Re: [PATCH v1 1/3] mm/hwpoison: find subpage in hugetlb HWPOISON list

[Jiaqi Yan]

On Fri, Jun 16, 2023 at 4:35 PM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> On 06/16/23 14:19, Jiaqi Yan wrote:
> > On Sun, Jun 11, 2023 at 9:19 PM Naoya Horiguchi
> > <naoya.horiguchi@xxxxxxxxx> wrote:
> > >
> > > On Fri, Jun 09, 2023 at 10:48:47PM -0700, Jiaqi Yan wrote:
> > > > On Thu, May 25, 2023 at 5:28 PM Jiaqi Yan <jiaqiyan@xxxxxxxxxx> wrote:
> > > > >
> > > > > On Mon, May 22, 2023 at 7:43 PM HORIGUCHI NAOYA(堀口　直也)
> > > > > <naoya.horiguchi@xxxxxxx> wrote:
> > > > > >
> > > > > > On Mon, May 22, 2023 at 11:22:49AM -0700, Jiaqi Yan wrote:
> > > > > > > On Sun, May 21, 2023 at 9:50 PM HORIGUCHI NAOYA(堀口　直也)
> > > > > > > <naoya.horiguchi@xxxxxxx> wrote:
> > > > > > > >
> > > > > > > > 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@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > > > > > > > > > > ---
> > > > > > > > > > > >  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?
> >
> > Hi Mike,
> >
> > Now looking again this, I think concurrent adding and deleting are
> > fine with each other and with themselves, because raw_hwp_list is
> > lock-less llist.
>
> Correct.
>
> > As for synchronizing traversal with adding and deleting, I wonder is
> > it a good idea to make __update_and_free_hugetlb_folio hold
> > hugetlb_lock before it folio_clear_hugetlb_hwpoison(which traverse +
> > delete raw_hwp_list)? In hugetlb, get_huge_page_for_hwpoison already
> > takes hugetlb_lock; it seems to me __update_and_free_hugetlb_folio is
> > missing the lock.
>
> I do not think the lock is needed.  However, while looking more closely
> at this I think I discovered another issue.
> This is VERY subtle.
> Perhaps Naoya can help verify if my reasoning below is correct.
>
> In __update_and_free_hugetlb_folio we are not operating on a hugetlb page.
> Why is this?
> Before calling update_and_free_hugetlb_folio we call remove_hugetlb_folio.
> The purpose of remove_hugetlb_folio is to remove the huge page from the
> list AND compound page destructor indicating this is a hugetlb page is changed.
> This is all done while holding the hugetlb lock.  So, the test for
> folio_test_hugetlb(folio) is false.
>
> We have technically a compound non-hugetlb page with a non-null raw_hwp_list.
>
> Important note: at this time we have not reallocated vmemmap pages if
> hugetlb page was vmemmap optimized.  That is done later in
> __update_and_free_hugetlb_folio.


>
> The 'good news' is that after this point get_huge_page_for_hwpoison will
> not recognize this as a hugetlb page, so nothing will be added to the
> list.  There is no need to worry about entries being added to the list
> during traversal.
>
> The 'bad news' is that if we get a memory error at this time we will
> treat it as a memory error on a regular compound page.  So,
> TestSetPageHWPoison(p) in memory_failure() may try to write a read only
> struct page. :(

At least I think this is an issue.

Would it help if dissolve_free_huge_page doesn't unlock hugetlb_lock
until update_and_free_hugetlb_folio is done, or basically until
dissolve_free_huge_page is done?

TestSetPageHWPoison in memory_failure is called after
try_memory_failure_hugetlb, and folio_test_hugetlb is tested within
__get_huge_page_for_hwpoison, which is wrapped by the hugetlb_lock. So
by the time dissolve_free_huge_page returns, subpages already go
through hugetlb_vmemmap_restore and __destroy_compound_gigantic_folio
and become non-compound raw pages (folios). Now
folio_test_hugetlb(p)=false will be correct for memory_failure, and it
can recover p as a dissolved non-hugetlb page.


> --
> Mike Kravetz