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.
> 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>
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.
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
