linux-kernel - Re: [PATCH v1 6/7] mm/ksm: convert break_ksm() to use walk_page_range

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Date:   Thu, 20 Oct 2022 10:59:05 +0200
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>, Hugh Dickins <hughd@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Peter Xu <peterx@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        Janosch Frank <frankja@...ux.ibm.com>
Subject: Re: [PATCH v1 6/7] mm/ksm: convert break_ksm() to use
 walk_page_range_vma()

On 30.09.22 16:19, David Hildenbrand wrote:
> FOLL_MIGRATION exists only for the purpose of break_ksm(), and
> actually, there is not even the need to wait for the migration to
> finish, we only want to know if we're dealing with a KSM page.
> 
> Using follow_page() just to identify a KSM page overcomplicates GUP
> code. Let's use walk_page_range_vma() instead, because we don't actually
> care about the page itself, we only need to know a single property --
> no need to even grab a reference on the page.
> 
> In my setup (AMD Ryzen 9 3900X), running the KSM selftest to test unmerge
> performance on 2 GiB (taskset 0x8 ./ksm_tests -D -s 2048), this results in
> a performance degradation of ~4% (old: ~5010 MiB/s, new: ~4800 MiB/s).
> I don't think we particularly care for now.
> 
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>   mm/ksm.c | 70 +++++++++++++++++++++++++++++++++++++++++++++++++-------
>   1 file changed, 62 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 4d7bcf7da7c3..814c1a37c323 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -39,6 +39,7 @@
>   #include <linux/freezer.h>
>   #include <linux/oom.h>
>   #include <linux/numa.h>
> +#include <linux/pagewalk.h>
>   
>   #include <asm/tlbflush.h>
>   #include "internal.h"
> @@ -452,6 +453,60 @@ static inline bool ksm_test_exit(struct mm_struct *mm)
>   	return atomic_read(&mm->mm_users) == 0;
>   }
>   
> +int break_ksm_pud_entry(pud_t *pud, unsigned long addr, unsigned long next,
> +			struct mm_walk *walk)
> +{
> +	/* We only care about page tables to walk to a single base page. */
> +	if (pud_leaf(*pud) || !pud_present(*pud))
> +		return 1;
> +	return 0;
> +}
> +
> +int break_ksm_pmd_entry(pmd_t *pmd, unsigned long addr, unsigned long next,
> +			struct mm_walk *walk)
> +{
> +	bool *ksm_page = walk->private;
> +	struct page *page = NULL;
> +	pte_t *pte, ptent;
> +	spinlock_t *ptl;
> +
> +	/* We only care about page tables to walk to a single base page. */
> +	if (pmd_leaf(*pmd) || !pmd_present(*pmd))
> +		return 1;
> +
> +	/*
> +	 * We only lookup a single page (a) no need to iterate; and (b)
> +	 * always return 1 to exit immediately and not iterate in the caller.
> +	 */
> +	pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
> +	ptent = *pte;
> +
> +	if (pte_none(ptent))
> +		return 1;

As reported by Janosch, we fail to drop the lock here.


t480s: ~/git/linux ksm_unshare $ git diff
diff --git a/mm/ksm.c b/mm/ksm.c
index 26aec41b127c..94f8e114c89f 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -435,7 +435,7 @@ int break_ksm_pmd_entry(pmd_t *pmd, unsigned long addr, unsigned long next,
         pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
  
         if (pte_none(*pte))
-               return 1;
+               goto out_unlock;
         if (!pte_present(*pte)) {
                 swp_entry_t entry = pte_to_swp_entry(*pte);
  
@@ -451,6 +451,7 @@ int break_ksm_pmd_entry(pmd_t *pmd, unsigned long addr, unsigned long next,
         }
         if (page && PageKsm(page))
                 *ksm_page = true;
+out_unlock:
         pte_unmap_unlock(pte, ptl);
         return 1;
  }



-- 
Thanks,

David / dhildenb