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

[Lorenzo Stoakes (ARM)]

(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.

See https://lore.kernel.org/all/?q=f%3ANguyen+Ngoc+Thang

>
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -2257,6 +2257,7 @@ 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 need_unlock = false;
> >
> >  	/*
> >  	 * MADV_COLLAPSE ignores shmem huge config, so do not check shmem
> > @@ -2271,6 +2272,15 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
> >  	if (result != SCAN_SUCCEED)
> >  		goto out;
> >
> > +	/*
> > +	 * Take invalidate_lock before any folio lock: the readahead below
> > +	 * needs it, and truncate holds it while waiting on folio locks.
> > +	 */
> > +	if (!is_shmem) {
>
> Is the shmem special-case a red flag?
>
> Probably this fix an acceptable minimal-thing-for-backporting.
>
> Question for maintainers as well as for yourself: but does this
> indicate a need for a more architected redo?
>

--
Cheers, Lorenzo