Message-ID: <20230522044557.GA845371@hori.linux.bs1.fc.nec.co.jp>
Date:   Mon, 22 May 2023 04:50:45 +0000
From:   HORIGUCHI NAOYA(堀口　直也) 
        <naoya.horiguchi@....com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
CC:     Jiaqi Yan <jiaqiyan@...gle.com>,
        "songmuchun@...edance.com" <songmuchun@...edance.com>,
        "shy828301@...il.com" <shy828301@...il.com>,
        "linmiaohe@...wei.com" <linmiaohe@...wei.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "duenwen@...gle.com" <duenwen@...gle.com>,
        "axelrasmussen@...gle.com" <axelrasmussen@...gle.com>,
        "jthoughton@...gle.com" <jthoughton@...gle.com>
Subject: Re: [PATCH v1 1/3] mm/hwpoison: find subpage in hugetlb HWPOISON list

On Fri, May 19, 2023 at 03:42:14PM -0700, Mike Kravetz wrote:
> On 05/19/23 13:54, Jiaqi Yan wrote:
> > On Wed, May 17, 2023 at 4:53 PM Mike Kravetz <mike.kravetz@...cle.com> wrote:
> > >
> > > On 05/17/23 16:09, Jiaqi Yan wrote:
> > > > Adds the functionality to search a subpage's corresponding raw_hwp_page
> > > > in hugetlb page's HWPOISON list. This functionality can also tell if a
> > > > subpage is a raw HWPOISON page.
> > > >
> > > > Exports this functionality to be immediately used in the read operation
> > > > for hugetlbfs.
> > > >
> > > > Signed-off-by: Jiaqi Yan <jiaqiyan@...gle.com>
> > > > ---
> > > >  include/linux/mm.h  | 23 +++++++++++++++++++++++
> > > >  mm/memory-failure.c | 26 ++++++++++++++++----------
> > > >  2 files changed, 39 insertions(+), 10 deletions(-)
> > > >
> > > > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > > > index 27ce77080c79..f191a4119719 100644
> > > > --- a/include/linux/mm.h
> > > > +++ b/include/linux/mm.h
> > >
> > > Any reason why you decided to add the following to linux/mm.h instead of
> > > linux/hugetlb.h?  Since it is hugetlb specific I would have thought
> > > hugetlb.h was more appropriate.
> > >
> > > > @@ -3683,6 +3683,29 @@ enum mf_action_page_type {
> > > >   */
> > > >  extern const struct attribute_group memory_failure_attr_group;
> > > >
> > > > +#ifdef CONFIG_HUGETLB_PAGE
> > > > +/*
> > > > + * Struct raw_hwp_page represents information about "raw error page",
> > > > + * constructing singly linked list from ->_hugetlb_hwpoison field of folio.
> > > > + */
> > > > +struct raw_hwp_page {
> > > > +     struct llist_node node;
> > > > +     struct page *page;
> > > > +};
> > > > +
> > > > +static inline struct llist_head *raw_hwp_list_head(struct folio *folio)
> > > > +{
> > > > +     return (struct llist_head *)&folio->_hugetlb_hwpoison;
> > > > +}
> > > > +
> > > > +/*
> > > > + * Given @subpage, a raw page in a hugepage, find its location in @folio's
> > > > + * _hugetlb_hwpoison list. Return NULL if @subpage is not in the list.
> > > > + */
> > > > +struct raw_hwp_page *find_raw_hwp_page(struct folio *folio,
> > > > +                                    struct page *subpage);
> > > > +#endif
> > > > +
> > > >  #if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
> > > >  extern void clear_huge_page(struct page *page,
> > > >                           unsigned long addr_hint,
> > > > diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> > > > index 5b663eca1f29..c49e6c2d1f07 100644
> > > > --- a/mm/memory-failure.c
> > > > +++ b/mm/memory-failure.c
> > > > @@ -1818,18 +1818,24 @@ EXPORT_SYMBOL_GPL(mf_dax_kill_procs);
> > > >  #endif /* CONFIG_FS_DAX */
> > > >
> > > >  #ifdef CONFIG_HUGETLB_PAGE
> > > > -/*
> > > > - * Struct raw_hwp_page represents information about "raw error page",
> > > > - * constructing singly linked list from ->_hugetlb_hwpoison field of folio.
> > > > - */
> > > > -struct raw_hwp_page {
> > > > -     struct llist_node node;
> > > > -     struct page *page;
> > > > -};
> > > >
> > > > -static inline struct llist_head *raw_hwp_list_head(struct folio *folio)
> > > > +struct raw_hwp_page *find_raw_hwp_page(struct folio *folio,
> > > > +                                    struct page *subpage)
> > > >  {
> > > > -     return (struct llist_head *)&folio->_hugetlb_hwpoison;
> > > > +     struct llist_node *t, *tnode;
> > > > +     struct llist_head *raw_hwp_head = raw_hwp_list_head(folio);
> > > > +     struct raw_hwp_page *hwp_page = NULL;
> > > > +     struct raw_hwp_page *p;
> > > > +
> > > > +     llist_for_each_safe(tnode, t, raw_hwp_head->first) {
> > >
> > > IIUC, in rare error cases a hugetlb page can be poisoned WITHOUT a
> > > raw_hwp_list.  This is indicated by the hugetlb page specific flag
> > > RawHwpUnreliable or folio_test_hugetlb_raw_hwp_unreliable().
> > >
> > > Looks like this routine does not consider that case.  Seems like it should
> > > always return the passed subpage if folio_test_hugetlb_raw_hwp_unreliable()
> > > is true?
> > 
> > Thanks for catching this. I wonder should this routine consider
> > RawHwpUnreliable or should the caller do.
> > 
> > find_raw_hwp_page now returns raw_hwp_page* in the llist entry to
> > caller (valid one at the moment), but once RawHwpUnreliable is set,
> > all the raw_hwp_page in the llist will be kfree(), and the returned
> > value becomes dangling pointer to caller (if the caller holds that
> > caller long enough). Maybe returning a bool would be safer to the
> > caller? If the routine returns bool, then checking RawHwpUnreliable
> > can definitely be within the routine.
> 
> I think the check for RawHwpUnreliable should be within this routine.
> Looking closer at the code, I do not see any way to synchronize this.
> It looks like manipulation in the memory-failure code would be
> synchronized via the mf_mutex.  However, I do not see how traversal and
> freeing of the raw_hwp_list  called from __update_and_free_hugetlb_folio
> is synchronized against memory-failure code modifying the list.
> 
> Naoya, can you provide some thoughts?

