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Commit 031e6e6b authored by Steve Capper's avatar Steve Capper Committed by Will Deacon
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arm64: hugetlb: Avoid unnecessary clearing in huge_ptep_set_access_flags 



For contiguous hugetlb, huge_ptep_set_access_flags performs a
get_clear_flush (which then flushes the TLBs) even when no change of ptes
is necessary.

Unfortunately, this behaviour can lead to back-to-back page faults being
generated when running with multiple threads that access the same
contiguous huge page.

Thread 1                     |  Thread 2
-----------------------------+------------------------------
hugetlb_fault                |
huge_ptep_set_access_flags   |
  -> invalidate pte range    | hugetlb_fault
continue processing          | wait for hugetlb_fault_mutex
release mutex and return     | huge_ptep_set_access_flags
                             |   -> invalidate pte range
hugetlb_fault
...

This patch changes huge_ptep_set_access_flags s.t. we first read the
contiguous range of ptes (whilst preserving dirty information); the pte
range is only then invalidated where necessary and this prevents further
spurious page faults.

Fixes: d8bdcff2 ("arm64: hugetlb: Add break-before-make logic for contiguous entries")
Reported-by: default avatarLei Zhang <zhang.lei@jp.fujitsu.com>
Signed-off-by: default avatarSteve Capper <steve.capper@arm.com>
Signed-off-by: default avatarWill Deacon <will.deacon@arm.com>
parent 469ed9d8

arch/arm64/mm/hugetlbpage.c

+34 −4
Original line number Diff line number Diff line
@@ -323,11 +323,40 @@ pte_t huge_ptep_get_and_clear(struct mm_struct *mm, 
	return get_clear_flush(mm, addr, ptep, pgsize, ncontig);

}



/*

 * huge_ptep_set_access_flags will update access flags (dirty, accesssed)

 * and write permission.

 *

 * For a contiguous huge pte range we need to check whether or not write

 * permission has to change only on the first pte in the set. Then for

 * all the contiguous ptes we need to check whether or not there is a

 * discrepancy between dirty or young.

 */

static int __cont_access_flags_changed(pte_t *ptep, pte_t pte, int ncontig)

{

	int i;



	if (pte_write(pte) != pte_write(huge_ptep_get(ptep)))

		return 1;



	for (i = 0; i < ncontig; i++) {

		pte_t orig_pte = huge_ptep_get(ptep + i);



		if (pte_dirty(pte) != pte_dirty(orig_pte))

			return 1;



		if (pte_young(pte) != pte_young(orig_pte))

			return 1;

	}



	return 0;

}



int huge_ptep_set_access_flags(struct vm_area_struct *vma,

			       unsigned long addr, pte_t *ptep,

			       pte_t pte, int dirty)

{

	int ncontig, i, changed = 0;

	int ncontig, i;

	size_t pgsize = 0;

	unsigned long pfn = pte_pfn(pte), dpfn;

	pgprot_t hugeprot;
@@ -339,9 +368,10 @@ int huge_ptep_set_access_flags(struct vm_area_struct *vma, 
	ncontig = find_num_contig(vma->vm_mm, addr, ptep, &pgsize);

	dpfn = pgsize >> PAGE_SHIFT;



	if (!__cont_access_flags_changed(ptep, pte, ncontig))

		return 0;



	orig_pte = get_clear_flush(vma->vm_mm, addr, ptep, pgsize, ncontig);

	if (!pte_same(orig_pte, pte))

		changed = 1;



	/* Make sure we don't lose the dirty or young state */

	if (pte_dirty(orig_pte))
@@ -354,7 +384,7 @@ int huge_ptep_set_access_flags(struct vm_area_struct *vma, 
	for (i = 0; i < ncontig; i++, ptep++, addr += pgsize, pfn += dpfn)

		set_pte_at(vma->vm_mm, addr, ptep, pfn_pte(pfn, hugeprot));



	return changed;

	return 1;

}



void huge_ptep_set_wrprotect(struct mm_struct *mm,