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

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Date:   Mon, 8 Feb 2021 17:39:36 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org, jgg@...pe.ca,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        dan.j.williams@...el.com
Subject: Re: [PATCH 1/2] mm: provide a sane PTE walking API for modules

> +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.