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

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Date:   Wed, 29 Sep 2021 18:08:26 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     David Hildenbrand <david@...hat.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

> > Could you please to be more specific? I mean how is it connected with huge
> > pages mappings? Huge-pages are which have order > 0. Or you mean that
> > a special alignments are needed for mapping huge pages?
>
> Let me try to clarify:
>
>
> KASAN does an exact allocation when onlining a memory block,
> __vmalloc_node_range() will try placing huge pages first, increasing the
> alignment to e.g., "1 << PMD_SHIFT".
>
> If we increase the search length in find_vmap_lowest_match(), that
> search will fail if the exact allocation is surrounded by other
> allocations. In that case, we won't place a huge page although we could
> -- because find_vmap_lowest_match() would be imprecise for alignments >
> PAGE_SIZE.
>
>
> Memory blocks we online/offline on x86 are at least 128MB. The KASAN
> "overhead" we have to allocate is 1/8 of that -- 16 MB, so essentially 8
> huge pages.
>
> __vmalloc_node_range() will increase the alignment to 2MB to try placing
> huge pages first. find_vmap_lowest_match() will search within the given
> exact 16MB are a 18MB area (size + align), which won't work. So
> __vmalloc_node_range() will fallback to the original PAGE_SIZE alignment
> and shift=PAGE_SHIFT.
>
> __vmalloc_area_node() will set the set_vm_area_page_order effectively to
> 0 --  small pages.
>
> Does that make sense or am I missing something?
>
Thank you for clarification. OK, we come back anyway to the "problem" with fixed
range and an exact allocation plus a special alignment > PAGE_SIZE. Thus the
KASAN will not make use of huge pages mappings and go with regular instead
as a fallback path. But we would like to utilize huge-mappings for KASAN.

I will send the patch you tested and add your "tested-by" tag. Does it
sound good?

--
Uladzislau Rezki