Re: Checking page_count(page) in invalidate_complete_page

[Chuck Lever <chuck.lever@oracle.com>]

Andrew Morton wrote:
> On Thu, 28 Sep 2006 12:26:36 -0400
> Chuck Lever <chuck.lever@oracle.com> wrote:
> 
>> Hi Nick-
>>
>> Nick Piggin wrote:
>>>> The page references I'm noting are:
>>>>
>>>> 1.  add_to_page_cache_lru  1 -> 2
>>>>
>>>> 2.  do_generic_mapping_read, released in nfs_do_filldir
>>>>
>>>> 3.  read_cache_page, released in nfs_readdir
>>>>
>>>> 4.  pagevec_lookup (from invalidate_inode_pages2_range)  2 -> 3
>>>>
>>>> The odd thing is some of the pages for the directory are released, so 
>>>> they must have a ref count of 2 *after* the pagevec_lookup.  Looks 
>>>> like more investigation is needed.
>>>
>>> Well they could get flushed out of the lru add pagevecs which drops 
>>> their count. Just to
>>> test that theory you could run lru_add_drain_all() at the start of the 
>>> invalidate function.
>>> That's not going to actually fix the problem, but it might help 2.6.18 
>>> limp along when
>>> combined with some other work.
>> I've verified that the pages that fail to be invalidated are indeed 
>> languishing in the per-CPU lru-add pagevec.  Adding a call to 
>> lru_add_drain_all() to the invalidate_inode_pages2() path does fix the 
>> NFS readdir problem.
>>
>> I think a call to lru_add_drain_all() belongs in both the 
>> invalidate_inode_pages() and the invalidate_inode_pages2() path.  Do you 
>> agree?
> 
> Yes.  But the page-pinned-by-vmscan scenario which you earlier identified
> can still happen, I believe.

Yes it can.  From what I gather from the conversation between you and 
Nick, that is a much harder problem to solve.  Given the debugging I've 
done, I don't think I was hitting that case, even though I still believe 
it is possible in the current code.

In this case, however, it seems like the non-empty LRU pagevecs will be 
a pretty common show-stopper for invalidate_inode_pages.  It will hit 
NFS directories and files quite a bit I would expect because the average 
size of these objects is only a few pages.

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