[RFC PATCH 06/13] iov_iter: support atomic copy_page_from_iter_iovec()

Nadav Amit
2020-11-29T00:51:35Z

copy_page_from_iter_iovec() cannot be used when preemption is enabled.

Change copy_page_from_iter_iovec() into __copy_page_from_iter_iovec()
with an additional parameter that says whether the caller runs in atomic
context. When __copy_page_from_iter_iovec() is used in an atomic context
it will gracefully fail but would not lead to a deadlock. The caller
is expected to recover from such failure gracefully.

Cc: Jens Axboe <axboe@kernel.dk>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: io-uring@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org
Signed-off-by: Nadav Amit <namit@vmware.com>
---
include/linux/uio.h | 3 +++
lib/iov_iter.c | 23 +++++++++++++++++------
2 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/include/linux/uio.h b/include/linux/uio.h
index 72d88566694e..7c90f7371a6f 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -121,6 +121,9 @@ size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
struct iov_iter *i);
size_t copy_page_from_iter(struct page *page, size_t offset, size_t bytes,
struct iov_iter *i);
+size_t __copy_page_from_iter(struct page *page, size_t offset, size_t bytes,
+ struct iov_iter *i, bool atomic);
+

size_t _copy_to_iter(const void *addr, size_t bytes, struct iov_iter *i);
size_t _copy_from_iter(void *addr, size_t bytes, struct iov_iter *i);
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 1635111c5bd2..e597df6a46a7 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -246,7 +246,7 @@ static size_t copy_page_to_iter_iovec(struct page *page, size_t offset, size_t b
}

static size_t copy_page_from_iter_iovec(struct page *page, size_t offset, size_t bytes,
- struct iov_iter *i)
+ struct iov_iter *i, bool atomic)
{
size_t skip, copy, left, wanted;
const struct iovec *iov;
@@ -259,14 +259,15 @@ static size_t copy_page_from_iter_iovec(struct page *page, size_t offset, size_t
if (unlikely(!bytes))
return 0;

- might_fault();
+ if (!atomic)
+ might_fault();
wanted = bytes;
iov = i->iov;
skip = i->iov_offset;
buf = iov->iov_base + skip;
copy = min(bytes, iov->iov_len - skip);

- if (IS_ENABLED(CONFIG_HIGHMEM) && !fault_in_pages_readable(buf, copy)) {
+ if (atomic || (IS_ENABLED(CONFIG_HIGHMEM) && !fault_in_pages_readable(buf, copy))) {
kaddr = kmap_atomic(page);
to = kaddr + offset;

@@ -295,6 +296,9 @@ static size_t copy_page_from_iter_iovec(struct page *page, size_t offset, size_t
buf += copy;
kunmap_atomic(kaddr);
copy = min(bytes, iov->iov_len - skip);
+ if (atomic)
+ goto done;
+
}
/* Too bad - revert to non-atomic kmap */

@@ -929,8 +933,8 @@ size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
}
EXPORT_SYMBOL(copy_page_to_iter);

-size_t copy_page_from_iter(struct page *page, size_t offset, size_t bytes,
- struct iov_iter *i)
+size_t __copy_page_from_iter(struct page *page, size_t offset, size_t bytes,
+ struct iov_iter *i, bool atomic)
{
if (unlikely(!page_copy_sane(page, offset, bytes)))
return 0;
@@ -944,7 +948,14 @@ size_t copy_page_from_iter(struct page *page, size_t offset, size_t bytes,
kunmap_atomic(kaddr);
return wanted;
} else
- return copy_page_from_iter_iovec(page, offset, bytes, i);
+ return copy_page_from_iter_iovec(page, offset, bytes, i, atomic);
+}
+EXPORT_SYMBOL(__copy_page_from_iter);
+
+size_t copy_page_from_iter(struct page *page, size_t offset, size_t bytes,
+ struct iov_iter *i)
+{
+ return __copy_page_from_iter(page, offset, bytes, i, false);
}
EXPORT_SYMBOL(copy_page_from_iter);

--
2.25.1