linux-kernel - Re: [PATCH v2] sparc64 mm: Fix more TSB sizing issues

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Message-Id: <20160928.113018.46166819947237523.davem@davemloft.net>
Date:   Wed, 28 Sep 2016 11:30:18 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     mike.kravetz@...cle.com
Cc:     linux-kernel@...r.kernel.org, sparclinux@...r.kernel.org,
        akpm@...ux-foundation.org, kirill.shutemov@...ux.intel.com,
        nitin.m.gupta@...cle.com
Subject: Re: [PATCH v2] sparc64 mm: Fix more TSB sizing issues

From: Mike Kravetz <mike.kravetz@...cle.com>
Date: Wed, 31 Aug 2016 13:48:19 -0700

> Commit af1b1a9b36b8 ("sparc64 mm: Fix base TSB sizing when hugetlb
> pages are used") addressed the difference between hugetlb and THP
> pages when computing TSB sizes.  The following additional issues
> were also discovered while working with the code.
> 
> In order to save memory, THP makes use of a huge zero page.  This huge
> zero page does not count against a task's RSS, but it does consume TSB
> entries.  This is similar to hugetlb pages.  Therefore, count huge
> zero page entries in hugetlb_pte_count.
> 
> Accounting of THP pages is done in the routine set_pmd_at().
> Unfortunately, this does not catch the case where a THP page is split.
> To handle this case, decrement the count in pmdp_invalidate().
> pmdp_invalidate is only called when splitting a THP.  However, 'sanity
> checks' are added in case it is ever called for other purposes.
> 
> A more general issue exists with HPAGE_SIZE accounting.
> hugetlb_pte_count tracks the number of HPAGE_SIZE (8M) pages.  This
> value is used to size the TSB for HPAGE_SIZE pages.  However,
> each HPAGE_SIZE page consists of two REAL_HPAGE_SIZE (4M) pages.
> The TSB contains an entry for each REAL_HPAGE_SIZE page.  Therefore,
> the number of REAL_HPAGE_SIZE pages should be used to size the huge
> page TSB.  A new compile time constant REAL_HPAGE_PER_HPAGE is used
> to multiply hugetlb_pte_count before sizing the TSB.
> 
> Changes from V1
> - Fixed build issue if hugetlb or THP not configured
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>

Applied.