linux-kernel - Re: [External] Re: [PATCH v2] writeback: fix obtain a reference to a freeing memcg css

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAMZfGtX9V898aezb-huMEYU_-NjqfL6HbXeaZr2Q2MUa+VG3qQ@mail.gmail.com>
Date:   Thu, 1 Apr 2021 21:59:13 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Tejun Heo <tj@...nel.org>, axboe@...com,
        Matthew Wilcox <willy@...radead.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [External] Re: [PATCH v2] writeback: fix obtain a reference to a
 freeing memcg css

On Thu, Apr 1, 2021 at 6:26 PM Michal Hocko <mhocko@...e.com> wrote:
>
> On Thu 01-04-21 17:33:43, Muchun Song wrote:
> > The caller of wb_get_create() should pin the memcg, because
> > wb_get_create() relies on this guarantee. The rcu read lock
> > only can guarantee that the memcg css returned by css_from_id()
> > cannot be released, but the reference of the memcg can be zero.
> > Fix it by holding a reference to the css before calling
> > wb_get_create(). This is not a problem I encountered in the
> > real world. Just the result of a code review.
> >
> > And it is unnecessary to use GFP_ATOMIC, so replace it with
> > GFP_NOIO.
>
> This should go into it's own patch. With more explanation why NOIO is
> required.

OK. Replacing GFP_ATOMIC with NOIO should be a separate
patch. I will remove this in the next version.

>
> > Fixes: 682aa8e1a6a1 ("writeback: implement unlocked_inode_to_wb transaction and use it for stat updates")
> > Signed-off-by: Muchun Song <songmuchun@...edance.com>
>
> For the css part feel free to add
> Acked-by: Michal Hocko <mhocko@...e.com>

Thanks.

>
> Even if the css ref count is not really necessary it shouldn't cause any
> harm and it makes the code easier to understand. At least a comment
> explaining why that is not necessary would be required without it

OK. I will add a comment here to explain why we need to hold a
ref.

>
> Thanks!
>
> > ---
> > Changelog in v2:
> >  1. Replace GFP_ATOMIC with GFP_NOIO suggested by Matthew.
> >
> >  fs/fs-writeback.c | 11 ++++++++---
> >  1 file changed, 8 insertions(+), 3 deletions(-)
> >
> > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> > index e91980f49388..df7f89f8f771 100644
> > --- a/fs/fs-writeback.c
> > +++ b/fs/fs-writeback.c
> > @@ -501,16 +501,21 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
> >       if (atomic_read(&isw_nr_in_flight) > WB_FRN_MAX_IN_FLIGHT)
> >               return;
> >
> > -     isw = kzalloc(sizeof(*isw), GFP_ATOMIC);
> > +     isw = kzalloc(sizeof(*isw), GFP_NOIO);
> >       if (!isw)
> >               return;
> >
> >       /* find and pin the new wb */
> >       rcu_read_lock();
> >       memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys);
> > -     if (memcg_css)
> > -             isw->new_wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
> > +     if (memcg_css && !css_tryget(memcg_css))
> > +             memcg_css = NULL;
> >       rcu_read_unlock();
> > +     if (!memcg_css)
> > +             goto out_free;
> > +
> > +     isw->new_wb = wb_get_create(bdi, memcg_css, GFP_NOIO);
> > +     css_put(memcg_css);
> >       if (!isw->new_wb)
> >               goto out_free;
> >
> > --
> > 2.11.0
>
> --
> Michal Hocko
> SUSE Labs