opening for write performs:
if (f->f_mode & FMODE_WRITE) {
[snip]
smp_mb();
if (filemap_nr_thps(inode->i_mapping)) {
[snip]
}
}
filemap_nr_thps on kernels built without CONFIG_READ_ONLY_THP_FOR
expands to 0, allowing the compiler to eliminate the entire thing, with
exception of the fence (and the branch leading there).
So happens required synchronisation between i_writecount and nr_thps
changes is already provided by the full fence coming from
get_write_access -> atomic_inc_unless_negative, thus the smp_mb instance
above can be removed regardless of CONFIG_READ_ONLY_THP_FOR.
While I updated commentary in places claiming to match the now-removed
fence, I did not try to patch them to act on the compile option.
I did not bother benchmarking it, not issuing a spurious full fence in
the fast path does not warrant justification from perf standpoint.
Signed-off-by: Mateusz Guzik <mjguzik@xxxxxxxxx>
---
v2:
- just whack the fence instead of ifdefing
- change To recipient, the person who committed the original change is
no longer active
fs/open.c | 9 ++++-----
mm/khugepaged.c | 10 +++++-----
2 files changed, 9 insertions(+), 10 deletions(-)
diff --git a/fs/open.c b/fs/open.c
index 28f2fcbebb1b..64976b6dc75f 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -986,12 +986,11 @@ static int do_dentry_open(struct file *f,
*/
if (f->f_mode & FMODE_WRITE) {
/*
- * Paired with smp_mb() in collapse_file() to ensure nr_thps
- * is up to date and the update to i_writecount by
- * get_write_access() is visible. Ensures subsequent insertion
- * of THPs into the page cache will fail.
+ * Depends on full fence from get_write_access() to synchronize
+ * against collapse_file() regarding i_writecount and nr_thps
+ * updates. Ensures subsequent insertion of THPs into the page
+ * cache will fail.
*/
- smp_mb();
if (filemap_nr_thps(inode->i_mapping)) {
struct address_space *mapping = inode->i_mapping;
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 409f67a817f1..2e017585f813 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1997,9 +1997,9 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
if (!is_shmem) {
filemap_nr_thps_inc(mapping);
/*
- * Paired with smp_mb() in do_dentry_open() to ensure
- * i_writecount is up to date and the update to nr_thps is
- * visible. Ensures the page cache will be truncated if the
+ * Paired with the fence in do_dentry_open() -> get_write_access()
+ * to ensure i_writecount is up to date and the update to nr_thps
+ * is visible. Ensures the page cache will be truncated if the
* file is opened writable.
*/
smp_mb();
@@ -2187,8 +2187,8 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
if (!is_shmem && result == SCAN_COPY_MC) {
filemap_nr_thps_dec(mapping);
/*
- * Paired with smp_mb() in do_dentry_open() to
- * ensure the update to nr_thps is visible.
+ * Paired with the fence in do_dentry_open() -> get_write_access()
+ * to ensure the update to nr_thps is visible.
*/
smp_mb();
}
--
2.43.0
