Re: [PATCH] mm: swap: Avoid infinite loop if no valid swap entry found during do_swap_page

[mawupeng <mawupeng1@huawei.com>]

On 2025/2/23 14:18, Barry Song wrote:
> On Sun, Feb 23, 2025 at 3:42 PM Matthew Wilcox <willy@infradead.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.

Yes, log flooding is not the main issue here, endless #PF is rather a more serious
problem.

> 
> 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