On 10/26/2012 05:48 AM, Hugh Dickins wrote:
On Thu, 25 Oct 2012, Johannes Weiner wrote:
On Wed, Oct 24, 2012 at 09:36:27PM -0700, Hugh Dickins wrote:
On Wed, 24 Oct 2012, Dave Jones wrote:
Machine under significant load (4gb memory used, swap usage fluctuating)
triggered this...
WARNING: at mm/shmem.c:1151 shmem_getpage_gfp+0xa5c/0xa70()
Pid: 29795, comm: trinity-child4 Not tainted 3.7.0-rc2+ #49
1148 error = shmem_add_to_page_cache(page, mapping, index,
1149 gfp, swp_to_radix_entry(swap));
1150 /* We already confirmed swap, and make no allocation */
1151 VM_BUG_ON(error);
1152 }
That's very surprising. Easy enough to handle an error there, but
of course I made it a VM_BUG_ON because it violates my assumptions:
I rather need to understand how this can be, and I've no idea.
Could it be concurrent truncation clearing out the entry between
shmem_confirm_swap() and shmem_add_to_page_cache()? I don't see
anything preventing that.
The empty slot would not match the expected swap entry this call
passes in and the returned error would be -ENOENT.
Excellent notion, many thanks Hannes, I believe you've got it.
I've hit that truncation problem in swapoff (and commented on it
in shmem_unuse_inode), but never hit it or considered it here.
I think of the page lock as holding it stable, but truncation's
free_swap_and_cache only does a trylock on the swapcache page,
so we're not secured against that possibility.
Hi Hugh,
Even though free_swap_and_cache only does a trylock on the swapcache
page, but it doens't call delete_from_swap_cache and the associated
entry should still be there, I am interested in what you have already
introduce to protect it?
So I'd like to change it to VM_BUG_ON(error && error != -ENOENT),
but there's a little tidying up to do in the -ENOENT case, which
Do you mean radix_tree_insert will return -ENOENT if the associated
entry is not present? Why I can't find this return value in the function
radix_tree_insert?
needs more thought. A delete_from_swap_cache(page) - though we
can be lazy and leave that to reclaim for such a rare occurrence -
and probably a mem_cgroup uncharge; but the memcg hooks are always
the hardest to get right, I'll have think about that one carefully.
Hugh
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