Re: [PATCH v3] mm: fix race between MADV_FREE reclaim and blkdev direct IO read

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

Yu Zhao <yuzhao@google.com> writes:

> On Wed, Feb 16, 2022 at 02:48:19PM +0800, Huang, Ying wrote:
>> Yu Zhao <yuzhao@google.com> writes:
>> 
>> > On Wed, Feb 02, 2022 at 06:27:47PM -0300, Mauricio Faria de Oliveira wrote:
>> >> On Wed, Feb 2, 2022 at 4:56 PM Yu Zhao <yuzhao@google.com> wrote:
>> >> >
>> >> > On Mon, Jan 31, 2022 at 08:02:55PM -0300, Mauricio Faria de Oliveira wrote:
>> >> > > Problem:
>> >> > > =======
>> >> >
>> >> > Thanks for the update. A couple of quick questions:
>> >> >
>> >> > > Userspace might read the zero-page instead of actual data from a
>> >> > > direct IO read on a block device if the buffers have been called
>> >> > > madvise(MADV_FREE) on earlier (this is discussed below) due to a
>> >> > > race between page reclaim on MADV_FREE and blkdev direct IO read.
>> >> >
>> >> > 1) would page migration be affected as well?
>> >> 
>> >> Could you please elaborate on the potential problem you considered?
>> >> 
>> >> I checked migrate_pages() -> try_to_migrate() holds the page lock,
>> >> thus shouldn't race with shrink_page_list() -> with try_to_unmap()
>> >> (where the issue with MADV_FREE is), but maybe I didn't get you
>> >> correctly.
>> >
>> > Could the race exist between DIO and migration? While DIO is writing
>> > to a page, could migration unmap it and copy the data from this page
>> > to a new page?
>> 
>> Check the migrate_pages() code,
>> 
>>   migrate_pages
>>     unmap_and_move
>>       __unmap_and_move
>>         try_to_migrate // set PTE to swap entry with PTL
>>         move_to_new_page
>>           migrate_page
>>             folio_migrate_mapping
>>               folio_ref_count(folio) != expected_count // check page ref count
>>             folio_migrate_copy
>> 
>> The page ref count is checked after unmapping and before copying.  This
>> is good, but it appears that we need a memory barrier between checking
>> page ref count and copying page.
>
> I didn't look into this but, off the top of head, this should be
> similar if not identical to the DIO case. Therefore, it requires two
> barriers -- before and after the refcnt check (which may or may not
> exist).

Yes.  I think so too.

Best Regards,
Huang, Ying