> On Mar 3, 2022, at 11:05 AM, Nadav Amit <namit@xxxxxxxxxx> wrote:
>
>
>
>> On Mar 3, 2022, at 12:03 AM, Peter Xu <peterx@xxxxxxxxxx> wrote:
>>
>> On Sat, Feb 26, 2022 at 02:26:55AM +0000, Nadav Amit wrote:
>>> From: Nadav Amit <namit@xxxxxxxxxx>
>>>
>>> Userfaultfd is supposed to provide the full address (i.e., unmasked) of
>>> the faulting access back to userspace. However, that is not the case for
>>> quite some time.
>>>
>>> Even running "userfaultfd_demo" from the userfaultfd man page provides
>>> the wrong output (and contradicts the man page). Notice that
>>> "UFFD_EVENT_PAGEFAULT event" shows the masked address (7fc5e30b3000)
>>> and not the first read address (0x7fc5e30b300f).
>>>
>>> Address returned by mmap() = 0x7fc5e30b3000
>>>
>>> fault_handler_thread():
>>> poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
>>> UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fc5e30b3000
>>> (uffdio_copy.copy returned 4096)
>>> Read address 0x7fc5e30b300f in main(): A
>>> Read address 0x7fc5e30b340f in main(): A
>>> Read address 0x7fc5e30b380f in main(): A
>>> Read address 0x7fc5e30b3c0f in main(): A
>>>
>>> The exact address is useful for various reasons and specifically for
>>> prefetching decisions. If it is known that the memory is populated by
>>> certain objects whose size is not page-aligned, then based on the
>>> faulting address, the uffd-monitor can decide whether to prefetch and
>>> prefault the adjacent page.
>>>
>>> This bug has been for quite some time in the kernel: since commit
>>> 1a29d85eb0f1 ("mm: use vmf->address instead of of vmf->virtual_address")
>>> vmf->virtual_address"), which dates back to 2016. A concern has been
>>> raised that existing userspace application might rely on the old/wrong
>>> behavior in which the address is masked. Therefore, it was suggested to
>>> provide the masked address unless the user explicitly asks for the exact
>>> address.
>>>
>>> Add a new userfaultfd feature UFFD_FEATURE_EXACT_ADDRESS to direct
>>> userfaultfd to provide the exact address. Add a new "real_address" field
>>> to vmf to hold the unmasked address. Provide the address to userspace
>>> accordingly.
>>>
>>> Initialize real_address in various code-paths to be consistent with
>>> address, even when it is not used, to be on the safe side.
>>>
>>> Acked-by: Peter Xu <peterx@xxxxxxxxxx>
>>> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
>>> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
>>> Cc: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>
>>> Cc: Jan Kara <jack@xxxxxxx>
>>> Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
>>
>> Hi, Andrew,
>>
>> Just a heads-up that this version has not yet been updated in -mm I think,
>> while the queued one is the old version.
>>
>> IOW, uffd is currently broken on latest linux-next on hugetlb.
>
> Thanks Peter for reminding Andrew.
>
> Andrew, please acknowledge it would be queue for the next version and
> I will submit a patch to the man pages.
Peter (et. al),
I’ll send it in a more orderly fashion later, but let me know if I got
something completely wrong for the man page change:
[ Thanks as usual; sorry - limited experience changing man pages ]
-- >8 --
From: Nadav Amit <namit@xxxxxxxxxx>
Date: Thu, 3 Mar 2022 19:44:37 +0000
Subject: [PATCH] ioctl_userfaultfd: add UFFD_FEATURE_EXACT_ADDRESS
Describe the new UFFD_FEATURE_EXACT_ADDRESS API feature.
Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
---
man2/ioctl_userfaultfd.2 | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
index 504f61d4b..2d065504e 100644
--- a/man2/ioctl_userfaultfd.2
+++ b/man2/ioctl_userfaultfd.2
@@ -214,6 +214,12 @@ memory accesses to the regions registered with userfaultfd.
If this feature bit is set,
.I uffd_msg.pagefault.feat.ptid
will be set to the faulted thread ID for each page-fault message.
+.TP
+.BR UFFD_FEATURE_EXACT_ADDRESS " (since Linux 5.18)"
+If this feature bit is set,
+.I uffd_msg.pagefault.address
+will be set to the exact page-fault address that was reported by the hardware,
+and will not mask the offset within the page.
.PP
The returned
.I ioctls
--
2.25.1
