above only says thos are _unsupported_ - in other words the behavior
>>>>>>> is not defined. Even if flags are passed down to kmalloc resp. vmalloc
>>>>>>> it doesn't mean they are used that way.  Remember that vmalloc uses
>>>>>>> some hardcoded GFP_KERNEL allocations.  So while I could be really
>>>>>>> strict about this and mask away these flags I doubt this is worth the
>>>>>>> additional code.
>>>>>>
>>>>>> I do wonder about passing those flags through to kmalloc. Maybe it is worth
>>>>>> stripping out __GFP_NORETRY and __GFP_NOFAIL, after all. It provides some
>>>>>> insulation from any future changes to the implementation of kmalloc, and it
>>>>>> also makes the documentation more believable.
>>>>>
>>>>> I am not really convinced that we should take an extra steps for these
>>>>> flags. There are no existing users for those flags and new users should
>>>>> follow the documentation.
>>>>
>>>> OK, let's just fortify the documentation ever so slightly, then, so that
>>>> users are more likely to do the right thing. How's this sound:
>>>>
>>>> * Reclaim modifiers - __GFP_NORETRY and __GFP_NOFAIL are not supported. (Even
>>>> * though the current implementation passes the flags on through to kmalloc and
>>>> * vmalloc, that is done for efficiency and to avoid unnecessary code. The caller
>>>> * should not pass in these flags.)
>>>> *
>>>> * __GFP_REPEAT is supported, but only for large (>64kB) allocations.
>>>>
>>>>
>>>> ? Or is that documentation overkill?
>>>
>>> Dunno, it sounds like an overkill to me. It is telling more than
>>> necessary. If we want to be so vocal about gfp flags then we would have
>>> to say much more I suspect. E.g. what about __GFP_HIGHMEM? This flag is
>>> supported for vmalloc while unsupported for kmalloc. I am pretty sure
>>> there would be other gfp flags to consider and then this would grow
>>> borringly large and uninteresting to the point when people simply stop
>>> reading it. Let's just be as simple as possible.
>>
>> Agreed, on the simplicity point: simple and clear is ideal. But here, it's
>> merely short, and not quite simple. :)  People will look at that short bit
>> of documentation, and then notice that the flags are, in fact, all passed
>> right on through down to both kmalloc_node and __vmalloc_node_flags.
>>
>> If you don't want too much documentation, then I'd be inclined to say
>> something higher-level, about the intent, rather than mentioning those two
>> flags directly. Because as it stands, the documentation contradicts what the
>> code does.
>
> Feel free to suggest a better wording. I am, of course, open to any
> changes.

OK, here's the best I've got, I tried to keep it concise, but (as you suspected) I'm not sure it's 
actually any better than the original:

  * Reclaim modifiers - __GFP_NORETRY and __GFP_NOFAIL should not be passed in.
  * Passing in __GFP_REPEAT is supported, but note that it is ignored for small
  * (<=64KB) allocations, during the kmalloc attempt. __GFP_REPEAT is fully
  * honored for  all allocation sizes during the second part: the vmalloc attempt.


>
> --
> Michal Hocko
> SUSE Labs
>