On Tue, Mar 30, 2021 at 05:29:33PM +0800, Muchun Song wrote:
> +++ b/fs/fs-writeback.c
> @@ -506,8 +506,10 @@ static void inode_switch_wbs(struct inode *inode, int
> new_wb_id)
> /* find and pin the new wb */
> rcu_read_lock();
> memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys);
> - if (memcg_css)
> + if (memcg_css && css_tryget(memcg_css)) {
> isw->new_wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC);
> + css_put(memcg_css);
> + }
> rcu_read_unlock();
> if (!isw->new_wb)
> goto out_free;
This seems like an unnecessary use of GFP_ATOMIC. Why not:
rcu_read_lock();
memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys);
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;
(inode_switch_wbs can't be called in interrupt context because it takes
inode->i_lock, which is not interrupt-safe. it's not clear to me whether
it is allowed to start IO or do FS reclaim, given where it is in the
I/O path, so i went with GFP_NOIO rather than GFP_KERNEL)
(also there's another use of GFP_ATOMIC in that function, which is
probably wrong)
