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

[Pedro Falcato]

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@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=16bf7cd0ebeb1de93aa5
> Fixes: 730633f0b7f9 ("mm: Protect operations adding pages to page cache with invalidate_lock")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>

Thanks for the patch! This looks almost perfect!

> ---
> 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@xxxxxxxxx/
> 
>  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.

With that addressed:

Reviewed-by: Pedro Falcato <pfalcato@xxxxxxx>

> +			if (IS_ERR(folio)) {
> +				page_cache_sync_readahead(mapping, &file->f_ra,
> +							  file, index, end - index);
> +				folio = filemap_lock_folio(mapping, index);
> +				if (IS_ERR(folio)) {
> +					result = SCAN_FAIL;
> +					goto out_put;
> +				}
> +			}
> +			index = folio_next_index(folio);
> +			folio_unlock(folio);
> +			folio_put(folio);
> +		}
> +		/* Drain the local LRU batch before retrying isolation. */
> +		lru_add_drain();
> +		readahead_done = true;
> +		goto retry;
> +	}
> +out_put:
>  	folio_put(new_folio);
>  out:
>  	VM_BUG_ON(!list_empty(&pagelist));
> -- 
> 2.49.0
> 

-- 
Pedro