From: John Hubbard <jhubbard@nvidia.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Vlastimil Babka <vbabka@suse.cz>,
David Rientjes <rientjes@google.com>,
Mel Gorman <mgorman@suse.de>,
Johannes Weiner <hannes@cmpxchg.org>,
Al Viro <viro@zeniv.linux.org.uk>,
linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>,
Anatoly Stepanov <astepanov@cloudlinux.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Mike Snitzer <snitzer@redhat.com>,
"Michael S. Tsirkin" <mst@redhat.com>,
Theodore Ts'o <tytso@mit.edu>
Subject: Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers
Date: Mon, 16 Jan 2017 11:09:37 -0800 [thread overview]
Message-ID: <0ca8a212-c651-7915-af25-23925e1c1cc3@nvidia.com> (raw)
In-Reply-To: <20170116084717.GA13641@dhcp22.suse.cz>
On 01/16/2017 12:47 AM, Michal Hocko wrote:
> On Sun 15-01-17 20:34:13, John Hubbard wrote:
>>
>>
>> On 01/12/2017 07:37 AM, Michal Hocko wrote:
> [...]
>>> diff --git a/mm/util.c b/mm/util.c
>>> index 3cb2164f4099..7e0c240b5760 100644
>>> --- a/mm/util.c
>>> +++ b/mm/util.c
>>> @@ -324,6 +324,48 @@ unsigned long vm_mmap(struct file *file, unsigned long addr,
>>> }
>>> EXPORT_SYMBOL(vm_mmap);
>>>
>>> +/**
>>> + * kvmalloc_node - allocate contiguous memory from SLAB with vmalloc fallback
>>
>> Hi Michal,
>>
>> How about this wording instead:
>>
>> kvmalloc_node - attempt to allocate physically contiguous memory, but upon
>> failure, fall back to non-contiguous (vmalloc) allocation.
>
> OK, why not.
>
>>> + * @size: size of the request.
>>> + * @flags: gfp mask for the allocation - must be compatible (superset) with GFP_KERNEL.
>>> + * @node: numa node to allocate from
>>> + *
>>> + * Uses kmalloc to get the memory but if the allocation fails then falls back
>>> + * to the vmalloc allocator. Use kvfree for freeing the memory.
>>> + *
>>> + * Reclaim modifiers - __GFP_NORETRY, __GFP_REPEAT and __GFP_NOFAIL are not supported
>>
>> 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.
>
>> b) In patch 6/6, you are in fact passing in __GFP_REPEAT to the wrappers
>> (kvzalloc, for example), and again, only adding, not removing flags.
>
> Patch 2 adds a support for __GFP_REPEAT and updates the above line as
> well.
OK, I see.
>
>>> + */
>>> +void *kvmalloc_node(size_t size, gfp_t flags, int node)
>>> +{
>>> + gfp_t kmalloc_flags = flags;
>>> + void *ret;
>>> +
>>> + /*
>>> + * vmalloc uses GFP_KERNEL for some internal allocations (e.g page tables)
>>> + * so the given set of flags has to be compatible.
>>> + */
>>> + WARN_ON_ONCE((flags & GFP_KERNEL) != GFP_KERNEL);
>>> +
>>> + /*
>>> + * Make sure that larger requests are not too disruptive - no OOM
>>> + * killer and no allocation failure warnings as we have a fallback
>>> + */
>>> + if (size > PAGE_SIZE)
>>> + kmalloc_flags |= __GFP_NORETRY | __GFP_NOWARN;
>>> +
>>> + ret = kmalloc_node(size, kmalloc_flags, node);
>>
>> Along those lines (dealing with larger requests), is there any value in
>> picking some threshold value, and going straight to vmalloc if size is
>> greater than that threshold?
>
> I am not a fan of thresholds. PAGE_ALLOC_COSTLY_ORDER which is
> internally used by the page allocator has turned out to be a major pain.
> I do not want to repeat the same mistake again here. Besides that you
> could hard find a "one suits all" value so it would have to be a part of
> the API. If we ever grow users who would really like to do something
> like that then a specialized API should be added.
