linux-ext4 - Re: [RFC PATCH] mm: introduce kv[mz]alloc helpers

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Date:   Thu, 8 Dec 2016 14:51:46 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-mm@...ck.org, 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>,
        Anatoly Stepanov <astepanov@...udlinux.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Theodore Ts'o <tytso@....edu>, kvm@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        linux-security-module@...r.kernel.org
Subject: Re: [RFC PATCH] mm: introduce kv[mz]alloc helpers

On Thu 08-12-16 14:00:20, David Hildenbrand wrote:
> Am 08.12.2016 um 11:33 schrieb Michal Hocko:
> > From: Michal Hocko <mhocko@...e.com>
> > 
> > Using kmalloc with the vmalloc fallback for larger allocations is a
> > common pattern in the kernel code. Yet we do not have any common helper
> > for that and so users have invented their own helpers. Some of them are
> > really creative when doing so. Let's just add kv[mz]alloc and make sure
> > it is implemented properly. This implementation makes sure to not make
> > a large memory pressure for > PAGE_SZE requests (__GFP_NORETRY) and also
> > to not warn about allocation failures. This also rules out the OOM
> > killer as the vmalloc is a more approapriate fallback than a disruptive
> > user visible action.
> > 
> > This patch also changes some existing users and removes helpers which
> > are specific for them. In some cases this is not possible (e.g.
> > ext4_kvmalloc, libcfs_kvzalloc, __aa_kvmalloc) because those seems to be
> > broken and require GFP_NO{FS,IO} context which is not vmalloc compatible
> > in general (note that the page table allocation is GFP_KERNEL). Those
> > need to be fixed separately.
> > 
> > apparmor has already claimed kv[mz]alloc so remove those and use
> > __aa_kvmalloc instead to prevent from the naming clashes.
> > 
> > Cc: Paolo Bonzini <pbonzini@...hat.com>
> > Cc: Mike Snitzer <snitzer@...hat.com>
> > Cc: dm-devel@...hat.com
> > Cc: "Michael S. Tsirkin" <mst@...hat.com>
> > Cc: "Theodore Ts'o" <tytso@....edu>
> > Cc: kvm@...r.kernel.org
> > Cc: linux-ext4@...r.kernel.org
> > Cc: linux-f2fs-devel@...ts.sourceforge.net
> > Cc: linux-security-module@...r.kernel.org
> > Signed-off-by: Michal Hocko <mhocko@...e.com>
> 
> I remember yet another similar user in arch/s390/kvm/kvm-s390.c
> -> kvm_s390_set_skeys()
> 
> ...
> keys = kmalloc_array(args->count, sizeof(uint8_t),
>                      GFP_KERNEL | __GFP_NOWARN);
> if (!keys)
>         vmalloc(sizeof(uint8_t) * args->count);
> ...
> 
> would kvmalloc_array make sense? (it would even make the code here
> less error prone and better to read)

Well, if there are more users like that then why not. I just do not want
to duplicate the whole kmalloc API right now. The above could be
trivially changed to kvmalloc(args->count * sizeof(uint8_t), GFP_KERNEL)
so a special API might not be really needed. 

-- 
Michal Hocko
SUSE Labs
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