Re: [RFC PATCH 1/4] mm: Move FOLL_* defs to mm_types.h
From: John Hubbard
Date: Thu Nov 17 2022 - 18:16:04 EST
On 11/17/22 06:54, David Howells wrote:
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].
Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
cc: John Hubbard <jhubbard@xxxxxxxxxx>
cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
cc: linux-mm@xxxxxxxxx
cc: linux-fsdevel@xxxxxxxxxxxxxxx
Link: https://lore.kernel.org/linux-fsdevel/Y1%2FhSO+7kAJhGShG@xxxxxxxxxxxxxxxxxxxx/ [1]
---
include/linux/mm.h | 74 ----------------------------------------------
include/linux/mm_types.h | 73 +++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 73 insertions(+), 74 deletions(-)
OK, I've verified that this is a "mostly identical" movement: the only
thing that changes is that the comments now come before the defines.
And because mm.h includes mm_types.h, it is unlikely that moving a
define from mm.h to mm_types.h would cause build failures. It's not
completely impossible: ordering issues are sometimes involved in this
sort of change. But unlikely.
Anyway, this is a good move. The users of various mm APIs should not
have to pull in quite so much of the internals of mm, and this is a step
in that direction. FOLL_* items are used by filesystems and other
subsystems that definitely do not need all of mm.h.
Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
thanks,
--
John Hubbard
NVIDIA
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8bbcccbc5565..7a7a287818ad 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2941,80 +2941,6 @@ static inline vm_fault_t vmf_error(int err)
struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
unsigned int foll_flags);
-#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_MIGRATION 0x400 /* wait for page to replace migration entry */
-#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 */
-
-/*
- * 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.
- */
-
static inline int vm_fault_to_errno(vm_fault_t vm_fault, int foll_flags)
{
if (vm_fault & VM_FAULT_OOM)
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 500e536796ca..0c80a5ad6e6a 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1003,4 +1003,77 @@ 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_MIGRATION 0x400 /* wait for page to replace migration entry */
+#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 */
+
#endif /* _LINUX_MM_TYPES_H */