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
