linux-kernel - Re: [RESEND PATCH] mm: align larger anonymous mappings on THP boundaries

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Date: Sat, 20 Jan 2024 16:39:44 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Yang Shi <yang@...amperecomputing.com>, riel@...riel.com,
	shy828301@...il.com, cl@...ux.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RESEND PATCH] mm: align larger anonymous mappings on THP
 boundaries

On Sat, Jan 20, 2024 at 12:04:27PM +0000, Ryan Roberts wrote:
> However, after this patch, each allocation is in its own VMA, and there is a 2M
> gap between each VMA. This causes 2 problems: 1) mmap becomes MUCH slower
> because there are so many VMAs to check to find a new 1G gap. 2) It fails once
> it hits the VMA limit (/proc/sys/vm/max_map_count). Hitting this limit then
> causes a subsequent calloc() to fail, which causes the test to fail.
> 
> Looking at the code, I think the problem is that arm64 selects
> ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT. But __thp_get_unmapped_area() allocates
> len+2M then always aligns to the bottom of the discovered gap. That causes the
> 2M hole. As far as I can see, x86 allocates bottom up, so you don't get a hole.

As a quick hack, perhaps
#ifdef ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
take-the-top-half
#else
current-take-bottom-half-code
#endif

?