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

From: David Miller
Date: Wed Sep 28 2016 - 11:30:29 EST

From: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
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@xxxxxxxxxx>