[PATCH v3 4/4] mm/gup: remove get_user_pages_locked()

John Hubbard
2022-02-03T09:33:21Z

There is only one caller of get_user_pages_locked(). The purpose of
get_user_pages_locked() is to allow for unlocking the mmap_lock when
reading a page from the disk during a page fault (hidden behind
VM_FAULT_RETRY). The idea is to reduce contention on the heavily-used
mmap_lock. (Thanks to Jan Kara for clearly pointing that out, and in
fact I've used some of his wording here.)

However, it is unlikely for lookup_node() to take a page fault. With
that in mind, change over to calling get_user_pages_fast(). This
simplifies the code, runs a little faster in the expected case, and
allows removing get_user_pages_locked() entirely.

Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
include/linux/mm.h | 2 --
mm/gup.c | 59 ----------------------------------------------
mm/mempolicy.c | 21 +++++++----------
3 files changed, 9 insertions(+), 73 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 80c540c17d83..528ef1cb4f3a 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1916,8 +1916,6 @@ long get_user_pages(unsigned long start, unsigned long nr_pages,
long pin_user_pages(unsigned long start, unsigned long nr_pages,
unsigned int gup_flags, struct page **pages,
struct vm_area_struct **vmas);
-long get_user_pages_locked(unsigned long start, unsigned long nr_pages,
- unsigned int gup_flags, struct page **pages, int *locked);
long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
struct page **pages, unsigned int gup_flags);
long pin_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
diff --git a/mm/gup.c b/mm/gup.c
index b0ecbfe03928..7da49df59110 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2118,65 +2118,6 @@ long get_user_pages(unsigned long start, unsigned long nr_pages,
}
EXPORT_SYMBOL(get_user_pages);

-/**
- * get_user_pages_locked() - variant of get_user_pages()
- *
- * @start: starting user address
- * @nr_pages: number of pages from start to pin
- * @gup_flags: flags modifying lookup behaviour
- * @pages: array that receives pointers to the pages pinned.
- * Should be at least nr_pages long. Or NULL, if caller
- * only intends to ensure the pages are faulted in.
- * @locked: pointer to lock flag indicating whether lock is held and
- * subsequently whether VM_FAULT_RETRY functionality can be
- * utilised. Lock must initially be held.
- *
- * It is suitable to replace the form:
- *
- * mmap_read_lock(mm);
- * do_something()
- * get_user_pages(mm, ..., pages, NULL);
- * mmap_read_unlock(mm);
- *
- * to:
- *
- * int locked = 1;
- * mmap_read_lock(mm);
- * do_something()
- * get_user_pages_locked(mm, ..., pages, &locked);
- * if (locked)
- * mmap_read_unlock(mm);
- *
- * We can leverage the VM_FAULT_RETRY functionality in the page fault
- * paths better by using either get_user_pages_locked() or
- * get_user_pages_unlocked().
- *
- */
-long get_user_pages_locked(unsigned long start, unsigned long nr_pages,
- unsigned int gup_flags, struct page **pages,
- int *locked)
-{
- /*
- * FIXME: Current FOLL_LONGTERM behavior is incompatible with
- * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on
- * vmas. As there are no users of this flag in this call we simply
- * disallow this option for now.
- */
- if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM))
- return -EINVAL;
- /*
- * FOLL_PIN must only be set internally by the pin_user_pages*() APIs,
- * never directly by the caller, so enforce that:
- */
- if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
- return -EINVAL;
-
- return __get_user_pages_locked(current->mm, start, nr_pages,
- pages, NULL, locked,
- gup_flags | FOLL_TOUCH);
-}
-EXPORT_SYMBOL(get_user_pages_locked);
-
/*
* get_user_pages_unlocked() is suitable to replace the form:
*
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 028e8dd82b44..3f8dc58da3e8 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -907,17 +907,14 @@ static void get_policy_nodemask(struct mempolicy *p, nodemask_t *nodes)
static int lookup_node(struct mm_struct *mm, unsigned long addr)
{
struct page *p = NULL;
- int err;
+ int ret;

- int locked = 1;
- err = get_user_pages_locked(addr & PAGE_MASK, 1, 0, &p, &locked);
- if (err > 0) {
- err = page_to_nid(p);
+ ret = get_user_pages_fast(addr & PAGE_MASK, 1, 0, &p);
+ if (ret > 0) {
+ ret = page_to_nid(p);
put_page(p);
}
- if (locked)
- mmap_read_unlock(mm);
- return err;
+ return ret;
}

/* Retrieve NUMA policy */
@@ -968,14 +965,14 @@ static long do_get_mempolicy(int *policy, nodemask_t *nmask,
if (flags & MPOL_F_NODE) {
if (flags & MPOL_F_ADDR) {
/*
- * Take a refcount on the mpol, lookup_node()
- * will drop the mmap_lock, so after calling
- * lookup_node() only "pol" remains valid, "vma"
- * is stale.
+ * Take a refcount on the mpol, because we are about to
+ * drop the mmap_lock, after which only "pol" remains
+ * valid, "vma" is stale.
*/
pol_refcount = pol;
vma = NULL;
mpol_get(pol);
+ mmap_read_unlock(mm);
err = lookup_node(mm, addr);
if (err < 0)
goto out;
--
2.35.1