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Subject[128/129] SHM_UNLOCK: fix long unpreemptible section
3.2-stable review patch.  If anyone has any objections, please let me know.

------------------
Content-Length: 5974
Lines: 176
From: Hugh Dickins <hughd@google.com>

commit 85046579bde15e532983438f86b36856e358f417 upstream.

scan_mapping_unevictable_pages() is used to make SysV SHM_LOCKed pages
evictable again once the shared memory is unlocked. It does this with
pagevec_lookup()s across the whole object (which might occupy most of
memory), and takes 300ms to unlock 7GB here. A cond_resched() every
PAGEVEC_SIZE pages would be good.

However, KOSAKI-san points out that this is called under shmem.c's
info->lock, and it's also under shm.c's shm_lock(), both spinlocks.
There is no strong reason for that: we need to take these pages off the
unevictable list soonish, but those locks are not required for it.

So move the call to scan_mapping_unevictable_pages() from shmem.c's
unlock handling up to shm.c's unlock handling. Remove the recently
added barrier, not needed now we have spin_unlock() before the scan.

Use get_file(), with subsequent fput(), to make sure we have a reference
to mapping throughout scan_mapping_unevictable_pages(): that's something
that was previously guaranteed by the shm_lock().

Remove shmctl's lru_add_drain_all(): we don't fault in pages at SHM_LOCK
time, and we lazily discover them to be Unevictable later, so it serves
no purpose for SHM_LOCK; and serves no purpose for SHM_UNLOCK, since
pages still on pagevec are not marked Unevictable.

The original code avoided redundant rescans by checking VM_LOCKED flag
at its level: now avoid them by checking shp's SHM_LOCKED.

The original code called scan_mapping_unevictable_pages() on a locked
area at shm_destroy() time: perhaps we once had accounting cross-checks
which required that, but not now, so skip the overhead and just let
inode eviction deal with them.

Put check_move_unevictable_page() and scan_mapping_unevictable_pages()
under CONFIG_SHMEM (with stub for the TINY case when ramfs is used),
more as comment than to save space; comment them used for SHM_UNLOCK.

Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Minchan Kim <minchan.kim@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Shaohua Li <shaohua.li@intel.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michel Lespinasse <walken@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
ipc/shm.c | 37 ++++++++++++++++++++++---------------
mm/shmem.c | 7 -------
mm/vmscan.c | 12 +++++++++++-
3 files changed, 33 insertions(+), 23 deletions(-)
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -870,9 +870,7 @@ SYSCALL_DEFINE3(shmctl, int, shmid, int,
case SHM_LOCK:
case SHM_UNLOCK:
{
- struct file *uninitialized_var(shm_file);
-
- lru_add_drain_all(); /* drain pagevecs to lru lists */
+ struct file *shm_file;

shp = shm_lock_check(ns, shmid);
if (IS_ERR(shp)) {
@@ -895,22 +893,31 @@ SYSCALL_DEFINE3(shmctl, int, shmid, int,
err = security_shm_shmctl(shp, cmd);
if (err)
goto out_unlock;
-
- if(cmd==SHM_LOCK) {
+
+ shm_file = shp->shm_file;
+ if (is_file_hugepages(shm_file))
+ goto out_unlock;
+
+ if (cmd == SHM_LOCK) {
struct user_struct *user = current_user();
- if (!is_file_hugepages(shp->shm_file)) {
- err = shmem_lock(shp->shm_file, 1, user);
- if (!err && !(shp->shm_perm.mode & SHM_LOCKED)){
- shp->shm_perm.mode |= SHM_LOCKED;
- shp->mlock_user = user;
- }
+ err = shmem_lock(shm_file, 1, user);
+ if (!err && !(shp->shm_perm.mode & SHM_LOCKED)) {
+ shp->shm_perm.mode |= SHM_LOCKED;
+ shp->mlock_user = user;
}
- } else if (!is_file_hugepages(shp->shm_file)) {
- shmem_lock(shp->shm_file, 0, shp->mlock_user);
- shp->shm_perm.mode &= ~SHM_LOCKED;
- shp->mlock_user = NULL;
+ goto out_unlock;
}
+
+ /* SHM_UNLOCK */
+ if (!(shp->shm_perm.mode & SHM_LOCKED))
+ goto out_unlock;
+ shmem_lock(shm_file, 0, shp->mlock_user);
+ shp->shm_perm.mode &= ~SHM_LOCKED;
+ shp->mlock_user = NULL;
+ get_file(shm_file);
shm_unlock(shp);
+ scan_mapping_unevictable_pages(shm_file->f_mapping);
+ fput(shm_file);
goto out;
}
case IPC_RMID:
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1068,13 +1068,6 @@ int shmem_lock(struct file *file, int lo
user_shm_unlock(inode->i_size, user);
info->flags &= ~VM_LOCKED;
mapping_clear_unevictable(file->f_mapping);
- /*
- * Ensure that a racing putback_lru_page() can see
- * the pages of this mapping are evictable when we
- * skip them due to !PageLRU during the scan.
- */
- smp_mb__after_clear_bit();
- scan_mapping_unevictable_pages(file->f_mapping);
}
retval = 0;

--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3353,6 +3353,7 @@ int page_evictable(struct page *page, st
return 1;
}

+#ifdef CONFIG_SHMEM
/**
* check_move_unevictable_page - check page for evictability and move to appropriate zone lru list
* @page: page to check evictability and move to appropriate lru list
@@ -3363,6 +3364,8 @@ int page_evictable(struct page *page, st
*
* Restrictions: zone->lru_lock must be held, page must be on LRU and must
* have PageUnevictable set.
+ *
+ * This function is only used for SysV IPC SHM_UNLOCK.
*/
static void check_move_unevictable_page(struct page *page, struct zone *zone)
{
@@ -3396,6 +3399,8 @@ retry:
*
* Scan all pages in mapping. Check unevictable pages for
* evictability and move them to the appropriate zone lru list.
+ *
+ * This function is only used for SysV IPC SHM_UNLOCK.
*/
void scan_mapping_unevictable_pages(struct address_space *mapping)
{
@@ -3441,9 +3446,14 @@ void scan_mapping_unevictable_pages(stru
pagevec_release(&pvec);

count_vm_events(UNEVICTABLE_PGSCANNED, pg_scanned);
+ cond_resched();
}
-
}
+#else
+void scan_mapping_unevictable_pages(struct address_space *mapping)
+{
+}
+#endif /* CONFIG_SHMEM */

static void warn_scan_unevictable_pages(void)
{



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