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

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Date:   Thu, 19 Jan 2017 13:28:32 -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 01/19/2017 01:56 AM, Michal Hocko wrote:
> 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?

Yes, that is good. I like that it helps guide the user. Here's some proposed optional grammar 
tweaks, but even without these, the above is understandable, so either way, I'm happy now:

  * Reclaim modifiers - __GFP_NORETRY and __GFP_NOFAIL are not supported. __GFP_REPEAT
  * is supported only for large (>32kB) allocations, and it should be used only if
  * kmalloc is preferable to the vmalloc fallback, due to visible performance drawbacks.
  *
  * Please consult with mm people before using any gfp flags other than GFP_KERNEL.

thanks
john h

> --
> Michal Hocko
> SUSE Labs
>
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