Re: [PATCH linux-next] Fix shmem huge page failed to set F_SEAL_WRITE attribute problem

[Hugh Dickins <hughd@google.com>]

On Wed, 16 Feb 2022, Mike Kravetz wrote:
> On 2/14/22 23:37, cgel.zte@gmail.com wrote:
> > From: wangyong <wang.yong12@zte.com.cn>
> > 
> > After enabling tmpfs filesystem to support transparent hugepage with the
> > following command:
> >  echo always > /sys/kernel/mm/transparent_hugepage/shmem_enabled
> > The docker program adds F_SEAL_WRITE through the following command will
> > prompt EBUSY.
> >  fcntl(5, F_ADD_SEALS, F_SEAL_WRITE)=-1.
> > 
> > It is found that in memfd_wait_for_pins function, the page_count of
> > hugepage is 512 and page_mapcount is 0, which does not meet the
> > conditions:
> >  page_count(page) - page_mapcount(page) != 1.
> > But the page is not busy at this time, therefore, the page_order of
> > hugepage should be taken into account in the calculation.
> > 
> > Reported-by: Zeal Robot <zealci@zte.com.cn>
> > Signed-off-by: wangyong <wang.yong12@zte.com.cn>
> > ---
> >  mm/memfd.c | 16 +++++++++++++---
> >  1 file changed, 13 insertions(+), 3 deletions(-)
> > 
> > diff --git a/mm/memfd.c b/mm/memfd.c
> > index 9f80f162791a..26d1d390a22a 100644
> > --- a/mm/memfd.c
> > +++ b/mm/memfd.c
> > @@ -31,6 +31,7 @@
> >  static void memfd_tag_pins(struct xa_state *xas)
> >  {
> >  	struct page *page;
> > +	int count = 0;
> >  	unsigned int tagged = 0;
> >  
> >  	lru_add_drain();
> > @@ -39,8 +40,12 @@ static void memfd_tag_pins(struct xa_state *xas)
> >  	xas_for_each(xas, page, ULONG_MAX) {
> >  		if (xa_is_value(page))
> >  			continue;
> > +
> >  		page = find_subpage(page, xas->xa_index);
> > -		if (page_count(page) - page_mapcount(page) > 1)
> > +		count = page_count(page);
> > +		if (PageTransCompound(page))
> 
> PageTransCompound() is true for hugetlb pages as well as THP.  And, hugetlb
> pages will not have a ref per subpage as THP does.  So, I believe this will
> break hugetlb seal usage.

Yes, I think so too; and that is not the only issue with the patch
(I don't think page_mapcount is enough, I had to use total_mapcount).

It's a good find, and thank you WangYong for the report.
I found the same issue when testing my MFD_HUGEPAGE patch last year,
and devised a patch to fix it (and keep MFD_HUGETLB working) then; but
never sent that in because there wasn't time to re-present MFD_HUGEPAGE.

I'm currently retesting my patch: just found something failing which
I thought should pass; but maybe I'm confused, or maybe the xarray is
working differently now.  I'm rushing to reply now because I don't want
others to waste their own time on it.

Andrew, please expect a replacement patch for this issue, but
I certainly have more testing and checking to do before sending.

Hugh

> 
> I was trying to do some testing via the memfd selftests, but those have some
> other issues for hugetlb that need to be fixed. :(
> -- 
> Mike Kravetz