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Subject: Re: [RFC] mm/gup.c: Updated return value of
 {get|pin}_user_pages_fast()
To: Souptick Joarder <jrdr.linux@gmail.com>, <tony.luck@intel.com>,
	<fenghua.yu@intel.com>, <rspringer@google.com>, <toddpoynor@google.com>,
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References: <1588706059-4208-1-git-send-email-jrdr.linux@gmail.com>
From: John Hubbard <jhubbard@nvidia.com>
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Date: Tue, 5 May 2020 12:38:17 -0700
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On 2020-05-05 12:14, Souptick Joarder wrote:
> Currently {get|pin}_user_pages_fast() have 3 return value 0, -errno
> and no of pinned pages. The only case where these two functions will
> return 0, is for nr_pages <= 0, which doesn't find a valid use case.
> But if at all any, then a -ERRNO will be returned instead of 0, which
> means {get|pin}_user_pages_fast() will have 2 return values -errno &
> no of pinned pages.
> 
> Update all the callers which deals with return value 0 accordingly.

Hmmm, seems a little shaky. In order to do this safely, I'd recommend
first changing gup_fast/pup_fast so so that they return -EINVAL if
the caller specified nr_pages==0, and of course auditing all callers,
to ensure that this won't cause problems.

The gup.c documentation would also need updating in a couple of comment
blocks, above get_user_pages_remote(), and __get_user_pages(), because
those talk about a zero return value.

This might be practical without slowing down the existing code, because
there is already a check in place, so just tweaking it like this (untested)
won't change performance at all:

diff --git a/mm/gup.c b/mm/gup.c
index 11fda538c9d9..708eed79ae29 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2787,7 +2787,7 @@ static int internal_get_user_pages_fast(unsigned long start, 
int nr_pages,
         end = start + len;

         if (end <= start)
-               return 0;
+               return -EINVAL;
         if (unlikely(!access_ok((void __user *)start, len)))
                 return -EFAULT;

...although I might be missing some other things that need a similar change,
so you should look carefully for yourself.


Once that change (and anything I missed) is in place, then you could go
ahead and stop handling ret==0 cases at the call sites.


thanks,
-- 
John Hubbard
NVIDIA

> 
> Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
> ---
>   arch/ia64/kernel/err_inject.c              | 2 +-
>   drivers/platform/goldfish/goldfish_pipe.c  | 2 +-
>   drivers/staging/gasket/gasket_page_table.c | 4 ++--
>   drivers/tee/tee_shm.c                      | 2 +-
>   mm/gup.c                                   | 6 +++---
>   net/rds/rdma.c                             | 2 +-
>   6 files changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/ia64/kernel/err_inject.c b/arch/ia64/kernel/err_inject.c
> index 8b5b8e6b..fd72218 100644
> --- a/arch/ia64/kernel/err_inject.c
> +++ b/arch/ia64/kernel/err_inject.c
> @@ -143,7 +143,7 @@ static DEVICE_ATTR(name, 0644, show_##name, store_##name)
>   	int ret;
>   
>   	ret = get_user_pages_fast(virt_addr, 1, FOLL_WRITE, NULL);
> -	if (ret<=0) {
> +	if (ret < 0) {
>   #ifdef ERR_INJ_DEBUG
>   		printk("Virtual address %lx is not existing.\n",virt_addr);
>   #endif
> diff --git a/drivers/platform/goldfish/goldfish_pipe.c b/drivers/platform/goldfish/goldfish_pipe.c
> index 1ab207e..831449d 100644
> --- a/drivers/platform/goldfish/goldfish_pipe.c
> +++ b/drivers/platform/goldfish/goldfish_pipe.c
> @@ -277,7 +277,7 @@ static int goldfish_pin_pages(unsigned long first_page,
>   	ret = pin_user_pages_fast(first_page, requested_pages,
>   				  !is_write ? FOLL_WRITE : 0,
>   				  pages);
> -	if (ret <= 0)
> +	if (ret < 0)
>   		return -EFAULT;
>   	if (ret < requested_pages)
>   		*iter_last_page_size = PAGE_SIZE;
> diff --git a/drivers/staging/gasket/gasket_page_table.c b/drivers/staging/gasket/gasket_page_table.c
> index f6d7157..1d08e1d 100644
> --- a/drivers/staging/gasket/gasket_page_table.c
> +++ b/drivers/staging/gasket/gasket_page_table.c
> @@ -489,11 +489,11 @@ static int gasket_perform_mapping(struct gasket_page_table *pg_tbl,
>   			ret = get_user_pages_fast(page_addr - offset, 1,
>   						  FOLL_WRITE, &page);
>   
> -			if (ret <= 0) {
> +			if (ret < 0) {
>   				dev_err(pg_tbl->device,
>   					"get user pages failed for addr=0x%lx, offset=0x%lx [ret=%d]\n",
>   					page_addr, offset, ret);
> -				return ret ? ret : -ENOMEM;
> +				return ret;
>   			}
>   			++pg_tbl->num_active_pages;
>   
> diff --git a/drivers/tee/tee_shm.c b/drivers/tee/tee_shm.c
> index bd679b7..2706a1f 100644
> --- a/drivers/tee/tee_shm.c
> +++ b/drivers/tee/tee_shm.c
> @@ -230,7 +230,7 @@ struct tee_shm *tee_shm_register(struct tee_context *ctx, unsigned long addr,
>   	if (rc > 0)
>   		shm->num_pages = rc;
>   	if (rc != num_pages) {
> -		if (rc >= 0)
> +		if (rc > 0)
>   			rc = -ENOMEM;
>   		ret = ERR_PTR(rc);
>   		goto err;
> diff --git a/mm/gup.c b/mm/gup.c
> index 50681f0..8d293ed 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2760,7 +2760,7 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
>   	end = start + len;
>   
>   	if (end <= start)
> -		return 0;
> +		return -EINVAL;
>   	if (unlikely(!access_ok((void __user *)start, len)))
>   		return -EFAULT;
>   
> @@ -2805,8 +2805,8 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
>    * calling get_user_pages().
>    *
>    * Returns number of pages pinned. This may be fewer than the number requested.
> - * If nr_pages is 0 or negative, returns 0. If no pages were pinned, returns
> - * -errno.
> + * If nr_pages is 0 or negative, returns -errno. If no pages were pinned,
> + * returns -errno.
>    */
>   int get_user_pages_fast(unsigned long start, int nr_pages,
>   			unsigned int gup_flags, struct page **pages)
> diff --git a/net/rds/rdma.c b/net/rds/rdma.c
> index a7ae118..44b96e6 100644
> --- a/net/rds/rdma.c
> +++ b/net/rds/rdma.c
> @@ -161,7 +161,7 @@ static int rds_pin_pages(unsigned long user_addr, unsigned int nr_pages,
>   		gup_flags |= FOLL_WRITE;
>   
>   	ret = pin_user_pages_fast(user_addr, nr_pages, gup_flags, pages);
> -	if (ret >= 0 && ret < nr_pages) {
> +	if (ret > 0 && ret < nr_pages) {
>   		unpin_user_pages(pages, ret);
>   		ret = -EFAULT;
>   	}
>