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Date: Thu, 19 Jan 2017 10:56:12 +0100
From: Michal Hocko <mhocko@kernel.org>
To: John Hubbard <jhubbard@nvidia.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>, David Rientjes <rientjes@google.com>,
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        "Michael S. Tsirkin" <mst@redhat.com>, "Theodore Ts'o" <tytso@mit.edu>
Subject: Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers
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On Thu 19-01-17 01:09:35, John Hubbard wrote:
[...]
> So that leaves us with maybe this for documentation?
> 
>  * Reclaim modifiers - __GFP_NORETRY and __GFP_NOFAIL should not be passed in.
>  * Passing in __GFP_REPEAT is supported, and will cause the following behavior:
>  * for larger (>64KB) allocations, the first part (kmalloc) will do some
>  * retrying, before falling back to vmalloc.

I am worried this is just too vague. It doesn't really help user to
decide whether "do some retrying" is what he really want's or needs.

So I would rather see the following.
"
 * Reclaim modifiers - __GFP_NORETRY and __GFP_NOFAIL are not supported. __GFP_REPEAT
 * is supported only for large (>32kB) allocations and it should be used when using
 * kmalloc is preferable because vmalloc fallback has visible performance drawbacks.
"

I would also add
"
Any use of gfp flags outside of GFP_KERNEL should be consulted with mm people.
"

Does it sound any better?
-- 
Michal Hocko
SUSE Labs