Commit 4f831457 authored by Nadav Amit's avatar Nadav Amit Committed by Andrew Morton
Browse files

mm: avoid unnecessary flush on change_huge_pmd() 

Calls to change_protection_range() on THP can trigger, at least on x86,
two TLB flushes for one page: one immediately, when pmdp_invalidate() is
called by change_huge_pmd(), and then another one later (that can be
batched) when change_protection_range() finishes.

The first TLB flush is only necessary to prevent the dirty bit (and with a
lesser importance the access bit) from changing while the PTE is modified.
However, this is not necessary as the x86 CPUs set the dirty-bit
atomically with an additional check that the PTE is (still) present.  One
caveat is Intel's Knights Landing that has a bug and does not do so.

Leverage this behavior to eliminate the unnecessary TLB flush in
change_huge_pmd().  Introduce a new arch specific pmdp_invalidate_ad()
that only invalidates the access and dirty bit from further changes.

Link: https://lkml.kernel.org/r/20220401180821.1986781-4-namit@vmware.com


Signed-off-by: default avatarNadav Amit <namit@vmware.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will@kernel.org>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Nick Piggin <npiggin@gmail.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent c9fe6656

arch/x86/include/asm/pgtable.h

+5 −0
Original line number Diff line number Diff line
@@ -1168,6 +1168,11 @@ static inline pmd_t pmdp_establish(struct vm_area_struct *vma, 
	}

}

#endif



#define __HAVE_ARCH_PMDP_INVALIDATE_AD

extern pmd_t pmdp_invalidate_ad(struct vm_area_struct *vma,

				unsigned long address, pmd_t *pmdp);



/*

 * Page table pages are page-aligned.  The lower half of the top

 * level is used for userspace and the top half for the kernel.

arch/x86/mm/pgtable.c

+10 −0
Original line number Diff line number Diff line
@@ -608,6 +608,16 @@ int pmdp_clear_flush_young(struct vm_area_struct *vma, 


	return young;

}



pmd_t pmdp_invalidate_ad(struct vm_area_struct *vma, unsigned long address,

			 pmd_t *pmdp)

{

	/*

	 * No flush is necessary. Once an invalid PTE is established, the PTE's

	 * access and dirty bits cannot be updated.

	 */

	return pmdp_establish(vma, address, pmdp, pmd_mkinvalid(*pmdp));

}

#endif



/**

include/linux/pgtable.h

+20 −0
Original line number Diff line number Diff line
@@ -570,6 +570,26 @@ extern pmd_t pmdp_invalidate(struct vm_area_struct *vma, unsigned long address, 
			    pmd_t *pmdp);

#endif



#ifndef __HAVE_ARCH_PMDP_INVALIDATE_AD



/*

 * pmdp_invalidate_ad() invalidates the PMD while changing a transparent

 * hugepage mapping in the page tables. This function is similar to

 * pmdp_invalidate(), but should only be used if the access and dirty bits would

 * not be cleared by the software in the new PMD value. The function ensures

 * that hardware changes of the access and dirty bits updates would not be lost.

 *

 * Doing so can allow in certain architectures to avoid a TLB flush in most

 * cases. Yet, another TLB flush might be necessary later if the PMD update

 * itself requires such flush (e.g., if protection was set to be stricter). Yet,

 * even when a TLB flush is needed because of the update, the caller may be able

 * to batch these TLB flushing operations, so fewer TLB flush operations are

 * needed.

 */

extern pmd_t pmdp_invalidate_ad(struct vm_area_struct *vma,

				unsigned long address, pmd_t *pmdp);

#endif



#ifndef __HAVE_ARCH_PTE_SAME

static inline int pte_same(pte_t pte_a, pte_t pte_b)

{

mm/huge_memory.c

+2 −2
Original line number Diff line number Diff line
@@ -1801,10 +1801,10 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma, 
	 * The race makes MADV_DONTNEED miss the huge pmd and don't clear it

	 * which may break userspace.

	 *

	 * pmdp_invalidate() is required to make sure we don't miss

	 * pmdp_invalidate_ad() is required to make sure we don't miss

	 * dirty/young flags set by hardware.

	 */

	oldpmd = pmdp_invalidate(vma, addr, pmd);

	oldpmd = pmdp_invalidate_ad(vma, addr, pmd);



	entry = pmd_modify(oldpmd, newprot);

	if (preserve_write)

mm/pgtable-generic.c

+8 −0
Original line number Diff line number Diff line
@@ -201,6 +201,14 @@ pmd_t pmdp_invalidate(struct vm_area_struct *vma, unsigned long address, 
}

#endif



#ifndef __HAVE_ARCH_PMDP_INVALIDATE_AD

pmd_t pmdp_invalidate_ad(struct vm_area_struct *vma, unsigned long address,

			 pmd_t *pmdp)

{

	return pmdp_invalidate(vma, address, pmdp);

}

#endif



#ifndef pmdp_collapse_flush

pmd_t pmdp_collapse_flush(struct vm_area_struct *vma, unsigned long address,

			  pmd_t *pmdp)