Re: [PATCH 3/3] mm/memory-failure: send SIGBUS in the event of thp split fail

[Jane Chu]

On 5/5/2024 12:00 AM, Miaohe Lin wrote:

> On 2024/5/2 7:24, Jane Chu wrote:
> > When handle hwpoison in a GUP longterm pin'ed thp page,
> > try_to_split_thp_page() will fail. And at this point, there is little else
> > the kernel could do except sending a SIGBUS to the user process, thus
> > give it a chance to recover.
> >
> > Signed-off-by: Jane Chu <jane.chu@xxxxxxxxxx>
> Thanks for your patch. Some comments below.
>
> > ---
> >   mm/memory-failure.c | 36 ++++++++++++++++++++++++++++++++++++
> >   1 file changed, 36 insertions(+)
> >
> > diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> > index 7fcf182abb96..67f4d24a98e7 100644
> > --- a/mm/memory-failure.c
> > +++ b/mm/memory-failure.c
> > @@ -2168,6 +2168,37 @@ static int memory_failure_dev_pagemap(unsigned long pfn, int flags,
> >   	return rc;
> >   }
> >   
> > +/*
> > + * The calling condition is as such: thp split failed, page might have
> > + * been GUP longterm pinned, not much can be done for recovery.
> > + * But a SIGBUS should be delivered with vaddr provided so that the user
> > + * application has a chance to recover. Also, application processes'
> > + * election for MCE early killed will be honored.
> > + */
> > +static int kill_procs_now(struct page *p, unsigned long pfn, int flags,
> > +			struct page *hpage)
> > +{
> > +	struct folio *folio = page_folio(hpage);
> > +	LIST_HEAD(tokill);
> > +	int res = -EHWPOISON;
> > +
> > +	/* deal with user pages only */
> > +	if (PageReserved(p) || PageSlab(p) || PageTable(p) || PageOffline(p))
> > +		res = -EBUSY;
> > +	if (!(PageLRU(hpage) || PageHuge(p)))
> > +		res = -EBUSY;
> Above checks seems unneeded. We already know it's thp?

Agreed.

I  lifted these checks from hwpoison_user_mapping() with a hope to make 
kill_procs_now() more generic,

such as, potentially replacing kill_accessing_processes() for 
re-accessing hwpoisoned page.

But I backed out at last, due to concerns that my tests might not have 
covered sufficient number of scenarios.

> > +
> > +	if (res == -EHWPOISON) {
> > +		collect_procs(folio, p, &tokill, flags & MF_ACTION_REQUIRED);
> > +		kill_procs(&tokill, true, pfn, flags);
> > +	}
> > +
> > +	if (flags & MF_COUNT_INCREASED)
> > +		put_page(p);
> This if block is broken. put_page() has been done when try_to_split_thp_page() fails?

put_page() has not been done if try_to_split_thp_page() fails, and I 
think it should.

I will revise the code so that put_page() is called regardless 
MF_ACTION_REQUIRED is set or not.

> > +
> action_result is missing?

Indeed,  action_result() isn't always called, referring to the 
re-accessing hwpoison scenarios.

In this case, I think the reason  is that, we just killed the process 
and there is nothing

else to do or to report.

> > +	return res;
> > +}
> > +
> >   /**
> >    * memory_failure - Handle memory failure of a page.
> >    * @pfn: Page Number of the corrupted page
> > @@ -2297,6 +2328,11 @@ int memory_failure(unsigned long pfn, int flags)
> >   		 */
> >   		SetPageHasHWPoisoned(hpage);
> >   		if (try_to_split_thp_page(p) < 0) {
> Should hwpoison_filter() be called in this case?
Yes, it should. I will add the hwpoison_filter check.
> > +			if (flags & MF_ACTION_REQUIRED) {
> > +				pr_err("%#lx: thp split failed\n", pfn);
> > +				res = kill_procs_now(p, pfn, flags, hpage);
> Can we use hwpoison_user_mappings() directly here?

I thought about using hwpoison_user_mappings() with an extra flag, but 
gave up in the end.

Because most of the code there are not needed, such as the checks you 
mentioned above,

and umapping etc.

thanks!

-jane

> Thanks.
> .
>
> > +				goto unlock_mutex;
> > +			}
> >   			res = action_result(pfn, MF_MSG_UNSPLIT_THP, MF_IGNORED);
> >   			goto unlock_mutex;
> >   		}