Re: [PATCH -V4 -mm] mm, swap: Fix race between swapoff and some swap operations

[Mel Gorman <mgorman@suse.de>]

On Sat, Dec 23, 2017 at 10:36:53AM +0900, Minchan Kim wrote:
> > code path.  It appears that similar situation is possible for them too.
> > 
> > The file cache pages will be delete from file cache address_space before
> > address_space (embedded in inode) is freed.  But they will be deleted
> > from LRU list only when its refcount dropped to zero, please take a look
> > at put_page() and release_pages().  While address_space will be freed
> > after putting reference to all file cache pages.  If someone holds a
> > reference to a file cache page for quite long time, it is possible for a
> > file cache page to be in LRU list after the inode/address_space is
> > freed.
> > 
> > And I found inode/address_space is freed witch call_rcu().  I don't know
> > whether this is related to page_mapping().
> > 
> > This is just my understanding.
> 
> Hmm, it smells like a bug of __isolate_lru_page.
> 
> Ccing Mel:
> 
> What locks protects address_space destroying when race happens between
> inode trauncation and __isolate_lru_page?
> 

I'm just back online and have a lot of catching up to do so this is a rushed
answer and I didn't read the background of this. However the question is
somewhat ambiguous and the scope is broad as I'm not sure which race you
refer to. For file cache pages, I wouldnt' expect the address_space to be
destroyed specifically as long as the inode exists which is the structure
containing the address_space in this case. A page on the LRU being isolated
in __isolate_lru_page will have an elevated reference count which will
pin the inode until remove_mapping is called which holds the page lock
while inode truncation looking at a page for truncation also only checks
page_mapping under the page lock. Very broadly speaking, pages avoid being
added back to an inode being freed by checking the I_FREEING state.

Hopefully that helps while I go back to the TODO mountain.

-- 
Mel Gorman
SUSE Labs

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