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

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Message-ID: <20170119095610.GL30786@dhcp22.suse.cz>
Date:   Thu, 19 Jan 2017 10:56:12 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     John Hubbard <jhubbard@...dia.com>
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 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