Re: [PATCH 4/4] mm: swapoff: shmem_unuse() stop eviction without igrab()
From: Konstantin Khlebnikov
Date: Tue Apr 09 2019 - 03:52:16 EST
On 08.04.2019 23:01, Hugh Dickins wrote:
The igrab() in shmem_unuse() looks good, but we forgot that it gives no
protection against concurrent unmounting: a point made by Konstantin
Khlebnikov eight years ago, and then fixed in 2.6.39 by 778dd893ae78
("tmpfs: fix race between umount and swapoff"). The current 5.1-rc
swapoff is liable to hit "VFS: Busy inodes after unmount of tmpfs.
Self-destruct in 5 seconds. Have a nice day..." followed by GPF.
Once again, give up on using igrab(); but don't go back to making such
heavy-handed use of shmem_swaplist_mutex as last time: that would spoil
the new design, and I expect could deadlock inside shmem_swapin_page().
Instead, shmem_unuse() just raise a "stop_eviction" count in the shmem-
specific inode, and shmem_evict_inode() wait for that to go down to 0.
Call it "stop_eviction" rather than "swapoff_busy" because it can be
put to use for others later (huge tmpfs patches expect to use it).
That simplifies shmem_unuse(), protecting it from both unlink and unmount;
and in practice lets it locate all the swap in its first try. But do not
rely on that: there's still a theoretical case, when shmem_writepage()
might have been preempted after its get_swap_page(), before making the
swap entry visible to swapoff.
Fixes: b56a2d8af914 ("mm: rid swapoff of quadratic complexity")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---
include/linux/shmem_fs.h | 1 +
mm/shmem.c | 39 ++++++++++++++++++---------------------
mm/swapfile.c | 11 +++++------
3 files changed, 24 insertions(+), 27 deletions(-)
--- 5.1-rc4/include/linux/shmem_fs.h 2019-03-17 16:18:15.181820820 -0700
+++ linux/include/linux/shmem_fs.h 2019-04-07 19:18:43.248639711 -0700
@@ -21,6 +21,7 @@ struct shmem_inode_info {
struct list_head swaplist; /* chain of maybes on swap */
struct shared_policy policy; /* NUMA memory alloc policy */
struct simple_xattrs xattrs; /* list of xattrs */
+ atomic_t stop_eviction; /* hold when working on inode */
struct inode vfs_inode;
};
--- 5.1-rc4/mm/shmem.c 2019-04-07 19:12:23.603858531 -0700
+++ linux/mm/shmem.c 2019-04-07 19:18:43.248639711 -0700
@@ -1081,9 +1081,15 @@ static void shmem_evict_inode(struct ino
}
spin_unlock(&sbinfo->shrinklist_lock);
}
- if (!list_empty(&info->swaplist)) {
+ while (!list_empty(&info->swaplist)) {
+ /* Wait while shmem_unuse() is scanning this inode... */
+ wait_var_event(&info->stop_eviction,
+ !atomic_read(&info->stop_eviction));
mutex_lock(&shmem_swaplist_mutex);
list_del_init(&info->swaplist);
Obviously, line above should be deleted.
+ /* ...but beware of the race if we peeked too early */
+ if (!atomic_read(&info->stop_eviction))
+ list_del_init(&info->swaplist);
mutex_unlock(&shmem_swaplist_mutex);
}
}
@@ -1227,36 +1233,27 @@ int shmem_unuse(unsigned int type, bool
unsigned long *fs_pages_to_unuse)
{
struct shmem_inode_info *info, *next;
- struct inode *inode;
- struct inode *prev_inode = NULL;
int error = 0;
if (list_empty(&shmem_swaplist))
return 0;
mutex_lock(&shmem_swaplist_mutex);
-
- /*
- * The extra refcount on the inode is necessary to safely dereference
- * p->next after re-acquiring the lock. New shmem inodes with swap
- * get added to the end of the list and we will scan them all.
- */
list_for_each_entry_safe(info, next, &shmem_swaplist, swaplist) {
if (!info->swapped) {
list_del_init(&info->swaplist);
continue;
}
-
- inode = igrab(&info->vfs_inode);
- if (!inode)
- continue;
-
+ /*
+ * Drop the swaplist mutex while searching the inode for swap;
+ * but before doing so, make sure shmem_evict_inode() will not
+ * remove placeholder inode from swaplist, nor let it be freed
+ * (igrab() would protect from unlink, but not from unmount).
+ */
+ atomic_inc(&info->stop_eviction);
mutex_unlock(&shmem_swaplist_mutex);
- if (prev_inode)
- iput(prev_inode);
- prev_inode = inode;
- error = shmem_unuse_inode(inode, type, frontswap,
+ error = shmem_unuse_inode(&info->vfs_inode, type, frontswap,
fs_pages_to_unuse);
cond_resched();
@@ -1264,14 +1261,13 @@ int shmem_unuse(unsigned int type, bool
next = list_next_entry(info, swaplist);
if (!info->swapped)
list_del_init(&info->swaplist);
+ if (atomic_dec_and_test(&info->stop_eviction))
+ wake_up_var(&info->stop_eviction);
if (error)
break;
}
mutex_unlock(&shmem_swaplist_mutex);
- if (prev_inode)
- iput(prev_inode);
-
return error;
}
@@ -2238,6 +2234,7 @@ static struct inode *shmem_get_inode(str
info = SHMEM_I(inode);
memset(info, 0, (char *)inode - (char *)info);
spin_lock_init(&info->lock);
+ atomic_set(&info->stop_eviction, 0);
info->seals = F_SEAL_SEAL;
info->flags = flags & VM_NORESERVE;
INIT_LIST_HEAD(&info->shrinklist);
--- 5.1-rc4/mm/swapfile.c 2019-04-07 19:17:13.291957539 -0700
+++ linux/mm/swapfile.c 2019-04-07 19:18:43.248639711 -0700
@@ -2116,12 +2116,11 @@ retry:
* Under global memory pressure, swap entries can be reinserted back
* into process space after the mmlist loop above passes over them.
*
- * Limit the number of retries? No: when shmem_unuse()'s igrab() fails,
- * a shmem inode using swap is being evicted; and when mmget_not_zero()
- * above fails, that mm is likely to be freeing swap from exit_mmap().
- * Both proceed at their own independent pace: we could move them to
- * separate lists, and wait for those lists to be emptied; but it's
- * easier and more robust (though cpu-intensive) just to keep retrying.
+ * Limit the number of retries? No: when mmget_not_zero() above fails,
+ * that mm is likely to be freeing swap from exit_mmap(), which proceeds
+ * at its own independent pace; and even shmem_writepage() could have
+ * been preempted after get_swap_page(), temporarily hiding that swap.
+ * It's easy and robust (though cpu-intensive) just to keep retrying.
*/
if (si->inuse_pages) {
if (!signal_pending(current))