Under shmem swapping and swapoff load, I sometimes hit the
VM_BUG_ON_PAGE(!page->mapping) in mem_cgroup_commit_charge() at
mm/memcontrol.c:6502! Each time it has been a call from shmem_unuse().
Yes, there are some cases (most commonly when the page being unswapped
is in a file being unlinked and evicted at that time) when the charge
should not be committed. In the old scheme, the page got uncharged
again on release; but in the new scheme, it hits that BUG beforehand.
It's a useful BUG, so adapt shmem_unuse() to allow for it. Which needs
more info from shmem_unuse_inode(): so abuse -EAGAIN internally to
replace the previous !found state (-ENOENT would be a more natural
code, but that's exactly what you get when the swap has been evicted).
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---
mm/shmem.c | 14 +++++++-------
1 file changed, 7 insertions(+), 7 deletions(-)
--- 3.16-rc2-mm1/mm/shmem.c 2014-06-25 18:43:59.868588121 -0700
+++ linux/mm/shmem.c 2014-06-30 15:05:50.736335600 -0700
@@ -611,7 +611,7 @@ static int shmem_unuse_inode(struct shme
radswap = swp_to_radix_entry(swap);
index = radix_tree_locate_item(&mapping->page_tree, radswap);
if (index == -1)
- return 0;
+ return -EAGAIN;
/*
* Move _head_ to start search for next from here.
@@ -670,7 +670,6 @@ static int shmem_unuse_inode(struct shme
spin_unlock(&info->lock);
swap_free(swap);
}
- error = 1; /* not an error, but entry was found */
}
return error;
}
@@ -683,7 +682,6 @@ int shmem_unuse(swp_entry_t swap, struct
struct list_head *this, *next;
struct shmem_inode_info *info;
struct mem_cgroup *memcg;
- int found = 0;
int error = 0;
/*
@@ -702,22 +700,24 @@ int shmem_unuse(swp_entry_t swap, struct
if (error)
goto out;
/* No radix_tree_preload: swap entry keeps a place for page in tree */
+ error = -EAGAIN;
mutex_lock(&shmem_swaplist_mutex);
list_for_each_safe(this, next, &shmem_swaplist) {
info = list_entry(this, struct shmem_inode_info, swaplist);
if (info->swapped)
- found = shmem_unuse_inode(info, swap, &page);
+ error = shmem_unuse_inode(info, swap, &page);
else
list_del_init(&info->swaplist);
cond_resched();
- if (found)
+ if (error != -EAGAIN)
break;
}
mutex_unlock(&shmem_swaplist_mutex);
- if (found < 0) {
- error = found;
+ if (error) {
+ if (error != -ENOMEM)
+ error = 0;
mem_cgroup_cancel_charge(page, memcg);
} else
mem_cgroup_commit_charge(page, memcg, true);
--
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