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

From	John Hubbard <jhubbard@nvidia.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers
Date	2017-01-16 23:00 +0100
Message-ID	<t0ntL-6uv-1@gated-at.bofh.it> (permalink)
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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.

Sorry to go on and on about such a minor point. I'll let it go after this last note.

> --
> Michal Hocko
> SUSE Labs
>

Thread

[PATCH 0/6 v3] kvmalloc Michal Hocko <mhocko@kernel.org> - 2017-01-12 16:50 +0100
  [PATCH 5/6] treewide: use kv[mz]alloc* rather than opencoded variants Michal Hocko <mhocko@kernel.org> - 2017-01-12 16:50 +0100
    Re: [PATCH 5/6] treewide: use kv[mz]alloc* rather than opencoded  variants David Sterba <dsterba@suse.cz> - 2017-01-12 17:00 +0100
    Re: [PATCH 5/6] treewide: use kv[mz]alloc* rather than opencoded  variants Christian Borntraeger <borntraeger@de.ibm.com> - 2017-01-12 17:10 +0100
    Re: [PATCH 5/6] treewide: use kv[mz]alloc* rather than opencoded variants Ilya Dryomov <idryomov@gmail.com> - 2017-01-12 18:00 +0100
      Re: [PATCH 5/6] treewide: use kv[mz]alloc* rather than opencoded  variants Michal Hocko <mhocko@kernel.org> - 2017-01-12 18:20 +0100
    Re: [PATCH 5/6] treewide: use kv[mz]alloc* rather than opencoded variants Dan Williams <dan.j.williams@intel.com> - 2017-01-12 18:10 +0100
    Re: [PATCH 5/6] treewide: use kv[mz]alloc* rather than opencoded variants Kees Cook <keescook@chromium.org> - 2017-01-12 18:30 +0100
      Re: [PATCH 5/6] treewide: use kv[mz]alloc* rather than opencoded  variants Michal Hocko <mhocko@kernel.org> - 2017-01-12 18:40 +0100
        Re: [PATCH 5/6] treewide: use kv[mz]alloc* rather than opencoded  variants Vlastimil Babka <vbabka@suse.cz> - 2017-01-20 15:00 +0100
          Re: [PATCH 5/6] treewide: use kv[mz]alloc* rather than opencoded  variants Michal Hocko <mhocko@kernel.org> - 2017-01-24 16:10 +0100
            Re: [PATCH 5/6] treewide: use kv[mz]alloc* rather than opencoded  variants Vlastimil Babka <vbabka@suse.cz> - 2017-01-25 12:20 +0100
              Re: [PATCH 5/6] treewide: use kv[mz]alloc* rather than opencoded  variants Michal Hocko <mhocko@kernel.org> - 2017-01-25 14:10 +0100
                Re: [PATCH 5/6] treewide: use kv[mz]alloc* rather than opencoded variants Ilya Dryomov <idryomov@gmail.com> - 2017-01-25 14:50 +0100
    Re: [PATCH 5/6] treewide: use kv[mz]alloc* rather than opencoded  variants Michal Hocko <mhocko@kernel.org> - 2017-01-12 18:40 +0100
      Re: [PATCH 5/6] treewide: use kv[mz]alloc* rather than opencoded  variants Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> - 2017-01-14 04:10 +0100
        Re: [PATCH 5/6] treewide: use kv[mz]alloc* rather than opencoded  variants Michal Hocko <mhocko@kernel.org> - 2017-01-14 10:10 +0100
    Re: [PATCH 5/6] treewide: use kv[mz]alloc* rather than opencoded  variants Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-01-12 21:20 +0100
    Re: [PATCH 5/6] treewide: use kv[mz]alloc* rather than opencoded  variants "Dilger, Andreas" <andreas.dilger@intel.com> - 2017-01-13 02:20 +0100
    Re: [PATCH 5/6] treewide: use kv[mz]alloc* rather than opencoded  variants Leon Romanovsky <leon@kernel.org> - 2017-01-14 12:00 +0100
      Re: [PATCH 5/6] treewide: use kv[mz]alloc* rather than opencoded  variants Michal Hocko <mhocko@kernel.org> - 2017-01-16 08:40 +0100
        Re: [PATCH 5/6] treewide: use kv[mz]alloc* rather than opencoded  variants Leon Romanovsky <leon@kernel.org> - 2017-01-16 09:30 +0100
  [PATCH 4/6] ila: simplify a strange allocation pattern Michal Hocko <mhocko@kernel.org> - 2017-01-12 16:50 +0100
  [PATCH 3/6] rhashtable: simplify a strange allocation pattern Michal Hocko <mhocko@kernel.org> - 2017-01-12 16:50 +0100
  [PATCH 1/6] mm: introduce kv[mz]alloc helpers Michal Hocko <mhocko@kernel.org> - 2017-01-12 16:50 +0100
    Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers John Hubbard <jhubbard@nvidia.com> - 2017-01-16 05:40 +0100
      Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers Michal Hocko <mhocko@kernel.org> - 2017-01-16 09:50 +0100
        Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers John Hubbard <jhubbard@nvidia.com> - 2017-01-16 20:10 +0100
          Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers Michal Hocko <mhocko@kernel.org> - 2017-01-16 20:50 +0100
            Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers John Hubbard <jhubbard@nvidia.com> - 2017-01-16 22:30 +0100
              Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers Michal Hocko <mhocko@kernel.org> - 2017-01-16 22:50 +0100
                Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers John Hubbard <jhubbard@nvidia.com> - 2017-01-16 23:00 +0100
                Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers Michal Hocko <mhocko@kernel.org> - 2017-01-17 09:00 +0100
                Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers John Hubbard <jhubbard@nvidia.com> - 2017-01-18 07:10 +0100
                Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers Michal Hocko <mhocko@kernel.org> - 2017-01-18 09:30 +0100
                Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers John Hubbard <jhubbard@nvidia.com> - 2017-01-19 09:40 +0100
                Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers Michal Hocko <mhocko@kernel.org> - 2017-01-19 10:10 +0100
                Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers John Hubbard <jhubbard@nvidia.com> - 2017-01-19 10:20 +0100
                Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers Michal Hocko <mhocko@kernel.org> - 2017-01-19 11:00 +0100
                Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers John Hubbard <jhubbard@nvidia.com> - 2017-01-19 22:30 +0100
    Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers Michal Hocko <mhocko@kernel.org> - 2017-01-26 13:20 +0100
      Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers Vlastimil Babka <vbabka@suse.cz> - 2017-01-30 10:10 +0100
  Re: [PATCH 0/6 v3] kvmalloc Michal Hocko <mhocko@kernel.org> - 2017-01-24 16:20 +0100
    Re: [PATCH 0/6 v3] kvmalloc Eric Dumazet <eric.dumazet@gmail.com> - 2017-01-24 17:10 +0100
      Re: [PATCH 0/6 v3] kvmalloc Michal Hocko <mhocko@kernel.org> - 2017-01-25 14:20 +0100
    Re: [PATCH 0/6 v3] kvmalloc Alexei Starovoitov <alexei.starovoitov@gmail.com> - 2017-01-24 20:20 +0100
      Re: [PATCH 0/6 v3] kvmalloc Michal Hocko <mhocko@kernel.org> - 2017-01-25 14:20 +0100
        Re: [PATCH 0/6 v3] kvmalloc Michal Hocko <mhocko@kernel.org> - 2017-01-25 14:30 +0100

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