linux-kernel - Re: [PATCH v1] mm/vmalloc: fix exact allocations with an alignment

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Date:   Wed, 22 Sep 2021 10:34:55 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, Ping Fang <pifang@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>, Michal Hocko <mhocko@...e.com>,
        Oscar Salvador <osalvador@...e.de>,
        Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH v1] mm/vmalloc: fix exact allocations with an alignment >
 1

>> No, that's leaking implementation details to the caller. And no, increasing
>> the range and eventually allocating something bigger (e.g., placing a huge
>> page where it might not have been possible) is not acceptable for KASAN.
>>
>> If you're terribly unhappy with this patch,
> Sorry to say but it simple does not make sense.
> 

Let's agree to disagree.

find_vmap_lowest_match() is imprecise now and that's an issue for exact 
allocations. We can either make it fully precise again (eventually 
degrading allocation performance) or just special-case exact allocations 
to fix the regression.

I decided to go the easy path and do the latter; I do agree that making 
find_vmap_lowest_match() fully precise again might be preferred -- we 
could have other allocations failing right now although there are still 
suitable holes.

I briefly thought about performing the search in 
find_vmap_lowest_match() twice. First, start the search without an 
extended range, and fallback to the extended range if that search fails. 
Unfortunately, I think that still won't make the function completely 
precise due to the way we might miss searching some suitable subtrees.

>>
>> please suggest something reasonable to fix exact allocations:
>> a) Fixes the KASAN use case.
>> b) Allows for automatic placement of huge pages for exact allocations.
>> c) Doesn't leak implementation details into the caller.
>>
> I am looking at it.

Thanks!

-- 
Thanks,

David / dhildenb