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SubjectRe: [PATCH -V4 -mm] mm, swap: Fix race between swapoff and some swap operations
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Mel Gorman <mgorman@techsingularity.net> writes:

> On Tue, Jan 02, 2018 at 12:29:55PM +0100, Jan Kara wrote:
>> On Tue 02-01-18 10:21:03, Mel Gorman wrote:
>> > 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.
>>
>> So I'm wondering what prevents the following:
>>
>> CPU1 CPU2
>>
>> truncate(inode) __isolate_lru_page()
>> ...
>> truncate_inode_page(mapping, page);
>> delete_from_page_cache(page)
>> spin_lock_irqsave(&mapping->tree_lock, flags);
>> __delete_from_page_cache(page, NULL)
>> page_cache_tree_delete(..)
>> ... mapping = page_mapping(page);
>> page->mapping = NULL;
>> ...
>> spin_unlock_irqrestore(&mapping->tree_lock, flags);
>> page_cache_free_page(mapping, page)
>> put_page(page)
>> if (put_page_testzero(page)) -> false
>> - inode now has no pages and can be freed including embedded address_space
>>
>> if (mapping && !mapping->a_ops->migratepage)
>> - we've dereferenced mapping which is potentially already free.
>>
>
> Hmm, possible if unlikely.
>
> Before delete_from_page_cache, we called truncate_cleanup_page so the
> page is likely to be !PageDirty or PageWriteback which gets skipped by
> the only caller that checks the mappping in __isolate_lru_page. The race
> is tiny but it does exist. One way of closing it is to check the mapping
> under the page lock which will prevent races with truncation. The
> overhead is minimal as the calling context (compaction) is quite a heavy
> operation anyway.
>

I think another possible fix is to use call_rcu_sched() to free inode
(and address_space). Because __isolate_lru_page() will be called with
LRU spinlock held and IRQ disabled, call_rcu_sched() will wait
LRU spin_unlock and IRQ enabled.

Best Regards,
Huang, Ying

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