Thanks for elaborating the issue.  I think that making find_raw_hwp_page() and
folio_clear_hugetlb_hwpoison() do their works within mf_mutex can be one solution.
try_memory_failure_hugetlb(), one of the callers of folio_clear_hugetlb_hwpoison(),
already calls it within mf_mutex, so some wrapper might be needed to implement
calling path from __update_and_free_hugetlb_folio() to take mf_mutex.

It might be a concern that mf_mutex is a big lock to cover overall hwpoison
subsystem, but I think that the impact is not so big if the changed code paths
take mf_mutex only after checking hwpoisoned hugepage.  Maybe using folio_lock
to synchronize accesses to the raw_hwp_list could be possible, but currently
__get_huge_page_for_hwpoison() calls folio_set_hugetlb_hwpoison() without
folio_lock, so this approach needs update on locking rule and it sounds more
error-prone to me.

Thanks,
Naoya Horiguchi

> 
> > 
> > Another option is, this routine simply doesn one thing: find a
> > raw_hwp_page in raw_hwp_list for a subpage. But the caller needs to 1)
> > test RawHwpUnreliable before calls into the routine, and 2) test
> > RawHwpUnreliable before access returned raw_hwp_page*. I think 2nd
> > option will be error-prone and the 1st option is a better one.
> > 
> > Maybe I am over-thinking. What do you think?
> 
> I think racing code accessing the raw_hwp_list is very unlikely.
> However, it is possible and should be considered.
> -- 
> Mike Kravetz
> 
> > 
> > > --
> > > Mike Kravetz
> > >
> > > > +             p = container_of(tnode, struct raw_hwp_page, node);
> > > > +             if (subpage == p->page) {
> > > > +                     hwp_page = p;
> > > > +                     break;
> > > > +             }
> > > > +     }
> > > > +
> > > > +     return hwp_page;
> > > >  }
> > > >
> > > >  static unsigned long __folio_free_raw_hwp(struct folio *folio, bool move_flag)
> > > > --
> > > > 2.40.1.606.ga4b1b128d6-goog
> > > >

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