linux-kernel - Re: [PATCH] sparc64: mm: fix copy_tsb to correctly copy huge page TSBs

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Date:   Mon, 05 Jun 2017 14:55:23 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     mike.kravetz@...cle.com
Cc:     linux-kernel@...r.kernel.org, sparclinux@...r.kernel.org,
        nitin.m.gupta@...cle.com, bob.picco@...cle.com,
        anthony.yznaga@...cle.com
Subject: Re: [PATCH] sparc64: mm: fix copy_tsb to correctly copy huge page
 TSBs

From: Mike Kravetz <mike.kravetz@...cle.com>
Date: Fri,  2 Jun 2017 14:51:12 -0700

> When a TSB grows beyond its current capacity, a new TSB is allocated
> and copy_tsb is called to copy entries from the old TSB to the new.
> A hash shift based on page size is used to calculate the index of an
> entry in the TSB.  copy_tsb has hard coded PAGE_SHIFT in these
> calculations.  However, for huge page TSBs the value REAL_HPAGE_SHIFT
> should be used.  As a result, when copy_tsb is called for a huge page
> TSB the entries are placed at the incorrect index in the newly
> allocated TSB.  When doing hardware table walk, the MMU does not
> match these entries and we end up in the TSB miss handling code.
> This code will then create and write an entry to the correct index
> in the TSB.  We take a performance hit for the table walk miss and
> recreation of these entries.
> 
> Pass a new parameter to copy_tsb that is the page size shift to be
> used when copying the TSB.
> 
> Suggested-by: Anthony Yznaga <anthony.yznaga@...cle.com>
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>

Great find, applied and queued up for -stable.

Thanks!