Re: [RFC PATCH v2 4/5] userfaultfd: zero access/write hints

[Nadav Amit <namit@vmware.com>]

On Jun 21, 2022, at 10:04 AM, Peter Xu <peterx@redhat.com> wrote:

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> 
> On Sun, Jun 19, 2022 at 04:34:48PM -0700, Nadav Amit wrote:
>> From: Nadav Amit <namit@vmware.com>
>> 
>> When userfaultfd provides a zeropage in response to ioctl, it provides a
>> readonly alias to the zero page. If the page is later written (which is
>> the likely scenario), page-fault occurs and the page-fault allocator
>> allocates a page and rewires the page-tables.
>> 
>> This is an expensive flow for cases in which a page is likely be written
>> to. Users can use the copy ioctl to initialize zero page (by copying
>> zeros), but this is also wasteful.
>> 
>> Allow userfaultfd users to efficiently map initialized zero-pages that
>> are writable. Introduce UFFDIO_ZEROPAGE_MODE_WRITE_LIKELY, which, when
>> provided would map a clear page instead of an alias to the zero page.
>> 
>> For consistency, introduce also UFFDIO_ZEROPAGE_MODE_ACCESS_LIKELY.
>> 
>> Suggested-by: David Hildenbrand <david@redhat.com>
>> Cc: Mike Kravetz <mike.kravetz@oracle.com>
>> Cc: Hugh Dickins <hughd@google.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Axel Rasmussen <axelrasmussen@google.com>
>> Cc: Peter Xu <peterx@redhat.com>
>> Cc: Mike Rapoport <rppt@linux.ibm.com>
>> Signed-off-by: Nadav Amit <namit@vmware.com>
>> ---
>> fs/userfaultfd.c                 | 14 +++++++++++--
>> include/uapi/linux/userfaultfd.h |  2 ++
>> mm/userfaultfd.c                 | 36 ++++++++++++++++++++++++++++++++
>> 3 files changed, 50 insertions(+), 2 deletions(-)
>> 
>> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
>> index a56983b594d5..ff073de78ea8 100644
>> --- a/fs/userfaultfd.c
>> +++ b/fs/userfaultfd.c
>> @@ -1770,6 +1770,8 @@ static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx,
>>      struct uffdio_zeropage uffdio_zeropage;
>>      struct uffdio_zeropage __user *user_uffdio_zeropage;
>>      struct userfaultfd_wake_range range;
>> +     bool mode_dontwake, mode_access_likely, mode_write_likely;
>> +     uffd_flags_t uffd_flags;
>> 
>>      user_uffdio_zeropage = (struct uffdio_zeropage __user *) arg;
>> 
>> @@ -1788,8 +1790,16 @@ static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx,
>>      if (ret)
>>              goto out;
>>      ret = -EINVAL;
>> -     if (uffdio_zeropage.mode & ~UFFDIO_ZEROPAGE_MODE_DONTWAKE)
>> -             goto out;
>> +
>> +     mode_dontwake = uffdio_zeropage.mode & UFFDIO_ZEROPAGE_MODE_DONTWAKE;
>> +     mode_access_likely = uffdio_zeropage.mode & UFFDIO_ZEROPAGE_MODE_ACCESS_LIKELY;
>> +     mode_write_likely = uffdio_zeropage.mode & UFFDIO_ZEROPAGE_MODE_WRITE_LIKELY;
>> +
>> +     if (mode_dontwake)
>> +             return -EINVAL;
> 
> Hmm.. Why?
> 
> Note that the above uffdio_zeropage.mode check was for invalid mode flags
> only, and I think that should be kept, but still I don't see why we want to
> fail UFFDIO_ZEROPAGE_MODE_DONTWAKE users.
> 
>> +
>> +     uffd_flags = (mode_access_likely ? UFFD_FLAGS_ACCESS_LIKELY : 0) |
>> +                  (mode_write_likely ? UFFD_FLAGS_WRITE_LIKELY : 0);
>> 
>>      if (mmget_not_zero(ctx->mm)) {
>>              ret = mfill_zeropage(ctx->mm, uffdio_zeropage.range.start,
>> diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
>> index 6ad93a13282e..b586b7c1e265 100644
>> --- a/include/uapi/linux/userfaultfd.h
>> +++ b/include/uapi/linux/userfaultfd.h
>> @@ -286,6 +286,8 @@ struct uffdio_copy {
>> struct uffdio_zeropage {
>>      struct uffdio_range range;
>> #define UFFDIO_ZEROPAGE_MODE_DONTWAKE                ((__u64)1<<0)
>> +#define UFFDIO_ZEROPAGE_MODE_ACCESS_LIKELY   ((__u64)1<<2)
>> +#define UFFDIO_ZEROPAGE_MODE_WRITE_LIKELY    ((__u64)1<<3)
>>      __u64 mode;
>> 
>>      /*
>> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
>> index 3172158d8faa..5dfbb1e80369 100644
>> --- a/mm/userfaultfd.c
>> +++ b/mm/userfaultfd.c
>> @@ -249,6 +249,38 @@ static int mfill_zeropage_pte(struct mm_struct *dst_mm,
>>      return ret;
>> }
>> 
>> +static int mfill_clearpage_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
>> +                            struct vm_area_struct *dst_vma,
>> +                            unsigned long dst_addr,
>> +                            uffd_flags_t uffd_flags)
>> +{
>> +     struct page *page;
>> +     int ret;
>> +
>> +     ret = -ENOMEM;
>> +     page = alloc_zeroed_user_highpage_movable(dst_vma, dst_addr);
>> +     if (!page)
>> +             goto out;
>> +
>> +     /* The PTE is not marked as dirty unconditionally */
>> +     SetPageDirty(page);
>> +     __SetPageUptodate(page);
>> +
>> +     ret = -ENOMEM;
> 
> Nit: can drop this since ret will always be -ENOMEM here..

I noticed. Just thought it is clearer this way, and more robust against
future changes.