linux-kernel - Re: [PATCH v8 05/17] iov_iter: Introduce fault_in_iov_iter

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Date:   Wed, 20 Oct 2021 17:25:24 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Andreas Gruenbacher <agruenba@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Paul Mackerras <paulus@...abs.org>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <willy@...radead.org>, cluster-devel@...hat.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        ocfs2-devel@....oracle.com, kvm-ppc@...r.kernel.org,
        linux-btrfs@...r.kernel.org
Subject: Re: [PATCH v8 05/17] iov_iter: Introduce fault_in_iov_iter_writeable

On Tue, Oct 19, 2021 at 03:41:52PM +0200, Andreas Gruenbacher wrote:
> diff --git a/mm/gup.c b/mm/gup.c
> index a7efb027d6cf..614b8536b3b6 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1691,6 +1691,69 @@ size_t fault_in_writeable(char __user *uaddr, size_t size)
>  }
>  EXPORT_SYMBOL(fault_in_writeable);
>  
> +/*
> + * fault_in_safe_writeable - fault in an address range for writing
> + * @uaddr: start of address range
> + * @size: length of address range
> + *
> + * Faults in an address range using get_user_pages, i.e., without triggering
> + * hardware page faults.  This is primarily useful when we already know that
> + * some or all of the pages in the address range aren't in memory.
> + *
> + * Other than fault_in_writeable(), this function is non-destructive.
> + *
> + * Note that we don't pin or otherwise hold the pages referenced that we fault
> + * in.  There's no guarantee that they'll stay in memory for any duration of
> + * time.
> + *
> + * Returns the number of bytes not faulted in, like copy_to_user() and
> + * copy_from_user().
> + */
> +size_t fault_in_safe_writeable(const char __user *uaddr, size_t size)
> +{
> +	unsigned long start = (unsigned long)uaddr;
> +	unsigned long end, nstart, nend;
> +	struct mm_struct *mm = current->mm;
> +	struct vm_area_struct *vma = NULL;

For arm64 tagged addresses we need the diff below, otherwise the
subsequent find_vma() will fail:

diff --git a/mm/gup.c b/mm/gup.c
index f5f362cb4640..2c51e9748a6a 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1713,7 +1713,7 @@ EXPORT_SYMBOL(fault_in_writeable);
  */
 size_t fault_in_safe_writeable(const char __user *uaddr, size_t size)
 {
-	unsigned long start = (unsigned long)uaddr;
+	unsigned long start = (unsigned long)untagged_addr(uaddr);
 	unsigned long end, nstart, nend;
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *vma = NULL;

-- 
Catalin