Re: [PATCH v2] mm/z3fold.c: Lock z3fold page before __SetPageMovable()

[Henry Burns <henryburns@google.com>]

On Tue, Jul 2, 2019 at 2:19 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Mon, 1 Jul 2019 18:16:30 -0700 Henry Burns <henryburns@google.com> wrote:
>
> > Cc: Vitaly Wool <vitalywool@gmail.com>, Vitaly Vul <vitaly.vul@sony.com>
>
> Are these the same person?
I Think it's the same person, but i wasn't sure which email to include
because one was
in the list of maintainers and I had contacted the other earlier.
>
> > Subject: Re: [PATCH v2] mm/z3fold.c: Lock z3fold page before __SetPageMovable()
> > Date: Mon, 1 Jul 2019 18:16:30 -0700
> >
> > On Mon, Jul 1, 2019 at 6:00 PM Shakeel Butt <shakeelb@google.com> wrote:
> > >
> > > On Mon, Jul 1, 2019 at 5:51 PM Henry Burns <henryburns@google.com> wrote:
> > > >
> > > > __SetPageMovable() expects it's page to be locked, but z3fold.c doesn't
> > > > lock the page. Following zsmalloc.c's example we call trylock_page() and
> > > > unlock_page(). Also makes z3fold_page_migrate() assert that newpage is
> > > > passed in locked, as documentation.
>
> The changelog still doesn't mention that this bug triggers a
> VM_BUG_ON_PAGE().  It should do so.  I did this:
>
> : __SetPageMovable() expects its page to be locked, but z3fold.c doesn't
> : lock the page.  This triggers the VM_BUG_ON_PAGE(!PageLocked(page), page)
> : in __SetPageMovable().
> :
> : Following zsmalloc.c's example we call trylock_page() and unlock_page().
> : Also make z3fold_page_migrate() assert that newpage is passed in locked,
> : as per the documentation.
>
> I'll add a cc:stable to this fix.
>
> > > > Signed-off-by: Henry Burns <henryburns@google.com>
> > > > Suggested-by: Vitaly Wool <vitalywool@gmail.com>
> > > > ---
> > > >  Changelog since v1:
> > > >  - Added an if statement around WARN_ON(trylock_page(page)) to avoid
> > > >    unlocking a page locked by a someone else.
> > > >
> > > >  mm/z3fold.c | 6 +++++-
> > > >  1 file changed, 5 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/mm/z3fold.c b/mm/z3fold.c
> > > > index e174d1549734..6341435b9610 100644
> > > > --- a/mm/z3fold.c
> > > > +++ b/mm/z3fold.c
> > > > @@ -918,7 +918,10 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
> > > >                 set_bit(PAGE_HEADLESS, &page->private);
> > > >                 goto headless;
> > > >         }
> > > > -       __SetPageMovable(page, pool->inode->i_mapping);
> > > > +       if (!WARN_ON(!trylock_page(page))) {
> > > > +               __SetPageMovable(page, pool->inode->i_mapping);
> > > > +               unlock_page(page);
> > > > +       }
> > >
> > > Can you please comment why lock_page() is not used here?
>
> Shakeel asked "please comment" (ie, please add a code comment), not
> "please comment on".  Subtle ;)
>
> > Since z3fold_alloc can be called in atomic or non atomic context,
> > calling lock_page() could trigger a number of
> > warnings about might_sleep() being called in atomic context. WARN_ON
> > should avoid the problem described
> > above as well, and in any weird condition where someone else has the
> > page lock, we can avoid calling
> > __SetPageMovable().
>
> I think this will suffice:
>
> --- a/mm/z3fold.c~mm-z3foldc-lock-z3fold-page-before-__setpagemovable-fix
> +++ a/mm/z3fold.c
> @@ -919,6 +919,9 @@ retry:
>                 set_bit(PAGE_HEADLESS, &page->private);
>                 goto headless;
>         }
> +       /*
> +        * z3fold_alloc() can be called from atomic contexts, hence the trylock
> +        */
>         if (!WARN_ON(!trylock_page(page))) {
>                 __SetPageMovable(page, pool->inode->i_mapping);
>                 unlock_page(page);
>
> However this code would be more effective if z3fold_alloc() were to be
> told when it is running in non-atomic context so it can perform a
> sleeping lock_page() in that case.  That's an improvement to consider
> for later, please.
>

z3fold_alloc() can tell when its called in atomic context, new patch incoming!
I'm thinking something like this:

> > > > +       if (can_sleep) {
> > > > +               lock_page(page);
> > > > +               __SetPageMovable(page, pool->inode->i_mapping);
> > > > +               unlock_page(page);
> > > > +       } else {
> > > > +               if (!WARN_ON(!trylock_page(page))) {
> > > > +                       __SetPageMovable(page, pool->inode->i_mapping);
> > > > +                       unlock_page(page);
> > > > +               } else {
> > > > +                       pr_err("Newly allocated z3fold page is locked\n");
> > > > +                       WARN_ON(1);
> > > > +               }
> > > > +       }