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

[Lance Yang]

On 2026/9/14 00:12, Lance Yang wrote:
> Hi Nguyen,
>
> Good catch, thanks!
>
> On Sun, Sep 13, 2026 at 05:11:42PM +0700, Nguyen Ngoc Thang 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 next iteration, or via a nested
> > readahead call), 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.
>
> That can happen on the first readahead call.
>
> > 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, 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.
> >
> > Reported-by: syzbot+16bf7cd0ebeb1de93aa5@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://syzkaller.appspot.com/bug?extid=16bf7cd0ebeb1de93aa5
>
> Ouch, we should add a Fixes tag and Cc stable:
>
> Fixes: 730633f0b7f9 ("mm: Protect operations adding pages to page cache with invalidate_lock")
> Cc: stable@xxxxxxxxxxxxxxx
>
> 730633f0b7f9 added invalidate_lock acquisition to readahead but missed
> collapse_file(), which already called it with page locks held. The
> subsequent filesystem conversions made the deadlock possible by taking
> invalidate_lock before locking pages :( right?
>
> > Signed-off-by: Nguyen Ngoc Thang <ngocthang2710.1999@xxxxxxxxx>
> > ---

Forgot to mention:

I reproduced the hang too, and it goes away with this patch applied :)

Tested-by: Lance Yang <lance.yang@xxxxxxxxx>

> > mm/khugepaged.c | 25 ++++++++++++++++++++++---
> > 1 file changed, 22 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 11ff98d55c76..690ccbcdf593 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -2267,6 +2267,13 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
> > 	VM_WARN_ON_ONCE(!is_shmem && !mapping_pmd_folio_support(mapping));
> > 	VM_WARN_ON_ONCE(start & (HPAGE_PMD_NR - 1));
> >
> > +	/*
> > +	 * Take invalidate_lock before any folio lock: the readahead below
> > +	 * needs it, and truncate holds it while waiting on folio locks.
> > +	 */
> > +	if (!is_shmem)
> > +		filemap_invalidate_lock_shared(mapping);
>
> Could we take the lock after alloc_charge_folio() succeeds, before
> locking any folio? That would keep allocation and charging outside the
> critical section. And if allocation fails, we should skip the unlock :)
>
> Cheers, Lance
>
> > +
> > 	result = alloc_charge_folio(&new_folio, mm, cc, HPAGE_PMD_ORDER);
> > 	if (result != SCAN_SUCCEED)
> > 		goto out;
> > @@ -2337,10 +2344,20 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
> > 			}
> > 		} else {	/* !is_shmem */
> > 			if (!folio || xa_is_value(folio)) {
> > +				DEFINE_READAHEAD(ractl, file, &file->f_ra,
> > +						  mapping, index);
> > +				pgoff_t eof = DIV_ROUND_UP(i_size_read(mapping->host),
> > +							    PAGE_SIZE);
> > +
> > 				xas_unlock_irq(&xas);
> > -				page_cache_sync_readahead(mapping, &file->f_ra,
> > -							  file, index,
> > -							  end - index);
> > +				/*
> > +				 * invalidate_lock held above; don't retake it.
> > +				 * page_cache_ra_unbounded(), unlike the readahead
> > +				 * helper this replaces, does not clamp to EOF.
> > +				 */
> > +				if (index < eof)
> > +					page_cache_ra_unbounded(&ractl,
> > +						min(end, eof) - index, 0);
> > 				/* drain lru cache to help folio_isolate_lru() */
> > 				lru_add_drain();
> > 				folio = filemap_lock_folio(mapping, index);
> > @@ -2672,6 +2689,8 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
> > 	folio_unlock(new_folio);
> > 	folio_put(new_folio);
> > out:
> > +	if (!is_shmem)
> > +		filemap_invalidate_unlock_shared(mapping);
> > 	VM_BUG_ON(!list_empty(&pagelist));
> > 	trace_mm_khugepaged_collapse_file(mm, new_folio, index, addr, is_shmem, file, HPAGE_PMD_NR, result);
> > 	return result;
> > --
> > 2.43.0