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SubjectRe: [PATCH 4/4] mm: swapoff: shmem_unuse() stop eviction without igrab()
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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@google.com>
> ---
>
> 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))
>

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