mm: move FOLL_* defs to mm_types.h

Move FOLL_* definitions to linux/mm_types.h to make them more accessible
without having to drag in all of linux/mm.h and everything that drags in
too[1].

Link: https://lkml.kernel.org/r/2161258.1671657894@warthog.procyon.org.uk
Signed-off-by: David Howells <dhowells@redhat.com>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 75 insertions, 0 deletions

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 9757067c3053..1118e381fcdc 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1085,4 +1085,79 @@ enum fault_flag {
 
 typedef unsigned int __bitwise zap_flags_t;
 
+/*
+ * FOLL_PIN and FOLL_LONGTERM may be used in various combinations with each
+ * other. Here is what they mean, and how to use them:
+ *
+ * FOLL_LONGTERM indicates that the page will be held for an indefinite time
+ * period _often_ under userspace control.  This is in contrast to
+ * iov_iter_get_pages(), whose usages are transient.
+ *
+ * FIXME: For pages which are part of a filesystem, mappings are subject to the
+ * lifetime enforced by the filesystem and we need guarantees that longterm
+ * users like RDMA and V4L2 only establish mappings which coordinate usage with
+ * the filesystem.  Ideas for this coordination include revoking the longterm
+ * pin, delaying writeback, bounce buffer page writeback, etc.  As FS DAX was
+ * added after the problem with filesystems was found FS DAX VMAs are
+ * specifically failed.  Filesystem pages are still subject to bugs and use of
+ * FOLL_LONGTERM should be avoided on those pages.
+ *
+ * FIXME: Also NOTE that FOLL_LONGTERM is not supported in every GUP call.
+ * Currently only get_user_pages() and get_user_pages_fast() support this flag
+ * and calls to get_user_pages_[un]locked are specifically not allowed.  This
+ * is due to an incompatibility with the FS DAX check and
+ * FAULT_FLAG_ALLOW_RETRY.
+ *
+ * In the CMA case: long term pins in a CMA region would unnecessarily fragment
+ * that region.  And so, CMA attempts to migrate the page before pinning, when
+ * FOLL_LONGTERM is specified.
+ *
+ * FOLL_PIN indicates that a special kind of tracking (not just page->_refcount,
+ * but an additional pin counting system) will be invoked. This is intended for
+ * anything that gets a page reference and then touches page data (for example,
+ * Direct IO). This lets the filesystem know that some non-file-system entity is
+ * potentially changing the pages' data. In contrast to FOLL_GET (whose pages
+ * are released via put_page()), FOLL_PIN pages must be released, ultimately, by
+ * a call to unpin_user_page().
+ *
+ * FOLL_PIN is similar to FOLL_GET: both of these pin pages. They use different
+ * and separate refcounting mechanisms, however, and that means that each has
+ * its own acquire and release mechanisms:
+ *
+ *     FOLL_GET: get_user_pages*() to acquire, and put_page() to release.
+ *
+ *     FOLL_PIN: pin_user_pages*() to acquire, and unpin_user_pages to release.
+ *
+ * FOLL_PIN and FOLL_GET are mutually exclusive for a given function call.
+ * (The underlying pages may experience both FOLL_GET-based and FOLL_PIN-based
+ * calls applied to them, and that's perfectly OK. This is a constraint on the
+ * callers, not on the pages.)
+ *
+ * FOLL_PIN should be set internally by the pin_user_pages*() APIs, never
+ * directly by the caller. That's in order to help avoid mismatches when
+ * releasing pages: get_user_pages*() pages must be released via put_page(),
+ * while pin_user_pages*() pages must be released via unpin_user_page().
+ *
+ * Please see Documentation/core-api/pin_user_pages.rst for more information.
+ */
+
+#define FOLL_WRITE	0x01	/* check pte is writable */
+#define FOLL_TOUCH	0x02	/* mark page accessed */
+#define FOLL_GET	0x04	/* do get_page on page */
+#define FOLL_DUMP	0x08	/* give error on hole if it would be zero */
+#define FOLL_FORCE	0x10	/* get_user_pages read/write w/o permission */
+#define FOLL_NOWAIT	0x20	/* if a disk transfer is needed, start the IO
+				 * and return without waiting upon it */
+#define FOLL_NOFAULT	0x80	/* do not fault in pages */
+#define FOLL_HWPOISON	0x100	/* check page is hwpoisoned */
+#define FOLL_TRIED	0x800	/* a retry, previous pass started an IO */
+#define FOLL_REMOTE	0x2000	/* we are working on non-current tsk/mm */
+#define FOLL_ANON	0x8000	/* don't do file mappings */
+#define FOLL_LONGTERM	0x10000	/* mapping lifetime is indefinite: see below */
+#define FOLL_SPLIT_PMD	0x20000	/* split huge pmd before returning */
+#define FOLL_PIN	0x40000	/* pages must be released via unpin_user_page */
+#define FOLL_FAST_ONLY	0x80000	/* gup_fast: prevent fall-back to slow gup */
+#define FOLL_PCI_P2PDMA	0x100000 /* allow returning PCI P2PDMA pages */
+#define FOLL_INTERRUPTIBLE  0x200000 /* allow interrupts from generic signals */
+
 #endif /* _LINUX_MM_TYPES_H */