Re: [PATCHv2 1/1] mm: fix unproperly folio_put by changing API in read_pages

[David Hildenbrand]

On 03.04.24 07:50, Zhaoyang Huang wrote:
> On Tue, Apr 2, 2024 at 8:58 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > On 01.04.24 10:17, zhaoyang.huang wrote:
> > > From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> > >
> > > An VM_BUG_ON in step 9 of [1] could happen as the refcnt is dropped
> > > unproperly during the procedure of read_pages()->readahead_folio->folio_put.
> > > This is introduced by commit 9fd472af84ab ("mm: improve cleanup when
> > > ->readpages doesn't process all pages")'.
> > >
> > > key steps of[1] in brief:
> > > 2'. Thread_truncate get folio to its local fbatch by find_get_entry in step 2
> > > 7'. Last refcnt remained which is not as expect as from alloc_pages
> > >       but from thread_truncate's local fbatch in step 7
> > > 8'. Thread_reclaim succeed to isolate the folio by the wrong refcnt(not
> > >       the value but meaning) in step 8
> > > 9'. Thread_truncate hit the VM_BUG_ON in step 9
> > >
> > > [1]
> > > Thread_readahead:
> > > 0. folio = filemap_alloc_folio(gfp_mask, 0);
> > >          (refcount 1: alloc_pages)
> > > 1. ret = filemap_add_folio(mapping, folio, index + i, gfp_mask);
> > >          (refcount 2: alloc_pages, page_cache)

[not going into all details, just a high-level remark]

page_cache_ra_unbounded() does a filemap_invalidate_lock_shared(), which 
is a down_read_trylock(&mapping->invalidate_lock).

That is, all read_pages() calls in mm/readahead.c happen under 
mapping->invalidate_lock in read mode.

.. and ...

> > > Thread_truncate:
> > > 2. folio = find_get_entries(&fbatch_truncate);
> > >          (refcount 3: alloc_pages, page_cache, fbatch_truncate))

truncation, such as truncate_inode_pages() must be called under 
mapping->invalidate_lock held in write mode. So naive me would have 
thought that readahead and truncate cannot race in that way.

[...]


> > Something that would help here is an actual reproducer that triggersthis
> > issue.
> >
> > To me, it's unclear at this point if we are talking about an actual
> > issue or a theoretical issue?
> Thanks for feedback. Above callstack is a theoretical issue so far
> which is arised from an ongoing analysis of a practical livelock issue
> generated by folio_try_get_rcu which is related to abnormal folio
> refcnt state. So do you think this callstack makes sense?

I'm not an expert on that code, and only spent 5 min looking into the 
code. So my reasoning about invalidate_lock above might be completely wrong.

It would be a very rare race that was not reported so far in practice. 
And it certainly wouldn't be the easiest one to explain, because the 
call chain above is a bit elaborate and does not explain which locks are 
involved and how they fail to protect us from any such race.

For this case in particular, I think we really need a real reproducer to 
convince people that the actual issue does exist and the fix actually 
resolves the issue.

--
Cheers,

David / dhildenb