Thanks for explaining, and the note about the pain of dealing with PAGE_ALLOC_COSTLY_ORDER is
especially interesting. Sounds good, then.
thanks
john h
>
>> It's less flexible and might even require
>> occasional maintenance over the years, but it would save some time on *some*
>> systems in some cases...OK, I think I just talked myself out of the whole
>> idea. But I still want to put the question out there, because I think others
>> may also ask it, and I'd like to hear a more experienced opinion.
>
>
> --
> Michal Hocko
> SUSE Labs
>
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next prev parent reply other threads:[~2017-01-16 19:09 UTC|newest]
Thread overview: 55+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-12 15:37 [PATCH 0/6 v3] kvmalloc Michal Hocko
2017-01-12 15:37 ` [PATCH 1/6] mm: introduce kv[mz]alloc helpers Michal Hocko
2017-01-16 4:34 ` John Hubbard
2017-01-16 8:47 ` Michal Hocko
2017-01-16 19:09 ` John Hubbard [this message]
2017-01-16 19:40 ` Michal Hocko
2017-01-16 21:15 ` John Hubbard
2017-01-16 21:48 ` Michal Hocko
2017-01-16 21:57 ` John Hubbard
2017-01-17 7:51 ` Michal Hocko
2017-01-18 5:59 ` John Hubbard
2017-01-18 8:21 ` Michal Hocko
2017-01-19 8:37 ` John Hubbard
2017-01-19 8:45 ` Michal Hocko
2017-01-19 9:09 ` John Hubbard
2017-01-19 9:56 ` Michal Hocko
2017-01-19 21:28 ` John Hubbard
2017-01-26 12:09 ` Michal Hocko
2017-01-30 8:42 ` Vlastimil Babka
2017-01-12 15:37 ` [PATCH 2/6] mm: support __GFP_REPEAT in kvmalloc_node for >=64kB Michal Hocko
2017-01-12 16:12 ` Michael S. Tsirkin
2017-01-14 2:42 ` Tetsuo Handa
2017-01-14 8:45 ` Michal Hocko
2017-01-24 15:40 ` Michael S. Tsirkin
2017-01-12 15:37 ` [PATCH 3/6] rhashtable: simplify a strange allocation pattern Michal Hocko
2017-01-12 15:37 ` [PATCH 4/6] ila: " Michal Hocko
2017-01-12 15:37 ` [PATCH 5/6] treewide: use kv[mz]alloc* rather than opencoded variants Michal Hocko
2017-01-12 15:57 ` David Sterba
2017-01-12 16:05 ` Christian Borntraeger
2017-01-12 16:54 ` Ilya Dryomov
2017-01-12 17:18 ` Michal Hocko
2017-01-12 17:00 ` Dan Williams
2017-01-12 17:26 ` Kees Cook
2017-01-12 17:37 ` Michal Hocko
2017-01-20 13:41 ` Vlastimil Babka
2017-01-24 15:00 ` Michal Hocko
2017-01-25 11:15 ` Vlastimil Babka
2017-01-25 13:09 ` Michal Hocko
2017-01-25 13:40 ` Ilya Dryomov
2017-01-12 17:29 ` Michal Hocko
2017-01-14 3:01 ` Tetsuo Handa
2017-01-14 8:49 ` Michal Hocko
2017-01-12 20:14 ` Boris Ostrovsky
2017-01-13 1:11 ` Dilger, Andreas
2017-01-14 10:56 ` Leon Romanovsky
2017-01-16 7:33 ` Michal Hocko
2017-01-16 8:28 ` Leon Romanovsky
2017-01-16 8:18 ` Tariq Toukan
2017-01-12 15:37 ` [RFC PATCH 6/6] net: use kvmalloc with __GFP_REPEAT rather than open coded variant Michal Hocko
2017-01-24 15:17 ` [PATCH 0/6 v3] kvmalloc Michal Hocko
2017-01-24 16:00 ` Eric Dumazet
2017-01-25 13:10 ` Michal Hocko
2017-01-24 19:17 ` Alexei Starovoitov
2017-01-25 13:10 ` Michal Hocko
2017-01-25 13:21 ` Michal Hocko
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