Re: [PATCH mm-unstable v1 09/20] mm/gup: reliable R/O long-term pinning in COW mappings

John Hubbard
2022-11-24T01:30:10Z

On 11/16/22 02:26, David Hildenbrand wrote:
...
> With this change, the new R/O long-term pinning tests for non-anonymous
> memory succeed:
> # [RUN] R/O longterm GUP pin ... with shared zeropage
> ok 151 Longterm R/O pin is reliable
> # [RUN] R/O longterm GUP pin ... with memfd
> ok 152 Longterm R/O pin is reliable
> # [RUN] R/O longterm GUP pin ... with tmpfile
> ok 153 Longterm R/O pin is reliable
> # [RUN] R/O longterm GUP pin ... with huge zeropage
> ok 154 Longterm R/O pin is reliable
> # [RUN] R/O longterm GUP pin ... with memfd hugetlb (2048 kB)
> ok 155 Longterm R/O pin is reliable
> # [RUN] R/O longterm GUP pin ... with memfd hugetlb (1048576 kB)
> ok 156 Longterm R/O pin is reliable
> # [RUN] R/O longterm GUP-fast pin ... with shared zeropage
> ok 157 Longterm R/O pin is reliable
> # [RUN] R/O longterm GUP-fast pin ... with memfd
> ok 158 Longterm R/O pin is reliable
> # [RUN] R/O longterm GUP-fast pin ... with tmpfile
> ok 159 Longterm R/O pin is reliable
> # [RUN] R/O longterm GUP-fast pin ... with huge zeropage
> ok 160 Longterm R/O pin is reliable
> # [RUN] R/O longterm GUP-fast pin ... with memfd hugetlb (2048 kB)
> ok 161 Longterm R/O pin is reliable
> # [RUN] R/O longterm GUP-fast pin ... with memfd hugetlb (1048576 kB)
> ok 162 Longterm R/O pin is reliable

Yes. I was able to reproduce these results, after some minor distractions
involving huge pages, don't ask. :)

>
> Note 1: We don't care about short-term R/O-pinning, because they have
> snapshot semantics: they are not supposed to observe modifications that
> happen after pinning.
>
> As one example, assume we start direct I/O to read from a page and store
> page content into a file: modifications to page content after starting
> direct I/O are not guaranteed to end up in the file. So even if we'd pin
> the shared zeropage, the end result would be as expected -- getting zeroes
> stored to the file.
>
> Note 2: For shared mappings we'll now always fallback to the slow path to
> lookup the VMA when R/O long-term pining. While that's the necessary price
> we have to pay right now, it's actually not that bad in practice: most
> FOLL_LONGTERM users already specify FOLL_WRITE, for example, along with
> FOLL_FORCE because they tried dealing with COW mappings correctly ...
>
> Note 3: For users that use FOLL_LONGTERM right now without FOLL_WRITE,
> such as VFIO, we'd now no longer pin the shared zeropage. Instead, we'd
> populate exclusive anon pages that we can pin. There was a concern that
> this could affect the memlock limit of existing setups.
>
> For example, a VM running with VFIO could run into the memlock limit and
> fail to run. However, we essentially had the same behavior already in
> commit 17839856fd58 ("gup: document and work around "COW can break either
> way" issue") which got merged into some enterprise distros, and there were
> not any such complaints. So most probably, we're fine.
>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
> include/linux/mm.h | 27 ++++++++++++++++++++++++---
> mm/gup.c | 10 +++++-----
> mm/huge_memory.c | 2 +-
> mm/hugetlb.c | 7 ++++---
> 4 files changed, 34 insertions(+), 12 deletions(-)
>

