Re: [PATCH v2] khugepaged: hold invalidate_lock across collapse_file() readahead

[David Hildenbrand (Arm)]

On 9/13/26 20:48, Lorenzo Stoakes (ARM) wrote:
> (Nguyen - do not send v2 in reply to v1, look across mm and see how things are
> done here).
> 
> somebody who has a
> 
> On Sun, Sep 13, 2026 at 11:17:00AM -0700, Andrew Morton wrote:
>> On Sun, 13 Sep 2026 23:36:44 +0700 Nguyen Ngoc Thang <ngocthang2710.1999@xxxxxxxxx> wrote:
>>
>>> collapse_file() calls page_cache_sync_readahead() to fault in missing
>>> pages before collapsing them into a THP. That helper takes
>>> mapping->invalidate_lock itself for the duration of the call, then
>>> drops it -- but truncate (e.g. ext4_setattr() -> truncate_pagecache())
>>> takes invalidate_lock and then waits on each page's folio lock while
>>> holding it. If collapse_file() has already locked one of those folios
>>> by the time truncate reaches it, and then tries to acquire
>>> invalidate_lock again (e.g. on the first readahead call, since
>>> invalidate_lock is not yet held at that point), the two paths can
>>> deadlock/hang on each other's lock: truncate blocked on the folio lock
>>> collapse holds, and collapse blocked waiting for invalidate_lock that
>>> truncate holds.
>>>
>>> Reproducing this over ~150,000 collapse iterations with truncate
>>> racing concurrently reliably hits hung_task: blocked tasks within
>>> about 20 seconds on an unpatched kernel.
>>>
>>> Fix it by taking invalidate_lock_shared once for the whole scan, after
>>> alloc_charge_folio() succeeds and before locking any folio, and using
>>> page_cache_ra_unbounded() directly in the readahead call site instead
>>> of page_cache_sync_readahead(), since the latter would try to retake
>>> the lock we already hold. page_cache_ra_unbounded() does not clamp to
>>> EOF like the helper it replaces, so clamp the requested range
>>> explicitly.
>>>
>>> 730633f0b7f9 added invalidate_lock acquisition around readahead but
>>> missed collapse_file(), which already locks pages while calling
>>> readahead; later filesystem conversions made the deadlock reachable by
>>> taking invalidate_lock before waiting on page locks during truncate.
>>>
>>> Reported-by: syzbot+16bf7cd0ebeb1de93aa5@xxxxxxxxxxxxxxxxxxxxxxxxx
>>> Closes: https://syzkaller.appspot.com/bug?extid=16bf7cd0ebeb1de93aa5
>>> Fixes: 730633f0b7f9 ("mm: Protect operations adding pages to page cache with invalidate_lock")
>>
>> (You forgot to cc the original author)
> 
> Also a change log, and that mm doesn't like sending a respin in-reply-to a
> previous version. Which is all consistent with somebody using e.g. openclaw to
> pepper generated patches across the kernel...
> 
>>
>> Five years.
>>
>> I wonder why this hasn't been discovered by lockdep, AI, syzbot or any
>> other of the tools we've been using for so long.
>>
>> Thanks for doing all this.
> 
> See https://lore.kernel.org/linux-mm/aqbtfms0_2ULBIT7@gremlin/
> 
> I am not really entirely happy with somebody who has sent a flurry of patches
> across disparate subsystems with no previous track record being in charge of a
> potentially backported fix.
> 
> It's David's decision but I think this fix should be taken over by somebody
> else.

IIUC, this patch might be responsible for random CI failures (Mike still
investigating).

So I'd prefer if this is taken over by someone familiar with that code.

@Willy, @Zi, @Lance?


-- 
Cheers,

David