Re: [PATCH] mm: fix sleeping copy_huge_page called from atomic context

[Yang Shi]

On Fri, Oct 22, 2021 at 10:38 AM Yang Shi <shy828301@xxxxxxxxx> wrote:
>
> On Fri, Oct 22, 2021 at 4:16 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >
> > On Fri, Oct 22, 2021 at 09:46:19AM +0200, Andrea Righi wrote:
> > > copy_huge_page() can be called with mapping->private_lock held from
> > > __buffer_migrate_page() -> migrate_page_copy(), so it is not safe to
> > > do a cond_resched() in this context.
> > >
> > > Introduce migrate_page_copy_nowait() and copy_huge_page_nowait()
> > > variants that can be used from an atomic context.
> >
> > I think this is a consequence of THPs being created when they should not
> > be.  This is the wrong way to fix this problem; and I suspect it may
> > already be fixed at least in -mm.  We should have taken this path:
> >
> >         if (!page_has_buffers(page))
> >                 return migrate_page(mapping, newpage, page, mode);
> >
> > but since we didn't, we can infer that there's a THP which has buffers
> > (this should never occur).  It's the same root cause as the invalidatepage
> > problem, just with a very different signature.
>
> Yeah, exactly. And I replied to that syzbot report a few days ago
> (https://lore.kernel.org/linux-mm/CAHbLzkoFaowaG8AU6tg_WMPdjcAdyE+Wafs7TJz1Z23TRg_d8A@xxxxxxxxxxxxxx/)
> with the same conclusion.
>
> I'm not sure why Hugh didn't submit his patch, maybe he was waiting
> for the test result from the bug reporter of that invalidatepage
> issue? It should be fine, the fix is quite straightforward IMHO.

Anyway if Hugh doesn't have time to do it, I could prepare the patch
for formal review.

>
> >