linux-kernel - Re: Stuck looping on list_empty(list) in free_pcppages

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Date:   Tue, 31 Aug 2021 09:37:23 -0700
From:   Sultan Alsawaf <sultan@...neltoast.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     linux-mm@...ck.org, mhocko@...e.com, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: Stuck looping on list_empty(list) in free_pcppages_bulk()

On Tue, Aug 31, 2021 at 01:44:49PM +0100, Mel Gorman wrote:
> That's your answer -- the PCP count has been corrupted or misaccounted.
> Given this is a Fedora kernel, check for any patches affecting
> mm/page_alloc.c that could be accounting related or that would affect
> the IRQ disabling or zone lock acquisition for problems. Another
> possibility is memory corruption -- either kernel or the hardware
> itself.

Hmm, I don't see any changes to mm/page_alloc.c from Fedora for this kernel.

What about a memory allocation originating from inside an NMI? I think this
could occur quite easily with an eBPF program registered to a tracepoint, as
some of the eBPF helpers do allocate memory on the fly for certain map types.

> > I tried to find some way that this could happen, but the only thing I could
> > think of was that maybe an allocation had both __GFP_RECLAIMABLE and
> > __GFP_MOVABLE set in its gfp mask, in which case the rmqueue() call in
> > get_page_from_freelist() would pass in a migratetype equal to MIGRATE_PCPTYPES
> > and then pages could be added to an out-of-bounds pcp list while still
> > incrementing the overall pcp count. This seems pretty unlikely though.
> 
> It's unlikely because it would be an outright bug to specify both flags.

Perhaps that VM_WARN_ON should be changed to a VM_BUG_ON?

Thanks,
Sultan