Re: [PATCH] mm: khugepaged: fix potential page state corruption

[Yang Shi]

On 3/20/20 4:45 AM, Kirill A. Shutemov wrote:
> On Thu, Mar 19, 2020 at 09:57:47AM -0700, Yang Shi wrote:
> > On 3/19/20 3:49 AM, Kirill A. Shutemov wrote:
> > > On Wed, Mar 18, 2020 at 10:39:21PM -0700, Yang Shi wrote:
> > > > On 3/18/20 5:55 PM, Yang Shi wrote:
> > > > > On 3/18/20 5:12 PM, Kirill A. Shutemov wrote:
> > > > > > On Thu, Mar 19, 2020 at 07:19:42AM +0800, Yang Shi wrote:
> > > > > > > When khugepaged collapses anonymous pages, the base pages would
> > > > > > > be freed
> > > > > > > via pagevec or free_page_and_swap_cache(). But, the anonymous page may
> > > > > > > be added back to LRU, then it might result in the below race:
> > > > > > >
> > > > > > >   ÂÂÂÂCPU AÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ CPU B
> > > > > > > khugepaged:
> > > > > > >   ÂÂ unlock page
> > > > > > >   ÂÂ putback_lru_page
> > > > > > >   ÂÂÂÂ add to lru
> > > > > > >   ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ page reclaim:
> > > > > > >   ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ isolate this page
> > > > > > >   ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ try_to_unmap
> > > > > > >   ÂÂ page_remove_rmap <-- corrupt _mapcount
> > > > > > >
> > > > > > > It looks nothing would prevent the pages from isolating by reclaimer.
> > > > > > Hm. Why should it?
> > > > > >
> > > > > > try_to_unmap() doesn't exclude parallel page unmapping. _mapcount is
> > > > > > protected by ptl. And this particular _mapcount pin is reachable for
> > > > > > reclaim as it's not part of usual page table tree. Basically
> > > > > > try_to_unmap() will never succeeds until we give up the _mapcount on
> > > > > > khugepaged side.
> > > > > I don't quite get. What does "not part of usual page table tree" means?
> > > > >
> > > > > How's about try_to_unmap() acquires ptl before khugepaged?
> > > The page table we are dealing with was detached from the process' page
> > > table tree: see pmdp_collapse_flush(). try_to_unmap() will not see the
> > > pte.
> > >
> > > try_to_unmap() can only reach the ptl if split ptl is disabled
> > > (mm->page_table_lock is used), but it still will not be able to reach pte.
> > Aha, got it. Thanks for explaining. I definitely missed this point. Yes,
> > pmdp_collapse_flush() would clear the pmd, then others won't see the page
> > table.
> >
> > However, it looks the vmscan would not stop at try_to_unmap() at all,
> > try_to_unmap() would just return true since pmd_present() should return
> > false in pvmw. Then it would go all the way down to __remove_mapping(), but
> > freezing the page would fail since try_to_unmap() doesn't actually drop the
> > refcount from the pte map.
> No. try_to_unmap() checks mapcount at the end and only returns true if
> it's zero.

Aha, yes, you are right. It does check mapcount. It would not go that far.

> > It would not result in any critical problem AFAICT, but suboptimal and it
> > may causes some unnecessary I/O due to swap.
> >
> > > > > > I don't see the issue right away.
> > > > > >
> > > > > > > The other problem is the page's active or unevictable flag might be
> > > > > > > still set when freeing the page via free_page_and_swap_cache().
> > > > > > So what?
> > > > > The flags may leak to page free path then kernel may complain if
> > > > > DEBUG_VM is set.
> > > Could you elaborate on what codepath you are talking about?
> > __put_page ->
> >  ÂÂÂ __put_single_page ->
> >  ÂÂÂ ÂÂÂ free_unref_page ->
> >  ÂÂÂ ÂÂÂ ÂÂÂ put_unref_page_prepare ->
> >  ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ free_pcp_prepare ->
> >  ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ free_pages_prepare ->
> >  ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ free_pages_check
> >
> > This check would just be run when DEBUG_VM is enabled.
> I'm not 100% sure, but I belive these flags will ge cleared on adding into
> lru:
>
>    release_pte_page()
>      putback_lru_page()
>        lru_cache_add()
>         __lru_cache_add()
>           __pagevec_lru_add()
> 	   __pagevec_lru_add_fn()
> 	     __pagevec_lru_add_fn()

No, adding into lru would not clear the flags. But, I finally found 
where they get cleared. They get cleared by:

__put_single_page() ->
ÂÂÂ __page_cache_release() ->
ÂÂÂ ÂÂÂ if page is lru
ÂÂÂ ÂÂÂ del_page_from_lru_list(page, lruvec, page_off_lru(page));

page_off_lru() would clear active and unevictable flags.

The name (__page_cache_release) sounds a little bit confusing I 
misunderstood it just would done something for page cache.

So, it looks the code does depend on putting page back to lru to release 
it. Nothing is wrong, just a little bit unproductive IMHO. Sorry for the 
rash patch. And thank you for your time again.

>