Re: [External] Re: [bug report] mm/zswap :memory corruption after zswap_load().

[Barry Song <21cnbao@gmail.com>]

On Sat, Mar 23, 2024 at 12:09 PM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> On Fri, Mar 22, 2024 at 4:04 PM Barry Song <21cnbao@gmail.com> wrote:
> >
> > On Sat, Mar 23, 2024 at 8:35 AM Yosry Ahmed <yosryahmed@google.com> wrote:
> > >
> > > On Thu, Mar 21, 2024 at 8:04 PM Zhongkun He
> > > <hezhongkun.hzk@bytedance.com> wrote:
> > > >
> > > > On Thu, Mar 21, 2024 at 5:29 PM Chengming Zhou <chengming.zhou@linux.dev> wrote:
> > > > >
> > > > > On 2024/3/21 14:36, Zhongkun He wrote:
> > > > > > On Thu, Mar 21, 2024 at 1:24 PM Chengming Zhou <chengming.zhou@linux.dev> wrote:
> > > > > >>
> > > > > >> On 2024/3/21 13:09, Zhongkun He wrote:
> > > > > >>> On Thu, Mar 21, 2024 at 12:42 PM Chengming Zhou
> > > > > >>> <chengming.zhou@linux.dev> 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
> >
> > but zswap has freed the object when the do_swap_page finishes swap_read_folio
> > due to exclusive load feature of zswap?
> >
> > so WILLNEED will get corrupted data and put it into swapcache.
> > some other concurrent new forked process might get the new data
> > from the swapcache WILLNEED puts when the new-forked process
> > goes into do_swap_page.
>
> Oh I was wondering how synchronization with WILLNEED happens without
> zswap. It seems like we could end up with two copies of the same folio
> and one of them will be leaked unless I am missing something.
>
> >
> > so very likely a new process is forked right after do_swap_page finishes
> > swap_read_folio and before swapcache_clear.
> >
> > > 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.
> >
> > we only need to write when we know swap_read_folio() gets data
> > from zswap but not swapfile. is there a quick way to do this?
>
> The folio will be dirty when loaded from zswap, so we can check if the
> folio is dirty and write the page if fail after swap_read_folio().

Is it actually a bug of swapin_walk_pmd_entry? it only check pte
before read_swap_cache_async. but when read_swap_cache_async
is blocked by swapcache_prepare, after it gets the swapcache_prepare
successfully , someone else should have already set the pte and freed
the swap slot even if this is not zswap?

static int swapin_walk_pmd_entry(pmd_t *pmd, unsigned long start,
                unsigned long end, struct mm_walk *walk)
{
        struct vm_area_struct *vma = walk->private;
        struct swap_iocb *splug = NULL;
        pte_t *ptep = NULL;
        spinlock_t *ptl;
        unsigned long addr;

        for (addr = start; addr < end; addr += PAGE_SIZE) {
                pte_t pte;
                swp_entry_t entry;
                struct folio *folio;

                if (!ptep++) {
                        ptep = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
                        if (!ptep)
                                break;
                }

                pte = ptep_get(ptep);
                if (!is_swap_pte(pte))
                        continue;
                entry = pte_to_swp_entry(pte);
                if (unlikely(non_swap_entry(entry)))
                        continue;

                pte_unmap_unlock(ptep, ptl);
                ptep = NULL;

                folio = read_swap_cache_async(entry, GFP_HIGHUSER_MOVABLE,
                                             vma, addr, &splug);
                if (folio)
                        folio_put(folio);
        }

        if (ptep)c
                pte_unmap_unlock(ptep, ptl);
        swap_read_unplug(splug);
        cond_resched();

        return 0;
}

I mean pte can become non-swap within read_swap_cache_async(),
so no matter if it is zswap, we have the bug.