On 02.10.23 17:55, Lokesh Gidra wrote:
On Mon, Oct 2, 2023 at 4:46 PM Lokesh Gidra <lokeshgidra@xxxxxxxxxx> wrote:
On Mon, Oct 2, 2023 at 4:21 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
On Mon, Oct 02, 2023 at 10:00:03AM +0200, David Hildenbrand wrote:
In case we cannot simply remap the page, the fallback sequence (from the
cover letter) would be triggered.
1) UFFDIO_COPY
2) MADV_DONTNEED
So we would just handle the operation internally without a fallback.
Note that I think there will be a slight difference on whole remap
atomicity, on what happens if the page is modified after UFFDIO_COPY but
before DONTNEED.
UFFDIO_REMAP guarantees full atomicity when moving the page, IOW, threads
can be updating the pages when ioctl(UFFDIO_REMAP), data won't get lost
during movement, and it will generate a missing event after moved, with
latest data showing up on dest.
I'm not sure that means such a fallback is a problem, Suren may know
better with the use case.
Although there is no problem in using fallback with our use case but
as a user of userfaultfd, I'd suggest leaving it to the developer.
Failing with appropriate errno makes more sense. If handled in the
kernel, then the user may assume at the end of the operation that the
src vma is completely unmapped. And if not correctness issues, it
could lead to memory leaks.
I meant that in addition to the possibility of correctness issues due
to lack of atomicity, it could also lead to memory leaks, as the user
may assume that src vma is empty post-operation. IMHO, it's better to
fail with errno so that the user would fix the code with necessary
changes (like using DONTFORK, if forking).
Leaving the atomicity discussion out because I think this can just be
handled (e.g., the src_vma would always be empty post-operation):
It might not necessarily be a good idea to only expose micro-operations
to user space. If the user-space fallback will almost always be
"UFFDIO_COPY+MADV_DONTNEED", then clearly the logical operation
performed is moving data, ideally with zero-copy.
[as said as reply to Peter, one could still have magic flags for users
that really want to detect when zero-copy is impossible]
With a logical MOVE API users like compaction [as given in the cover
letter], not every such user has to eventually implement fallback paths.
But just my 2 cents, the UFFDIO_REMAP users probably can share what the
exact use cases are and if fallbacks are required at all or if no-KSM +
DONTFORK just does the trick.
--
Cheers,
David / dhildenb
