On Thu, Apr 1, 2021 at 6:26 PM Michal Hocko <mhocko@xxxxxxxx> 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@xxxxxxxxxxxxx>
>
> For the css part feel free to add
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
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
