Re: [PATCH v4] khugepaged: fix readahead lock inversion in collapse_file()

[Pedro Falcato <pfalcato@suse.de>]

On Tue, Oct 27, 2026 at 10:55:07AM +0800, Lance Yang wrote:
> 
> On Mon, Oct 26, 2026 at 04:33:08PM +0000, Pedro Falcato wrote:
> >On Mon, Oct 26, 2026 at 11:20:13PM +0800, Lance Yang wrote:
> >> collapse_file() can call page_cache_sync_readahead() while still holding
> >> source folios locked. If truncate holds mapping->invalidate_lock
> >> exclusively and waits for one of those folios, readahead waits for
> >> invalidate_lock and neither side can make progress.
> >> 
> >> We need to release those folio locks before starting readahead. On a page
> >> cache miss, roll back first, populate the collapse range, and retry once.
> >> Doing readahead after rollback avoids an extra pass when all folios are
> >> already present. If another miss occurs on retry, give up rather than keep
> >> retrying while truncate or reclaim removes folios.
> >> 
> >> Keep page_cache_sync_readahead() and wait for read I/O with
> >> filemap_lock_folio(), releasing each folio before moving to the next. The
> >> existing uptodate check during collapse still handles read errors.
> >> 
> >> Shmem uses shmem_get_folio() for swapin and fallocated folios without
> >> taking mapping->invalidate_lock, so it is unaffected.
> >> 
> >> Reported-by: syzbot+16bf7cd0ebeb1de93aa5@syzkaller.appspotmail.com
> >> Closes: https://syzkaller.appspot.com/bug?extid=16bf7cd0ebeb1de93aa5
> >> Fixes: 730633f0b7f9 ("mm: Protect operations adding pages to page cache with invalidate_lock")
> >> Cc: stable@vger.kernel.org
> >> Signed-off-by: Lance Yang <lance.yang@linux.dev>
> >
> >Thanks for the patch! This looks almost perfect!
> 
> Thanks Pedro!
> 
> >
> >> ---
> >> v3 -> v4:
> >>  - Move readahead after rollback and retry collapse once (per Pedro).
> >>  - Drop the preparation pass and its redundant uptodate check (per
> >>    Baolin and Pedro). Thanks!
> >>  - https://lore.kernel.org/all/20261016065920.4153-1-lance.yang@linux.dev/
> >> 
> >>  mm/khugepaged.c | 44 +++++++++++++++++++++++++++++++++-----------
> >>  1 file changed, 33 insertions(+), 11 deletions(-)
> >> 
> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> >> index da91762f6358..0d3bd9e23613 100644
> >> --- a/mm/khugepaged.c
> >> +++ b/mm/khugepaged.c
> >> @@ -2239,6 +2239,8 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
> >>  	enum scan_result result = SCAN_SUCCEED;
> >>  	int nr_none = 0;
> >>  	bool is_shmem = shmem_file(file);
> >> +	bool do_readahead = false;
> >> +	bool readahead_done = false;
> >>  
> >>  	/*
> >>  	 * MADV_COLLAPSE ignores shmem huge config, so do not check shmem
> >> @@ -2256,6 +2258,10 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
> >>  
> >>  	mapping_set_update(&xas, mapping);
> >>  
> >> +retry:
> >> +	result = SCAN_SUCCEED;
> >> +	do_readahead = false;
> >> +	xas_set_order(&xas, start, HPAGE_PMD_ORDER);
> >>  	__folio_set_locked(new_folio);
> >>  	if (is_shmem)
> >>  		__folio_set_swapbacked(new_folio);
> >> @@ -2320,17 +2326,9 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
> >>  			}
> >>  		} else {	/* !is_shmem */
> >>  			if (!folio || xa_is_value(folio)) {
> >> -				xas_unlock_irq(&xas);
> >> -				page_cache_sync_readahead(mapping, &file->f_ra,
> >> -							  file, index,
> >> -							  end - index);
> >> -				/* drain lru cache to help folio_isolate_lru() */
> >> -				lru_add_drain();
> >> -				folio = filemap_lock_folio(mapping, index);
> >> -				if (IS_ERR(folio)) {
> >> -					result = SCAN_FAIL;
> >> -					goto xa_unlocked;
> >> -				}
> >> +				do_readahead = !readahead_done;
> >> +				result = SCAN_FAIL;
> >> +				goto xa_locked;
> >>  			} else if (folio_test_dirty(folio)) {
> >>  				/*
> >>  				 * This page is dirty because it hasn't
> >> @@ -2657,6 +2655,30 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
> >>  	new_folio->mapping = NULL;
> >>  
> >>  	folio_unlock(new_folio);
> >> +
> >> +	/* Readahead may take invalidate_lock; source folios are unlocked. */
> >> +	if (do_readahead) {
> >> +		for (index = start; index < end;) {
> >> +			folio = filemap_lock_folio(mapping, index);
> >
> >I'm not sure if there's a point in locking the folio. If we are retrying,
> >then the logic above will lock it anyway and check uptodate. So I don't
> >think there's much of a point doing it here. Right?
> >
> >So I think we could get away with a filemap_get_folio() here, and no
> >filemap_lock_folio() after RA. Unless you're intent on waiting for RA
> >right before failing, but I don't see a point in that.
> 
> Right, waiting for RA is what I intended here. Without that wait, the
> retry can hit folio_trylock() while the read is still in flight and bail
> out with SCAN_PAGE_LOCK, before it even gets to the uptodate check.
> 
> If the read completes successfully, we can continue with collapse.
> That's why I kept the wait before retrying. Do you think waiting isn't
> worth it here, and we'd be better off leaving collapse to a later
> attempt?

Hmm, yes, thanks for the explanation. I had a fresh look at the code and your
patch preserves that bit of behavior. Isolating folios is simply trylocking,
and only does a hard folio_lock after RA. So, LGTM!

-- 
Pedro