Re: [PATCH v2] mm: fix race between MADV_FREE reclaim and blkdev direct IO read

["Huang, Ying" <ying.huang@intel.com>]

"Huang, Ying" <ying.huang@intel.com> writes:

> Minchan Kim <minchan@kernel.org> writes:
>
>> On Wed, Jan 12, 2022 at 06:53:07PM -0300, Mauricio Faria de Oliveira wrote:
>>> Hi Minchan Kim,
>>> 
>>> Thanks for handling the hard questions! :)
>>> 
>>> On Wed, Jan 12, 2022 at 2:33 PM Minchan Kim <minchan@kernel.org> wrote:
>>> >
>>> > On Wed, Jan 12, 2022 at 09:46:23AM +0800, Huang, Ying wrote:
>>> > > Yu Zhao <yuzhao@google.com> writes:
>>> > >
>>> > > > On Wed, Jan 05, 2022 at 08:34:40PM -0300, Mauricio Faria de Oliveira wrote:
>>> > > >> diff --git a/mm/rmap.c b/mm/rmap.c
>>> > > >> index 163ac4e6bcee..8671de473c25 100644
>>> > > >> --- a/mm/rmap.c
>>> > > >> +++ b/mm/rmap.c
>>> > > >> @@ -1570,7 +1570,20 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
>>> > > >>
>>> > > >>                    /* MADV_FREE page check */
>>> > > >>                    if (!PageSwapBacked(page)) {
>>> > > >> -                          if (!PageDirty(page)) {
>>> > > >> +                          int ref_count = page_ref_count(page);
>>> > > >> +                          int map_count = page_mapcount(page);
>>> > > >> +
>>> > > >> +                          /*
>>> > > >> +                           * The only page refs must be from the isolation
>>> > > >> +                           * (checked by the caller shrink_page_list() too)
>>> > > >> +                           * and one or more rmap's (dropped by discard:).
>>> > > >> +                           *
>>> > > >> +                           * Check the reference count before dirty flag
>>> > > >> +                           * with memory barrier; see __remove_mapping().
>>> > > >> +                           */
>>> > > >> +                          smp_rmb();
>>> > > >> +                          if ((ref_count - 1 == map_count) &&
>>> > > >> +                              !PageDirty(page)) {
>>> > > >>                                    /* Invalidate as we cleared the pte */
>>> > > >>                                    mmu_notifier_invalidate_range(mm,
>>> > > >>                                            address, address + PAGE_SIZE);
>>> > > >
>>> > > > Out of curiosity, how does it work with COW in terms of reordering?
>>> > > > Specifically, it seems to me get_page() and page_dup_rmap() in
>>> > > > copy_present_pte() can happen in any order, and if page_dup_rmap()
>>> > > > is seen first, and direct io is holding a refcnt, this check can still
>>> > > > pass?
>>> > >
>>> > > I think that you are correct.
>>> > >
>>> > > After more thoughts, it appears very tricky to compare page count and
>>> > > map count.  Even if we have added smp_rmb() between page_ref_count() and
>>> > > page_mapcount(), an interrupt may happen between them.  During the
>>> > > interrupt, the page count and map count may be changed, for example,
>>> > > unmapped, or do_swap_page().
>>> >
>>> > Yeah, it happens but what specific problem are you concerning from the
>>> > count change under race? The fork case Yu pointed out was already known
>>> > for breaking DIO so user should take care not to fork under DIO(Please
>>> > look at O_DIRECT section in man 2 open). If you could give a specific
>>> > example, it would be great to think over the issue.
>>> >
>>> > I agree it's little tricky but it seems to be way other place has used
>>> > for a long time(Please look at write_protect_page in ksm.c).
>>> 
>>> Ah, that's great to see it's being used elsewhere, for DIO particularly!
>>> 
>>> > So, here what we missing is tlb flush before the checking.
>>> 
>>> That shouldn't be required for this particular issue/case, IIUIC.
>>> One of the things we checked early on was disabling deferred TLB flush
>>> (similarly to what you've done), and it didn't help with the issue; also, the
>>> issue happens on uniprocessor mode too (thus no remote CPU involved.)
>>
>> I guess you didn't try it with page_mapcount + 1 == page_count at tha
>> time?  Anyway, I agree we don't need TLB flush here like KSM.
>> I think the reason KSM is doing TLB flush before the check it to
>> make sure trap trigger on the write from userprocess in other core.
>> However, this MADV_FREE case, HW already gaurantees the trap.
>> Please see below.
>>
>>> 
>>> 
>>> >
>>> > Something like this.
>>> >
>>> > diff --git a/mm/rmap.c b/mm/rmap.c
>>> > index b0fd9dc19eba..b4ad9faa17b2 100644
>>> > --- a/mm/rmap.c
>>> > +++ b/mm/rmap.c
>>> > @@ -1599,18 +1599,8 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
>>> >
>>> >                         /* MADV_FREE page check */
>>> >                         if (!PageSwapBacked(page)) {
>>> > -                               int refcount = page_ref_count(page);
>>> > -
>>> > -                               /*
>>> > -                                * The only page refs must be from the isolation
>>> > -                                * (checked by the caller shrink_page_list() too)
>>> > -                                * and the (single) rmap (dropped by discard:).
>>> > -                                *
>>> > -                                * Check the reference count before dirty flag
>>> > -                                * with memory barrier; see __remove_mapping().
>>> > -                                */
>>> > -                               smp_rmb();
>>> > -                               if (refcount == 2 && !PageDirty(page)) {
>>> > +                               if (!PageDirty(page) &&
>>> > +                                       page_mapcount(page) + 1 == page_count(page)) {
>>> 
>>> In the interest of avoiding a different race/bug, it seemed worth following the
>>> suggestion outlined in __remove_mapping(), i.e., checking PageDirty()
>>> after the page's reference count, with a memory barrier in between.
>>
>> True so it means your patch as-is is good for me.
>
> If my understanding were correct, a shared anonymous page will be mapped
> read-only.  If so, will a private anonymous page be called
> SetPageDirty() concurrently after direct IO case has been dealt with
> via comparing page_count()/page_mapcount()?

Sorry, I found that I am not quite right here.  When direct IO read
completes, it will call SetPageDirty() and put_page() finally.  And
MADV_FREE in try_to_unmap_one() needs to deal with that too.

Checking direct IO code, it appears that set_page_dirty_lock() is used
to set page dirty, which will use lock_page().

  dio_bio_complete
    bio_check_pages_dirty
      bio_dirty_fn  /* through workqueue */
        bio_release_pages
          set_page_dirty_lock
    bio_release_pages
      set_page_dirty_lock

So in theory, for direct IO, the memory barrier may be unnecessary.  But
I don't think it's a good idea to depend on this specific behavior of
direct IO.  The original code with memory barrier looks more generic and
I don't think it will introduce visible overhead.

Best Regards,
Huang, Ying