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

From	Michal Hocko <mhocko@kernel.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers
Date	2017-01-17 09:00 +0100
Message-ID	<t0wQq-4ey-7@gated-at.bofh.it> (permalink)
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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.

-- 
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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