linux-kernel - Re: [PATCH] mm: swap: Avoid infinite loop if no valid swap entry found during do_swap

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Message-ID: <CAGsJ_4xHaaf_DHsFZ_zEqEd3Nb9C=7JJjy5gGFo+RhEhQYX_tg@mail.gmail.com>
Date: Sun, 23 Feb 2025 19:18:31 +1300
From: Barry Song <21cnbao@...il.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: mawupeng <mawupeng1@...wei.com>, akpm@...ux-foundation.org, david@...hat.com, 
	kasong@...cent.com, ryan.roberts@....com, chrisl@...nel.org, 
	huang.ying.caritas@...il.com, schatzberg.dan@...il.com, hanchuanhua@...o.com, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: swap: Avoid infinite loop if no valid swap entry
 found during do_swap_page

On Sun, Feb 23, 2025 at 3:42 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Sat, Feb 22, 2025 at 11:59:53AM +0800, mawupeng wrote:
> >
> >
> > On 2025/2/22 11:45, Matthew Wilcox wrote:
> > > On Sat, Feb 22, 2025 at 10:46:17AM +0800, Wupeng Ma wrote:
> > >> Digging into the source, we found that the swap entry is invalid due to
> > >> unknown reason, and this lead to invalid swap_info_struct. Excessive log
> > >> printing can fill up the prioritized log space, leading to the purging of
> > >> originally valid logs and hindering problem troubleshooting. To make this
> > >> more robust, kill this task.
> > >
> > > this seems like a very bad way to fix this problem
> >
> > Sure, It's a bad way to fix this. Just a proper way to make it more robust?
> > Since it will produce lots of invalid and same log?
>
> We have a mechanism to prevent flooding the log: <linux/ratelimit.h>.
> If you grep for 'ratelimit' in include, you'll see a number of
> convenience functions exist; not sure whether you'll need to use the raw
> ratelilmit stuff, or if you can just use one of the prepared ones.
>

IMHO, I really don’t think log flooding is the issue here; rather, we’re dealing
with an endless page fault. For servers, that might mean server is unresponsive
, for phones, they could be quickly running out of battery.

It’s certainly better to identify the root cause, but it could be due
to a bit-flip in
DDR or memory corruption in the page table. Until we can properly fix it, the
patch seems somewhat reasonable—the wrong application gets killed, it at
least has a chance to be restarted by systemd, Android init, etc. A PTE pointing
to a non-existent swap file and never being enabled clearly indicates something
has gone seriously wrong - either a hardware issue or a kernel bug.
At the very least, it warrants a WARN_ON_ONCE(), even after we identify and fix
the root cause, as it still enhances the system's robustness.

Gaoxu will certainly encounter the same problem if do_swap_page() executes
earlier than swap_duplicate() where the PTE points to a non-existent swap
file [1]. That means the phone will heat up quickly.

[1] https://lore.kernel.org/linux-mm/e223b0e6ba2f4924984b1917cc717bd5@honor.com/

Thanks
Barry