On 20.01.23 18:55, David Howells wrote:
Add a function, iov_iter_extract_pages(), to extract a list of pages from
an iterator. The pages may be returned with a reference added or a pin
added or neither, depending on the type of iterator and the direction of
transfer. The caller must pass FOLL_READ_FROM_MEM or FOLL_WRITE_TO_MEM
as part of gup_flags to indicate how the iterator contents are to be used.
Add a second function, iov_iter_extract_mode(), to determine how the
cleanup should be done.
There are three cases:
(1) Transfer *into* an ITER_IOVEC or ITER_UBUF iterator.
Extracted pages will have pins obtained on them (but not references)
so that fork() doesn't CoW the pages incorrectly whilst the I/O is in
progress.
iov_iter_extract_mode() will return FOLL_PIN for this case. The
caller should use something like unpin_user_page() to dispose of the
page.
(2) Transfer is *out of* an ITER_IOVEC or ITER_UBUF iterator.
Extracted pages will have references obtained on them, but not pins.
iov_iter_extract_mode() will return FOLL_GET. The caller should use
something like put_page() for page disposal.
(3) Any other sort of iterator.
No refs or pins are obtained on the page, the assumption is made that
the caller will manage page retention. ITER_ALLOW_P2PDMA is not
permitted.
iov_iter_extract_mode() will return 0. The pages don't need
additional disposal.
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
cc: Christoph Hellwig <hch@xxxxxx>
cc: John Hubbard <jhubbard@xxxxxxxxxx>
cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
cc: linux-fsdevel@xxxxxxxxxxxxxxx
cc: linux-mm@xxxxxxxxx
Link: https://lore.kernel.org/r/166920903885.1461876.692029808682876184.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v2
Link: https://lore.kernel.org/r/166997421646.9475.14837976344157464997.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v3
Link: https://lore.kernel.org/r/167305163883.1521586.10777155475378874823.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v4
Link: https://lore.kernel.org/r/167344728530.2425628.9613910866466387722.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v5
Link: https://lore.kernel.org/r/167391053207.2311931.16398133457201442907.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v6
---
Notes:
ver #7)
- Switch to passing in iter-specific flags rather than FOLL_* flags.
- Drop the direction flags for now.
- Use ITER_ALLOW_P2PDMA to request FOLL_PCI_P2PDMA.
- Disallow use of ITER_ALLOW_P2PDMA with non-user-backed iter.
- Add support for extraction from KVEC-type iters.
- Use iov_iter_advance() rather than open-coding it.
- Make BVEC- and KVEC-type skip over initial empty vectors.
ver #6)
- Add back the function to indicate the cleanup mode.
- Drop the cleanup_mode return arg to iov_iter_extract_pages().
- Pass FOLL_SOURCE/DEST_BUF in gup_flags. Check this against the iter
data_source.
ver #4)
- Use ITER_SOURCE/DEST instead of WRITE/READ.
- Allow additional FOLL_* flags, such as FOLL_PCI_P2PDMA to be passed in.
ver #3)
- Switch to using EXPORT_SYMBOL_GPL to prevent indirect 3rd-party access
to get/pin_user_pages_fast()[1].
include/linux/uio.h | 28 +++
lib/iov_iter.c | 424 ++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 452 insertions(+)
diff --git a/include/linux/uio.h b/include/linux/uio.h
index 46d5080314c6..a4233049ab7a 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -363,4 +363,32 @@ static inline void iov_iter_ubuf(struct iov_iter *i, unsigned int direction,
/* Flags for iov_iter_get/extract_pages*() */
#define ITER_ALLOW_P2PDMA 0x01 /* Allow P2PDMA on the extracted pages */
+ssize_t iov_iter_extract_pages(struct iov_iter *i, struct page ***pages,
+ size_t maxsize, unsigned int maxpages,
+ unsigned int extract_flags, size_t *offset0);
+
+/**
+ * iov_iter_extract_mode - Indicate how pages from the iterator will be retained
+ * @iter: The iterator
+ * @extract_flags: How the iterator is to be used
+ *
+ * Examine the iterator and @extract_flags and indicate by returning FOLL_PIN,
+ * FOLL_GET or 0 as to how, if at all, pages extracted from the iterator will
+ * be retained by the extraction function.
+ *
+ * FOLL_GET indicates that the pages will have a reference taken on them that
+ * the caller must put. This can be done for DMA/async DIO write from a page.
+ *
+ * FOLL_PIN indicates that the pages will have a pin placed in them that the
+ * caller must unpin. This is must be done for DMA/async DIO read to a page to
+ * avoid CoW problems in fork.
+ *
+ * 0 indicates that no measures are taken and that it's up to the caller to
+ * retain the pages.
+ */
+#define iov_iter_extract_mode(iter, extract_flags) \
+ (user_backed_iter(iter) ? \
+ (iter->data_source == ITER_SOURCE) ? \
+ FOLL_GET : FOLL_PIN : 0)
+
How does this work align with the goal of no longer using FOLL_GET for
O_DIRECT? We should get rid of any FOLL_GET usage for accessing page
content.
@John, any comments?
--
Thanks,
David / dhildenb