Looks good,

Reviewed-by: John Hubbard <jhubbard@nvidia.com>

thanks,
--
John Hubbard
NVIDIA

> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 6bd2ee5872dd..e8cc838f42f9 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3095,8 +3095,12 @@ static inline int vm_fault_to_errno(vm_fault_t vm_fault, int foll_flags)
> * Must be called with the (sub)page that's actually referenced via the
> * page table entry, which might not necessarily be the head page for a
> * PTE-mapped THP.
> + *
> + * If the vma is NULL, we're coming from the GUP-fast path and might have
> + * to fallback to the slow path just to lookup the vma.
> */
> -static inline bool gup_must_unshare(unsigned int flags, struct page *page)
> +static inline bool gup_must_unshare(struct vm_area_struct *vma,
> + unsigned int flags, struct page *page)
> {
> /*
> * FOLL_WRITE is implicitly handled correctly as the page table entry
> @@ -3109,8 +3113,25 @@ static inline bool gup_must_unshare(unsigned int flags, struct page *page)
> * Note: PageAnon(page) is stable until the page is actually getting
> * freed.
> */
> - if (!PageAnon(page))
> - return false;
> + if (!PageAnon(page)) {
> + /*
> + * We only care about R/O long-term pining: R/O short-term
> + * pinning does not have the semantics to observe successive
> + * changes through the process page tables.
> + */
> + if (!(flags & FOLL_LONGTERM))
> + return false;
> +
> + /* We really need the vma ... */
> + if (!vma)
> + return true;
> +
> + /*
> + * ... because we only care about writable private ("COW")
> + * mappings where we have to break COW early.
> + */
> + return is_cow_mapping(vma->vm_flags);
> + }
>
> /* Paired with a memory barrier in page_try_share_anon_rmap(). */
> if (IS_ENABLED(CONFIG_HAVE_FAST_GUP))
> diff --git a/mm/gup.c b/mm/gup.c
> index 5182abaaecde..01116699c863 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -578,7 +578,7 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
> }
> }
>
> - if (!pte_write(pte) && gup_must_unshare(flags, page)) {
> + if (!pte_write(pte) && gup_must_unshare(vma, flags, page)) {
> page = ERR_PTR(-EMLINK);
> goto out;
> }
> @@ -2338,7 +2338,7 @@ static int gup_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,
> goto pte_unmap;
> }
>
> - if (!pte_write(pte) && gup_must_unshare(flags, page)) {
> + if (!pte_write(pte) && gup_must_unshare(NULL, flags, page)) {
> gup_put_folio(folio, 1, flags);
> goto pte_unmap;
> }
> @@ -2506,7 +2506,7 @@ static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
> return 0;
> }
>
> - if (!pte_write(pte) && gup_must_unshare(flags, &folio->page)) {
> + if (!pte_write(pte) && gup_must_unshare(NULL, flags, &folio->page)) {
> gup_put_folio(folio, refs, flags);
> return 0;
> }
> @@ -2572,7 +2572,7 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
> return 0;
> }
>
> - if (!pmd_write(orig) && gup_must_unshare(flags, &folio->page)) {
> + if (!pmd_write(orig) && gup_must_unshare(NULL, flags, &folio->page)) {
> gup_put_folio(folio, refs, flags);
> return 0;
> }
> @@ -2612,7 +2612,7 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
> return 0;
> }
>
> - if (!pud_write(orig) && gup_must_unshare(flags, &folio->page)) {
> + if (!pud_write(orig) && gup_must_unshare(NULL, flags, &folio->page)) {
> gup_put_folio(folio, refs, flags);
> return 0;
> }
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 68d00196b519..dec7a7c0eca8 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1434,7 +1434,7 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
> if (pmd_protnone(*pmd) && !gup_can_follow_protnone(flags))
> return NULL;
>
> - if (!pmd_write(*pmd) && gup_must_unshare(flags, page))
> + if (!pmd_write(*pmd) && gup_must_unshare(vma, flags, page))
> return ERR_PTR(-EMLINK);
>
> VM_BUG_ON_PAGE((flags & FOLL_PIN) && PageAnon(page) &&
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 383b26069b33..c3aab6d5b7aa 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6195,7 +6195,8 @@ static void record_subpages_vmas(struct page *page, struct vm_area_struct *vma,
> }
> }
>
> -static inline bool __follow_hugetlb_must_fault(unsigned int flags, pte_t *pte,
> +static inline bool __follow_hugetlb_must_fault(struct vm_area_struct *vma,
> + unsigned int flags, pte_t *pte,
> bool *unshare)
> {
> pte_t pteval = huge_ptep_get(pte);
> @@ -6207,7 +6208,7 @@ static inline bool __follow_hugetlb_must_fault(unsigned int flags, pte_t *pte,
> return false;
> if (flags & FOLL_WRITE)
> return true;
> - if (gup_must_unshare(flags, pte_page(pteval))) {
> + if (gup_must_unshare(vma, flags, pte_page(pteval))) {
> *unshare = true;
> return true;
> }
> @@ -6336,7 +6337,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
> * directly from any kind of swap entries.
> */
> if (absent ||
> - __follow_hugetlb_must_fault(flags, pte, &unshare)) {
> + __follow_hugetlb_must_fault(vma, flags, pte, &unshare)) {
> vm_fault_t ret;
> unsigned int fault_flags = 0;
>