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

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Date:   Mon, 16 Jan 2017 13:15:08 -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 01/16/2017 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?

thanks
john h

>
> --
> Michal Hocko
> SUSE Labs
>