To avoid bugs in userspace, we require that userspace provide
UFFD_FEATURE_EXACT_ADDRESS when using UFFD_FEATURE_MINOR_HUGETLBFS_HGM,
otherwise UFFDIO_API will fail with EINVAL.
The potential confusion is this: without EXACT_ADDRESS, the address
given in the userfaultfd message will be rounded down to the hugepage
size. Userspace may think that, because they're using HGM, just
UFFDIO_CONTINUE the interval [address, address+PAGE_SIZE), but for
faults that didn't occur in the first base page of the hugepage, this
won't resolve the fault. The only choice it has in this scenario is to
UFFDIO_CONTINUE the interval [address, address+hugepage_size), which
negates the purpose of using HGM in the first place.
By requiring userspace to provide UFFD_FEATURE_EXACT_ADDRESS, there is
no rounding, and userspace now has the information it needs to
appropriately resolve the fault.
Another potential solution here is to change the behavior when
UFFD_FEATURE_EXACT_ADDRESS is not provided: when HGM is enabled, start
rounding to PAGE_SIZE instead of to the hugepage size. I think requiring
UFFD_FEATURE_EXACT_ADDRESS is cleaner.
Signed-off-by: James Houghton <[email protected]>
---
fs/userfaultfd.c | 11 +++++++++++
1 file changed, 11 insertions(+)
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 0204108e3882..c8f21f53e37d 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1990,6 +1990,17 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
~(UFFD_FEATURE_MINOR_HUGETLBFS | UFFD_FEATURE_MINOR_SHMEM);
#ifndef CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING
uffdio_api.features &= ~UFFD_FEATURE_MINOR_HUGETLBFS_HGM;
+#else
+
+ ret = -EINVAL;
+ if ((uffdio_api.features & UFFD_FEATURE_MINOR_HUGETLBFS_HGM) &&
+ !(uffdio_api.features & UFFD_FEATURE_EXACT_ADDRESS))
+ /*
+ * UFFD_FEATURE_MINOR_HUGETLBFS_HGM is mostly
+ * useless without UFFD_FEATURE_EXACT_ADDRESS,
+ * so require userspace to provide both.
+ */
+ goto err_out;
#endif /* CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING */
#endif /* CONFIG_HAVE_ARCH_USERFAULTFD_MINOR */
--
2.38.0.135.g90850a2211-goog
On Fri, Oct 21, 2022 at 04:36:51PM +0000, James Houghton wrote:
> @@ -1990,6 +1990,17 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
> ~(UFFD_FEATURE_MINOR_HUGETLBFS | UFFD_FEATURE_MINOR_SHMEM);
> #ifndef CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING
> uffdio_api.features &= ~UFFD_FEATURE_MINOR_HUGETLBFS_HGM;
> +#else
> +
> + ret = -EINVAL;
> + if ((uffdio_api.features & UFFD_FEATURE_MINOR_HUGETLBFS_HGM) &&
> + !(uffdio_api.features & UFFD_FEATURE_EXACT_ADDRESS))
This check needs to be done upon "features" or "ctx_features", rather than
"uffdio_api.features". The latter is the one we'll report to the user only.
> + /*
> + * UFFD_FEATURE_MINOR_HUGETLBFS_HGM is mostly
> + * useless without UFFD_FEATURE_EXACT_ADDRESS,
> + * so require userspace to provide both.
> + */
> + goto err_out;
> #endif /* CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING */
> #endif /* CONFIG_HAVE_ARCH_USERFAULTFD_MINOR */
>
> --
> 2.38.0.135.g90850a2211-goog
>
>
--
Peter Xu
On Thu, Dec 22, 2022 at 4:47 PM Peter Xu <[email protected]> wrote:
>
> On Fri, Oct 21, 2022 at 04:36:51PM +0000, James Houghton wrote:
> > @@ -1990,6 +1990,17 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
> > ~(UFFD_FEATURE_MINOR_HUGETLBFS | UFFD_FEATURE_MINOR_SHMEM);
> > #ifndef CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING
> > uffdio_api.features &= ~UFFD_FEATURE_MINOR_HUGETLBFS_HGM;
> > +#else
> > +
> > + ret = -EINVAL;
> > + if ((uffdio_api.features & UFFD_FEATURE_MINOR_HUGETLBFS_HGM) &&
> > + !(uffdio_api.features & UFFD_FEATURE_EXACT_ADDRESS))
>
> This check needs to be done upon "features" or "ctx_features", rather than
> "uffdio_api.features". The latter is the one we'll report to the user only.
Ack, thanks Peter. I'm going to drop this patch given the API change
(switching to MADV_SPLIT).
- James