Re: [PATCH 2/4] mm/memory-failure.c: fix wrong user reference report

[Yang Shi]

On Mon, Mar 7, 2022 at 3:26 AM Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
>
> On 2022/3/4 16:27, HORIGUCHI NAOYA(堀口 直也) wrote:
> > On Mon, Feb 28, 2022 at 10:02:43PM +0800, Miaohe Lin wrote:
> >> The dirty swapcache page is still residing in the swap cache after it's
> >> hwpoisoned. So there is always one extra refcount for swap cache.
> >
> > The diff seems fine at a glance, but let me have a few question to
> > understand the issue more.
> >
> > - Is the behavior described above the effect of recent change on shmem where
> >   dirty pagecache is pinned on hwpoison (commit a76054266661 ("mm: shmem:
> >   don't truncate page if memory failure happens"). Or the older kernels
> >   behave as the same?
> >
> > - Is the behavior true for normal anonymous pages (not shmem pages)?
> >
>
> The behavior described above is aimed at swapcache not pagecache. So it should be
> irrelevant with the recent change on shmem.
>
> What I try to fix here is that me_swapcache_dirty holds one extra pin via SwapCache
> regardless of the return value of delete_from_lru_cache. We should try to report more
> accurate extra refcount for debugging purpose.

I think you misunderstood the code. The delete_from_lru_cache()
returning 0 means the page was on LRU and isolated from LRU
successfully now. Returning -EIO means the page was not on LRU, so it
should have at least an extra pin on it.

So MF_DELAYED means there is no other pin other than hwpoison and
swapcache which is expected, MF_FAILED means there might be extra
pins.

The has_extra_refcount() raised error then there is *unexpected* refcount.

>
> > I'm trying to test hwpoison hitting the dirty swapcache, but it seems that
> > in my testing memory_faliure() fails with "hwpoison: unhandlable page"
>
> Maybe memory_faliure is racing with page reclaim where page is isolated?
>
> > warning at get_any_page().  So I'm still not sure that me_pagecache_dirty()
> > fixes any visible problem.
>
> IIUC, me_pagecache_dirty can't do much except for the corresponding address_space supporting
> error_remove_page which can truncate the dirty pagecache page. But this may cause silent data
> loss. It's better to keep the page stay in the pagecache until the file is truncated, hole
> punched or removed as commit a76054266661 pointed out.
>
> Thanks.
>
> > > Thanks,
> > Naoya Horiguchi
> >
> >>
> >> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> >> ---
> >>  mm/memory-failure.c | 6 +-----
> >>  1 file changed, 1 insertion(+), 5 deletions(-)
> >>
> >> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> >> index 0d7c58340a98..5f9503573263 100644
> >> --- a/mm/memory-failure.c
> >> +++ b/mm/memory-failure.c
> >> @@ -984,7 +984,6 @@ static int me_pagecache_dirty(struct page_state *ps, struct page *p)
> >>  static int me_swapcache_dirty(struct page_state *ps, struct page *p)
> >>  {
> >>      int ret;
> >> -    bool extra_pins = false;
> >>
> >>      ClearPageDirty(p);
> >>      /* Trigger EIO in shmem: */
> >> @@ -993,10 +992,7 @@ static int me_swapcache_dirty(struct page_state *ps, struct page *p)
> >>      ret = delete_from_lru_cache(p) ? MF_FAILED : MF_DELAYED;
> >>      unlock_page(p);
> >>
> >> -    if (ret == MF_DELAYED)
> >> -            extra_pins = true;
> >> -
> >> -    if (has_extra_refcount(ps, p, extra_pins))
> >> +    if (has_extra_refcount(ps, p, true))
> >>              ret = MF_FAILED;
> >>
> >>      return ret;
> >> --
> >> 2.23.0
>
>