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

[Jane Chu]

On 5/9/2024 7:59 PM, Miaohe Lin wrote:
> On 2024/5/9 23:34, Jane Chu wrote:
> > On 5/9/2024 1:30 AM, Miaohe Lin wrote:
> > > On 2024/5/9 1:45, Jane Chu wrote:
> > > > On 5/8/2024 1:08 AM, Miaohe Lin wrote:
> > > >
> > > > > On 2024/5/7 4:26, Jane Chu wrote:
> > > > > > 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.
> > > > > In try_to_split_thp_page(), if split_huge_page fails, i.e. ret != 0, put_page() is called. See below:
> > > > >
> > > > > static int try_to_split_thp_page(struct page *page)
> > > > > {
> > > > >       int ret;
> > > > >
> > > > >       lock_page(page);
> > > > >       ret = split_huge_page(page);
> > > > >       unlock_page(page);
> > > > >
> > > > >       if (unlikely(ret))
> > > > >           put_page(page);
> > > > >       ^^^^^^^^^^^^^^^^^^^^^^^
> > > > >       return ret;
> > > > > }
> > > > >
> > > > > Or am I miss something?
> > > > I think you caught a bug in my code, thanks!
> > > >
> > > > How about moving put_page() outside try_to_split_thp_page() ?
> > > If you want to send SIGBUS in the event of thp split fail, it might be required to do so.
> > > I think kill_procs_now() needs extra thp refcnt to do its work.
> > Agreed.  I added an boolean to try_to_split_thp_page(),the boolean indicates whether to put_page().
> IMHO, it might be too complicated to add an extra boolean to indicate whether to put_page(). It might be
> more straightforward to always put_page outside try_to_split_thp_page?

Looks okay to me, let's see.  Will send out v2 in a while.

thanks,

-jane

> Thanks.
> .