On Thu, Mar 21, 2024 at 8:04 PM Zhongkun He
<hezhongkun.hzk@xxxxxxxxxxxxx> wrote:
>
> On Thu, Mar 21, 2024 at 5:29 PM Chengming Zhou <chengming.zhou@xxxxxxxxx> wrote:
> >
> > On 2024/3/21 14:36, Zhongkun He wrote:
> > > On Thu, Mar 21, 2024 at 1:24 PM Chengming Zhou <chengming.zhou@xxxxxxxxx> wrote:
> > >>
> > >> On 2024/3/21 13:09, Zhongkun He wrote:
> > >>> On Thu, Mar 21, 2024 at 12:42 PM Chengming Zhou
> > >>> <chengming.zhou@xxxxxxxxx> wrote:
> > >>>>
> > >>>> On 2024/3/21 12:34, Zhongkun He wrote:
> > >>>>> Hey folks,
> > >>>>>
> > >>>>> Recently, I tested the zswap with memory reclaiming in the mainline
> > >>>>> (6.8) and found a memory corruption issue related to exclusive loads.
> > >>>>
> > >>>> Is this fix included? 13ddaf26be32 ("mm/swap: fix race when skipping swapcache")
> > >>>> This fix avoids concurrent swapin using the same swap entry.
> > >>>>
> > >>>
> > >>> Yes, This fix avoids concurrent swapin from different cpu, but the
> > >>> reported issue occurs
> > >>> on the same cpu.
> > >>
> > >> I think you may misunderstand the race description in this fix changelog,
> > >> the CPU0 and CPU1 just mean two concurrent threads, not real two CPUs.
> > >>
> > >> Could you verify if the problem still exists with this fix?
> > >
> > > Yes，I'm sure the problem still exists with this patch.
> > > There is some debug info, not mainline.
> > >
> > > bpftrace -e'k:swap_readpage {printf("%lld, %lld,%ld,%ld,%ld\n%s",
> > > ((struct page *)arg0)->private,nsecs,tid,pid,cpu,kstack)}' --include
> > > linux/mm_types.h
> >
> > Ok, this problem seems only happen on SWP_SYNCHRONOUS_IO swap backends,
> > which now include zram, ramdisk, pmem, nvdimm.
>
> Yes.
>
> >
> > It maybe not good to use zswap on these swap backends?
> >
> > The problem here is the page fault handler tries to skip swapcache to
> > swapin the folio (swap entry count == 1), but then it can't install folio
> > to pte entry since some changes happened such as concurrent fork of entry.
> >
>
> The first page fault returned VM_FAULT_RETRY because
> folio_lock_or_retry() failed.
How so? The folio is newly allocated and not visible to any other
threads or CPUs. swap_read_folio() unlocks it and then returns and we
immediately try to lock it again with folio_lock_or_retry(). How does
this fail?
Let's go over what happens after swap_read_folio():
- The 'if (!folio)' code block will be skipped.
- folio_lock_or_retry() should succeed as I mentioned earlier.
- The 'if (swapcache)' code block will be skipped.
- The pte_same() check should succeed on first look because other
concurrent faulting threads should be held off by the newly introduced
swapcache_prepare() logic. But looking deeper I think this one may
fail due to a concurrent MADV_WILLNEED.
- The 'if (unlikely(!folio_test_uptodate(folio)))` part will be
skipped because swap_read_folio() marks the folio up-to-date.
- After that point there is no possible failure until we install the
pte, at which point concurrent faults will fail on !pte_same() and
retry.
So the only failure I think is possible is the pte_same() check. I see
how a concurrent MADV_WILLNEED could cause that check to fail. A
concurrent MADV_WILLNEED will block on swapcache_prepare(), but once
the fault resolves it will go ahead and read the folio again into the
swapcache. It seems like we will end up with two copies of the same
folio? Maybe this is harmless because the folio in the swacache will
never be used, but it is essentially leaked at that point, right?
I feel like I am missing something. Adding other folks that were
involved in the recent swapcache_prepare() synchronization thread.
Anyway, I agree that at least in theory the data corruption could
happen because of exclusive loads when skipping the swapcache, and we
should fix that.
Perhaps the right thing to do may be to write the folio again to zswap
before unlocking it and before calling swapcache_clear(). The need for
the write can be detected by checking if the folio is dirty, I think
this will only be true if the folio was loaded from zswap.
