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SubjectRe: [PATCH 4/4] mm/gup: adapt get_user_page_vma_remote() to never return NULL
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On 01.10.23 18:00, Lorenzo Stoakes wrote:
> get_user_pages_remote() will never return 0 except in the case of
> FOLL_NOWAIT being specified, which we explicitly disallow.
>
> This simplifies error handling for the caller and avoids the awkwardness of
> dealing with both errors and failing to pin. Failing to pin here is an
> error.
>
> Suggested-by: Arnd Bergmann <arnd@arndb.de>
> Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
> ---
> arch/arm64/kernel/mte.c | 4 ++--
> include/linux/mm.h | 16 +++++++++++++---
> kernel/events/uprobes.c | 4 ++--
> mm/memory.c | 3 +--
> 4 files changed, 18 insertions(+), 9 deletions(-)
>
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index 4edecaac8f91..8878b392df58 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -411,8 +411,8 @@ static int __access_remote_tags(struct mm_struct *mm, unsigned long addr,
> struct page *page = get_user_page_vma_remote(mm, addr,
> gup_flags, &vma);
>
> - if (IS_ERR_OR_NULL(page)) {
> - err = page == NULL ? -EIO : PTR_ERR(page);
> + if (IS_ERR(page)) {
> + err = PTR_ERR(page);
> break;
> }
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 7b89f7bd420d..da9631683d38 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2425,6 +2425,7 @@ long pin_user_pages_remote(struct mm_struct *mm,
> unsigned int gup_flags, struct page **pages,
> int *locked);
>
> +/* Either retrieve a single VMA and page, or an error. */
> static inline struct page *get_user_page_vma_remote(struct mm_struct *mm,
> unsigned long addr,
> int gup_flags,
> @@ -2432,12 +2433,21 @@ static inline struct page *get_user_page_vma_remote(struct mm_struct *mm,
> {
> struct page *page;
> struct vm_area_struct *vma;
> - int got = get_user_pages_remote(mm, addr, 1, gup_flags, &page, NULL);
> + int got;
> +
> + if (unlikely(gup_flags & FOLL_NOWAIT))
> + return ERR_PTR(-EINVAL);
> +

Do we have any callers or do we want to make that official (document it)
and use WARN_ON_ONCE() instead?

> + got = get_user_pages_remote(mm, addr, 1, gup_flags, &page, NULL);
>
> if (got < 0)
> return ERR_PTR(got);
> - if (got == 0)
> - return NULL;
> +
> + /*
> + * get_user_pages_remote() is guaranteed to not return 0 for
> + * non-FOLL_NOWAIT contexts, so this should never happen.
> + */
> + VM_WARN_ON(got == 0);

You should probably just drop that. Not worth the comment + code and its
better checked inside get_user_pages_remote().

Ideally, just document that behavior for get_user_pages_remote() "Will
never return 0 without FOLL_NOWAIT."

--
Cheers,

David / dhildenb

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