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SubjectRe: [PATCH v6 01/11] asm-generic/pgtable: Adds generic functions to track lockless pgtable walks
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Le 06/02/2020 à 04:08, Leonardo Bras a écrit :
> It's necessary to track lockless pagetable walks, in order to avoid doing
> THP splitting/collapsing during them.
>
> The default solution is to disable irq before lockless pagetable walks and
> enable it after it's finished.
>
> On code, this means you can find local_irq_disable() and local_irq_enable()
> around some pieces of code, usually without comments on why it is needed.
>
> This patch proposes a set of generic functions to be called before starting
> and after finishing a lockless pagetable walk. It is supposed to make clear
> that a lockless pagetable walk happens there, and also carries information
> on why the irq disable/enable is needed.
>
> begin_lockless_pgtbl_walk()
> Insert before starting any lockless pgtable walk
> end_lockless_pgtbl_walk()
> Insert after the end of any lockless pgtable walk
> (Mostly after the ptep is last used)
>
> A memory barrier was also added just to make sure there is no speculative
> read outside the interrupt disabled area. Other than that, it is not
> supposed to have any change of behavior from current code.

Is that speculative barrier necessary for all architectures ? Does it
impact performance ? Shouldn't this be another patch ?

>
> It is planned to allow arch-specific versions, so that additional steps can
> be added while keeping the code clean.
>
> Signed-off-by: Leonardo Bras <leonardo@linux.ibm.com>
> ---
> include/asm-generic/pgtable.h | 51 +++++++++++++++++++++++++++++++++++
> 1 file changed, 51 insertions(+)
>
> diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
> index e2e2bef07dd2..8d368d3c0974 100644
> --- a/include/asm-generic/pgtable.h
> +++ b/include/asm-generic/pgtable.h
> @@ -1222,6 +1222,57 @@ static inline bool arch_has_pfn_modify_check(void)
> #endif
> #endif
>
> +#ifndef __HAVE_ARCH_LOCKLESS_PGTBL_WALK_CONTROL
> +/*
> + * begin_lockless_pgtbl_walk: Must be inserted before a function call that does
> + * lockless pagetable walks, such as __find_linux_pte()
> + */
> +static inline
> +unsigned long begin_lockless_pgtbl_walk(void)

What about keeping the same syntax as local_irq_save(), something like:

#define begin_lockless_pgtbl_walk(flags) \
do {
local_irq_save(flags);
smp_mb();
} while (0)

> +{
> + unsigned long irq_mask;
> +
> + /*
> + * Interrupts must be disabled during the lockless page table walk.
> + * That's because the deleting or splitting involves flushing TLBs,
> + * which in turn issues interrupts, that will block when disabled.
> + */
> + local_irq_save(irq_mask);
> +
> + /*
> + * This memory barrier pairs with any code that is either trying to
> + * delete page tables, or split huge pages. Without this barrier,
> + * the page tables could be read speculatively outside of interrupt
> + * disabling.
> + */
> + smp_mb();
> +
> + return irq_mask;
> +}
> +
> +/*
> + * end_lockless_pgtbl_walk: Must be inserted after the last use of a pointer
> + * returned by a lockless pagetable walk, such as __find_linux_pte()
> + */
> +static inline void end_lockless_pgtbl_walk(unsigned long irq_mask)

Same

#define end_lockless_pgtbl_walk(flags) \
do {
smp_mb();
local_irq_restore(flags);
} while (0);

> +{
> + /*
> + * This memory barrier pairs with any code that is either trying to
> + * delete page tables, or split huge pages. Without this barrier,
> + * the page tables could be read speculatively outside of interrupt
> + * disabling.
> + */
> + smp_mb();
> +
> + /*
> + * Interrupts must be disabled during the lockless page table walk.
> + * That's because the deleting or splitting involves flushing TLBs,
> + * which in turn issues interrupts, that will block when disabled.
> + */
> + local_irq_restore(irq_mask);
> +}
> +#endif
> +
> /*
> * On some architectures it depends on the mm if the p4d/pud or pmd
> * layer of the page table hierarchy is folded or not.
>

Christophe

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