Re: shmem_getpage_gfp VM_BUG_ON triggered. [3.7rc2]
From: Hugh Dickins <hughd@google.com>
Date: 2012-10-25 21:48:40
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On Thu, 25 Oct 2012, Johannes Weiner wrote:
On Wed, Oct 24, 2012 at 09:36:27PM -0700, Hugh Dickins wrote:quoted
On Wed, 24 Oct 2012, Dave Jones wrote:quoted
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. 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 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>