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tmpfs: fix shmem_swaplist races

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Intensive swapoff testing shows shmem_unuse spinning on an entry in
shmem_swaplist pointing to itself: how does that come about?  Days pass...

First guess is this: shmem_delete_inode tests list_empty without taking the
global mutex (so the swapping case doesn't slow down the common case); but
there's an instant in shmem_unuse_inode's list_move_tail when the list entry
may appear empty (a rare case, because it's actually moving the head not the
the list member).  So there's a danger of leaving the inode on the swaplist
when it's freed, then reinitialized to point to itself when reused.  Fix that
by skipping the list_move_tail when it's a no-op, which happens to plug this.

But this same spinning then surfaces on another machine.  Ah, I'd never
suspected it, but shmem_writepage's swaplist manipulation is unsafe: though we
still hold page lock, which would hold off inode deletion if the page were in
pagecache, it doesn't hold off once it's in swapcache (free_swap_and_cache
doesn't wait on locked pages).  Hmm: we could put the the inode on swaplist
earlier, but then shmem_unuse_inode could never prune unswapped inodes.

Fix this with an igrab before dropping info->lock, as in shmem_unuse_inode;
though I am a little uneasy about the iput which has to follow - it works, and
I see nothing wrong with it, but it is surprising that shmem inode deletion
may now occur below shmem_writepage.  Revisit this fix later?

And while we're looking at these races: the way shmem_unuse tests swapped
without holding info->lock looks unsafe, if we've more than one swap area: a
racing shmem_writepage on another page of the same inode could be putting it
in swapcache, just as we're deciding to remove the inode from swaplist -
there's a danger of going on swap without being listed, so a later swapoff
would hang, being unable to locate the entry.  Move that test and removal down
into shmem_unuse_inode, once info->lock is held.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Hugh Dickins 2008-02-04 22:28:55 -08:00 committed by Linus Torvalds
parent b409f9fcf0
commit 1b1b32f2c6
1 changed files with 25 additions and 12 deletions
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37
mm/shmem.c
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@ -833,6 +833,10 @@ static int shmem_unuse_inode(struct shmem_inode_info *info, swp_entry_t entry, s
idx = 0;
ptr = info->i_direct;
spin_lock(&info->lock);
if (!info->swapped) {
list_del_init(&info->swaplist);
goto lost2;
}
limit = info->next_index;
size = limit;
if (size > SHMEM_NR_DIRECT)
@ -894,8 +898,15 @@ found:
inode = igrab(&info->vfs_inode);
spin_unlock(&info->lock);
/* move head to start search for next from here */
list_move_tail(&shmem_swaplist, &info->swaplist);
/*
* Move _head_ to start search for next from here.
* But be careful: shmem_delete_inode checks list_empty without taking
* mutex, and there's an instant in list_move_tail when info->swaplist
* would appear empty, if it were the only one on shmem_swaplist. We
* could avoid doing it if inode NULL; or use this minor optimization.
*/
if (shmem_swaplist.next != &info->swaplist)
list_move_tail(&shmem_swaplist, &info->swaplist);
mutex_unlock(&shmem_swaplist_mutex);
error = 1;
@ -955,10 +966,7 @@ int shmem_unuse(swp_entry_t entry, struct page *page)
mutex_lock(&shmem_swaplist_mutex);
list_for_each_safe(p, next, &shmem_swaplist) {
info = list_entry(p, struct shmem_inode_info, swaplist);
if (info->swapped)
found = shmem_unuse_inode(info, entry, page);
else
list_del_init(&info->swaplist);
found = shmem_unuse_inode(info, entry, page);
cond_resched();
if (found)
goto out;
@ -1021,18 +1029,23 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
remove_from_page_cache(page);
shmem_swp_set(info, entry, swap.val);
shmem_swp_unmap(entry);
if (list_empty(&info->swaplist))
inode = igrab(inode);
else
inode = NULL;
spin_unlock(&info->lock);
if (list_empty(&info->swaplist)) {
mutex_lock(&shmem_swaplist_mutex);
/* move instead of add in case we're racing */
list_move_tail(&info->swaplist, &shmem_swaplist);
mutex_unlock(&shmem_swaplist_mutex);
}
swap_duplicate(swap);
BUG_ON(page_mapped(page));
page_cache_release(page); /* pagecache ref */
set_page_dirty(page);
unlock_page(page);
if (inode) {
mutex_lock(&shmem_swaplist_mutex);
/* move instead of add in case we're racing */
list_move_tail(&info->swaplist, &shmem_swaplist);
mutex_unlock(&shmem_swaplist_mutex);
iput(inode);
}
return 0;
}