Re: BUG: KCSAN: data-race in folio_batch_move_lru / mpage_read_end_io

[Mirsad Todorovac]

On 9/18/23 16:53, Matthew Wilcox wrote:
> On Mon, Sep 18, 2023 at 02:15:05PM +0200, Mirsad Todorovac wrote:
> > > This is what I'm currently running with, and it doesn't trigger.
> > > I'd expect it to if we were going to hit the KCSAN bug.
> > >
> > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > index 0c5be12f9336..d22e8798c326 100644
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -4439,6 +4439,7 @@ struct page *__alloc_pages(gfp_t gfp, unsigned int order, int preferred_nid,
> > >    	page = __alloc_pages_slowpath(alloc_gfp, order, &ac);
> > >    out:
> > > +	VM_BUG_ON_PAGE(page && (page->flags & (PAGE_FLAGS_CHECK_AT_PREP &~ (1 << PG_head))), page);
> > >    	if (memcg_kmem_online() && (gfp & __GFP_ACCOUNT) && page &&
> > >    	    unlikely(__memcg_kmem_charge_page(page, gfp, order) != 0)) {
> > >    		__free_pages(page, order);
> >
> > Hi,
> >
> > Caught another instance of this bug involving folio_batch_move_lru: I don't seem that I can make it
> > happen reliably by the nature of the data racing conditions if I understood them well.
>
> Were you running with this patch at the time, or was this actually
> vanilla?  The problem is that, if my diagnosis is correct, both of the
> tasks mentioned are victims; we have a prematurely freed page.  While
> btrfs is clearly a user, it may not be btrfs's fault that the
> page was also allocated as an anon page.
>
> I'm trying to gather more data, and running with this patch will give
> us more -- because it'll dump the entire struct page instead of just
> the page->flags, like KCSAN is currently doing.

As my learning curve adapts, I seem to be more aware of what you are talking about.

I still have to learn to cope with patches, diffs, fixes and pulls all together and
consistent.

Sometimes I feel like in the BORG maturation chamber when I try to learn the Linux kernel,
and I wonder if this is the Author of my story trying to make up "for the years that locust
had eaten". Or is it that I am just losing the plot.

I learn that I was conceited and not respecting the work you guys have done in thirty years
I wasted for one reason or another: objective difficulties and personal weaknesses.

Forgive me this moment of truth.

I certainly feel more motivated to catch the real culprit, rather than just the symptoms.

I will rebuild with your patch again and try to reproduce the problem.

Best regards
Mirsad Todorovac