linux-kernel - Re: [PATCH 2/2] mm: mmap: map MAP_STACK to VM

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Date: Wed, 31 Jan 2024 08:53:40 +0100
From: Florian Weimer <fweimer@...hat.com>
To: Yang Shi <shy828301@...il.com>
Cc: oliver.sang@...el.com,  riel@...riel.com,  fengwei.yin@...el.com,
  willy@...radead.org,  cl@...ux.com,  ying.huang@...el.com,
  akpm@...ux-foundation.org,  linux-kernel@...r.kernel.org,
  linux-mm@...ck.org
Subject: Re: [PATCH 2/2] mm: mmap: map MAP_STACK to VM_NOHUGEPAGE

* Yang Shi:

> From: Yang Shi <yang@...amperecomputing.com>
>
> The commit efa7df3e3bb5 ("mm: align larger anonymous mappings on THP
> boundaries") incured regression for stress-ng pthread benchmark [1].
> It is because THP get allocated to pthread's stack area much more possible
> than before.  Pthread's stack area is allocated by mmap without VM_GROWSDOWN
> or VM_GROWSUP flag, so kernel can't tell whether it is a stack area or not.
>
> The MAP_STACK flag is used to mark the stack area, but it is a no-op on
> Linux.  Mapping MAP_STACK to VM_NOHUGEPAGE to prevent from allocating
> THP for such stack area.

Doesn't this introduce a regression in the other direction, where
workloads expect to use a hugepage TLB entry for the stack?

It's seems an odd approach to fixing the stress-ng regression.  Isn't it
very much coding to the benchmark?

Thanks,
Florian