Date:   Thu, 19 Jan 2017 00:37:08 -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 01/18/2017 12:21 AM, Michal Hocko wrote:
> On Tue 17-01-17 21:59:13, John Hubbard wrote:
>>
>> 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.
>
> this is not true to be really precise because vmalloc doesn't respect
> the given gfp mask all the way down (look at the pte initialization).
>

I'm having some difficulty in locating that pte initialization part, am I on the 
wrong code path? Here's what I checked, before making the claim about __GFP_REPEAT 
being honored:

kvmalloc_node
   __vmalloc_node_flags
     __vmalloc_node
       __vmalloc_node_range
         __vmalloc_area_node
             alloc_pages_node
               __alloc_pages_node
                 __alloc_pages
                   __alloc_pages_nodemask
                     __alloc_pages_slowpath


...and __alloc_pages_slowpath does the __GFP_REPEAT handling:

     /*
      * Do not retry costly high order allocations unless they are
      * __GFP_REPEAT
      */
     if (order > PAGE_ALLOC_COSTLY_ORDER && !(gfp_mask & __GFP_REPEAT))
         goto nopage;

thanks,
john h

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