Re: [PATCH v2] mm/gup: fix try_grab_compound_head() race with split_huge_page()

[Vlastimil Babka]

On 6/16/21 1:10 AM, Yang Shi wrote:
> On Tue, Jun 15, 2021 at 5:10 AM Jann Horn <jannh@xxxxxxxxxx> wrote:
>>
>> On Tue, Jun 15, 2021 at 8:37 AM John Hubbard <jhubbard@xxxxxxxxxx> wrote:
>> > On 6/14/21 6:20 PM, Jann Horn wrote:
>> > > try_grab_compound_head() is used to grab a reference to a page from
>> > > get_user_pages_fast(), which is only protected against concurrent
>> > > freeing of page tables (via local_irq_save()), but not against
>> > > concurrent TLB flushes, freeing of data pages, or splitting of compound
>> > > pages.
>> [...]
>> > Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
>>
>> Thanks!
>>
>> [...]
>> > > @@ -55,8 +72,23 @@ static inline struct page *try_get_compound_head(struct page *page, int refs)
>> > >       if (WARN_ON_ONCE(page_ref_count(head) < 0))
>> > >               return NULL;
>> > >       if (unlikely(!page_cache_add_speculative(head, refs)))
>> > >               return NULL;
>> > > +
>> > > +     /*
>> > > +      * At this point we have a stable reference to the head page; but it
>> > > +      * could be that between the compound_head() lookup and the refcount
>> > > +      * increment, the compound page was split, in which case we'd end up
>> > > +      * holding a reference on a page that has nothing to do with the page
>> > > +      * we were given anymore.
>> > > +      * So now that the head page is stable, recheck that the pages still
>> > > +      * belong together.
>> > > +      */
>> > > +     if (unlikely(compound_head(page) != head)) {
>> >
>> > I was just wondering about what all could happen here. Such as: page gets split,
>> > reallocated into a different-sized compound page, one that still has page pointing
>> > to head. I think that's OK, because we don't look at or change other huge page
>> > fields.
>> >
>> > But I thought I'd mention the idea in case anyone else has any clever ideas about
>> > how this simple check might be insufficient here. It seems fine to me, but I
>> > routinely lack enough imagination about concurrent operations. :)
>>
>> Hmmm... I think the scariest aspect here is probably the interaction
>> with concurrent allocation of a compound page on architectures with
>> store-store reordering (like ARM). *If* the page allocator handled
>> compound pages with lockless, non-atomic percpu freelists, I think it
>> might be possible that the zeroing of tail_page->compound_head in
>> put_page() could be reordered after the page has been freed,
>> reallocated and set to refcount 1 again?
>>
>> That shouldn't be possible at the moment, but it is still a bit scary.
> 
> It might be possible after Mel's "mm/page_alloc: Allow high-order
> pages to be stored on the per-cpu lists" patch
> (https://patchwork.kernel.org/project/linux-mm/patch/20210611135753.GC30378@xxxxxxxxxxxxxxxxxxx/).

Those would be percpu indeed, but not "lockless, non-atomic", no? They are
protected by a local_lock.

>>
>>
>> I think the lockless page cache code also has to deal with somewhat
>> similar ordering concerns when it uses page_cache_get_speculative(),
>> e.g. in mapping_get_entry() - first it looks up a page pointer with
>> xas_load(), and any access to the page later on would be a _dependent
>> load_, but if the page then gets freed, reallocated, and inserted into
>> the page cache again before the refcount increment and the re-check
>> using xas_reload(), then there would be no data dependency from
>> xas_reload() to the following use of the page...
>>
>