From: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
To: Michal Hocko <mhocko@kernel.org>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2] mm/kvmalloc: do not call kmalloc for size > KMALLOC_MAX_SIZE
Date: Thu, 1 Nov 2018 19:42:48 +0300 [thread overview]
Message-ID: <ae51e16b-459c-7d59-6277-b1a197dbf5ff@yandex-team.ru> (raw)
In-Reply-To: <20181101125543.GH23921@dhcp22.suse.cz>
On 01.11.2018 15:55, Michal Hocko wrote:
> On Thu 01-11-18 13:48:17, Konstantin Khlebnikov wrote:
>>
>>
>> On 01.11.2018 13:24, Michal Hocko wrote:
>>> On Thu 01-11-18 13:09:16, Konstantin Khlebnikov wrote:
>>>> Allocations over KMALLOC_MAX_SIZE could be served only by vmalloc.
>>>
>>> I would go on and say that allocations with sizes too large can actually
>>> trigger a warning (once you have posted in the previous version outside
>>> of the changelog area) because that might be interesting to people -
>>> there are deployments to panic on warning and then a warning is much
>>> more important.
>>
>> It seems that warning isn't completely valid.
>>
>>
>> __alloc_pages_slowpath() handles this more gracefully:
>>
>> /*
>> * In the slowpath, we sanity check order to avoid ever trying to
>> * reclaim >= MAX_ORDER areas which will never succeed. Callers may
>> * be using allocators in order of preference for an area that is
>> * too large.
>> */
>> if (order >= MAX_ORDER) {
>> WARN_ON_ONCE(!(gfp_mask & __GFP_NOWARN));
>> return NULL;
>> }
>>
>>
>> Fast path is ready for order >= MAX_ORDER
>>
>>
>> Problem is in node_reclaim() which is called earlier than __alloc_pages_slowpath()
>> from surprising place - get_page_from_freelist()
>>
>>
>> Probably node_reclaim() simply needs something like this:
>>
>> if (order >= MAX_ORDER)
>> return NODE_RECLAIM_NOSCAN;
>
> Maybe but the point is that triggering this warning is possible. Even if
> the warning is bogus it doesn't really make much sense to even try
> kmalloc if the size is not supported by the allocator.
>
But __GFP_NOWARN allocation (like in this case) should just fail silently
without warnings regardless of reason because caller can deal with that.
Without __GFP_NOWARN allocator should print standard warning.
Caller anyway must handle NULL\ENOMEM result - this error path
should be used for handling impossible sizes too.
Of course it could check size first, just as optimization.
next prev parent reply other threads:[~2018-11-01 16:42 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-01 9:12 [PATCH] mm/kvmalloc: do not confuse kmalloc with page order over MAX_ORDER Konstantin Khlebnikov
2018-11-01 9:33 ` Michal Hocko
2018-11-01 10:09 ` [PATCH 2] mm/kvmalloc: do not call kmalloc for size > KMALLOC_MAX_SIZE Konstantin Khlebnikov
2018-11-01 10:24 ` Michal Hocko
2018-11-01 10:48 ` Konstantin Khlebnikov
2018-11-01 12:55 ` Michal Hocko
2018-11-01 16:42 ` Konstantin Khlebnikov [this message]
2018-11-01 16:55 ` Michal Hocko
2018-11-05 13:03 ` Vlastimil Babka
2018-11-05 16:19 ` Konstantin Khlebnikov
2018-11-05 16:52 ` Vlastimil Babka
2018-11-05 16:57 ` Michal Hocko
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