Re: [PATCH v3] mm/gup: Allow real explicit breaking of COW

[Jann Horn <jannh@google.com>]

On Tue, Aug 11, 2020 at 8:39 PM Peter Xu <peterx@redhat.com> wrote:
> Starting from commit 17839856fd58 ("gup: document and work around "COW can
> break either way" issue", 2020-06-02), explicit copy-on-write behavior is
> enforced for private gup pages even if it's a read-only.  It is achieved by
> always passing FOLL_WRITE to emulate a write.
>
> That should fix the COW issue that we were facing, however above commit could
> also break userfaultfd-wp and applications like umapsort [1,2].
>
> One general routine of umap-like program is: userspace library will manage page
> allocations, and it will evict the least recently used pages from memory to
> external storages (e.g., file systems).  Below are the general steps to evict
> an in-memory page in the uffd service thread when the page pool is full:
>
>   (1) UFFDIO_WRITEPROTECT with mode=WP on some to-be-evicted page P, so that
>       further writes to page P will block (keep page P clean)
>   (2) Copy page P to external storage (e.g. file system)
>   (3) MADV_DONTNEED to evict page P
>
> Here step (1) makes sure that the page to dump will always be up-to-date, so
> that the page snapshot in the file system is consistent with the one that was
> in the memory.  However with commit 17839856fd58, step (2) can potentially hang
> itself because e.g. if we use write() to a file system fd to dump the page
> data, that will be a translated read gup request in the file system driver to
> read the page content, then the read gup will be translated to a write gup due
> to the new enforced COW behavior.  This write gup will further trigger
> handle_userfault() and hang the uffd service thread itself.
>
> I think the problem will go away too if we replace the write() to the file
> system into a memory write to a mmaped region in the userspace library, because
> normal page faults will not enforce COW, only gup is affected.  However we
> cannot forbid users to use write() or any form of kernel level read gup.
>
> One solution is actually already mentioned in commit 17839856fd58, which is to
> provide an explicit BREAK_COW scemantics for enforced COW.  Then we can still
> use FAULT_FLAG_WRITE to identify whether this is a "real write request" or an
> "enfornced COW (read) request".
>
> With the enforced COW, we also need to inherit UFFD_WP bit during COW because
> now COW can happen with UFFD_WP enabled (previously, it cannot).
>
> Since at it, rename the variable in __handle_mm_fault() from "dirty" to "cow"
> to better suite its functionality.
[...]
> diff --git a/mm/memory.c b/mm/memory.c
[...]
> +                        * copied due to enfornced COW.  When it happens, we

(typo here and in the huge_memory version)

[...]
> diff --git a/mm/gup.c b/mm/gup.c
> index d8a33dd1430d..c33e84ab9c36 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -870,6 +870,8 @@ static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
>                 return -ENOENT;
>         if (*flags & FOLL_WRITE)
>                 fault_flags |= FAULT_FLAG_WRITE;
> +       if (*flags & FOLL_BREAK_COW)
> +               fault_flags |= FAULT_FLAG_BREAK_COW;
>         if (*flags & FOLL_REMOTE)
>                 fault_flags |= FAULT_FLAG_REMOTE;
>         if (locked)
> @@ -1076,7 +1078,7 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
>                         }
>                         if (is_vm_hugetlb_page(vma)) {
>                                 if (should_force_cow_break(vma, foll_flags))
> -                                       foll_flags |= FOLL_WRITE;
> +                                       foll_flags |= FOLL_BREAK_COW;

How does this interact with the FOLL_WRITE check in follow_page_pte()?
If we want the COW to be broken, follow_page_pte() would have to treat
FOLL_BREAK_COW similarly to FOLL_WRITE, right?