Re: summarize all information again at bottom//reply: reply: [PATCH] mm: fix a race scenario in folio_isolate_lru

[Zhaoyang Huang]

On Mon, Mar 25, 2024 at 11:22 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Sun, Mar 24, 2024 at 07:14:27PM +0800, Zhaoyang Huang wrote:
> > ok. It seems like madvise is robust enough to leave no BUGs. I presume
> > another two scenarios which call folio_isloate_lru by any other ways
> > but PTE. Besides, scenario 2 reminds me of a previous bug reported by
> > me as find_get_entry entered in a livelock where the folio's refcnt ==
> > 0 but remains at xarray which causes the reset->retry loops forever. I
> > would like to reply in that thread for more details.
> >
> > Scenario 1:
> > 0. Thread_bad gets the folio by find_get_entry and preempted before
> > folio_lock (could be the second round scan of
> > truncate_inode_pages_range)
> >     refcnt == 2(page_cache, fbatch_bad), PG_lru == true, PG_lock == false
> >         folio = find_get_entry
> >         folio_try_get_rcu
> >         <preempted>
> >         folio_try_lock
> >
> > 1. Thread_truncate get the folio via
> > truncate_inode_pages_range->find_lock_entries
> >     refcnt == 3(page_cache, fbatch_bad, fbatch_truncate), PG_lru ==
> > true, PG_lock == true
>
> Hang on, you can't have two threads in truncate_inode_pages_range()
> at the same time.  I appreciate that we don't have any documentation
> of that, but if it were possible, we'd see other crashes.  Removing
> the folio from the page cache sets folio->mapping to NULL.  And
> __filemap_remove_folio() uses folio->mapping in
> filemap_unaccount_folio() and page_cache_delete(), so we'd get NULL
> pointer dereferences.
ok. I will check if it is possible to have another way of entering
this scenario.
>
> I see a hint in the DAX code that it's an fs-dependent lock:
>
>         /*
>          * This gets called from truncate / punch_hole path. As such, the caller
>          * must hold locks protecting against concurrent modifications of the
>          * page cache (usually fs-private i_mmap_sem for writing). Since the
>          * caller has seen a DAX entry for this index, we better find it
>          * at that index as well...
>          */
>
> so maybe that's why there's no lockdep_assert() in
> truncate_inode_pages_range(), but there should be a comment.
>
> > Scenario 2:
> > 0. Thread_bad gets the folio by find_get_entry and preempted before
> > folio_lock (could be the second round scan of
> > truncate_inode_pages_range)
> >     refcnt == 2(page_cache, fbatch_bad), PG_lru == true, PG_lock == false
> >         folio = find_get_entry
> >         folio_try_get_rcu
> >         <preempted>
> >         folio_try_lock
> >
> > 1. Thread_readahead remove the folio from page cache and drop one
> > refcnt by filemap_remove_folio(get rid of the folios which failed to
> > launch IO during readahead)
> >     refcnt == 1(fbatch_bad), PG_lru == true, PG_lock == true
>
> So readaahead inserts the folio locked, and then calls
> filemap_remove_folio() without having unlocked it.
> filemap_remove_folio() sets folio->mapping to NULL in
> page_cache_delete().  When "Thread_bad" wakes up, it gets the
> folio lock, calls truncate_inode_folio() and sees that
> folio->mapping != mapping, so it doesn't call filemap_remove_folio().
>
> > 4. Thread_bad schedule back from step 0 and clear one refcnt wrongly
> > when doing truncate_inode_folio->filemap_remove_folio as it take this
> > refcnt as the page cache one
> >     refcnt == 1'(thread_isolate), PG_lru == false, PG_lock == false
> >         find_get_entries
> >             folio = find_get_entry
> >             folio_try_get_rcu
> >         folio_lock
> >         <no check as folio->mapping != mapping as folio_lock_entries does>
> >         truncate_inode_folio
> >             filemap_remove_folio
> >             <preempted>
Please review the following scenario, where the folio dropped two
refcnt wrongly when cleaning Non-IO folios within ractl. Should we
change it to __readahead_folio?

0. Thread_bad gets the folio by find_get_entry and preempted after
folio_try_get_rcu (could be the second round scan of
truncate_inode_pages_range)
    refcnt == 2(page_cache, fbatch_bad), PG_lru == true, PG_lock == false
        folio = find_get_entry
        folio_try_get_rcu
        <preempted>

1. Thread_readahead remove the folio from page cache and drop 2 refcnt
by readahead_folio & filemap_remove_folio(get rid of the folios which
failed to launch IO during readahead)
    refcnt == 0, PG_lru == true, PG_lock == true
    read_pages
..
        folio = readahead_folio
        <one refcnt dropped here>
********For the folio which can not launch IO, we should NOT drop
refcnt here??? replaced by __readahead_folio???**********
        folio_get
        filemap_remove_folio(folio)
        folio_unlock
        <one refcnt dropped here>
        folio_put

2.  folio_unlock
    refcnt == 0, PG_lru == true, PG_lock == false

3. Thread_isolate get one refcnt and call folio_isolate_lru(could be
any process)
    refcnt == 1(thread_isolate), PG_lru == true, PG_lock == false

4. Thread_isolate proceed to clear PG_lru and get preempted before folio_get
    refcnt == 1(thread_isolate), PG_lru == false, PG_lock == false
        folio_test_clear_folio
        <preempted>
        folio_get

5. Thread_bad schedule back from step 0 and proceed to drop one refcnt
by release_pages and hit the BUG
    refcnt == 0'(thread_isolate), PG_lru == false, PG_lock == false