Re: [PATCH 1/2] mm: provide a sane PTE walking API for modules

[Christoph Hellwig <hch@infradead.org>]

> +int follow_invalidate_pte(struct mm_struct *mm, unsigned long address,
> +			  struct mmu_notifier_range *range, pte_t **ptepp, pmd_t **pmdpp,
> +			  spinlock_t **ptlp);

This adds a very pointless overy long line.

> +/**
> + * follow_pte - look up PTE at a user virtual address
> + * @vma: memory mapping
> + * @address: user virtual address
> + * @ptepp: location to store found PTE
> + * @ptlp: location to store the lock for the PTE
> + *
> + * On a successful return, the pointer to the PTE is stored in @ptepp;
> + * the corresponding lock is taken and its location is stored in @ptlp.
> + * The contents of the PTE are only stable until @ptlp is released;
> + * any further use, if any, must be protected against invalidation
> + * with MMU notifiers.
> + *
> + * Only IO mappings and raw PFN mappings are allowed.  The mmap semaphore
> + * should be taken for read.
> + *
> + * Return: zero on success, -ve otherwise.
> + */
> +int follow_pte(struct mm_struct *mm, unsigned long address,
> +	       pte_t **ptepp, spinlock_t **ptlp)
> +{
> +	return follow_invalidate_pte(mm, address, NULL, ptepp, NULL, ptlp);
> +}
> +EXPORT_SYMBOL_GPL(follow_pte);

I still don't think this is good as a general API.  Please document this
as KVM only for now, and hopefully next merge window I'll finish an
export variant restricting us to specific modules.