* [PATCH v2] writeback: fix obtain a reference to a freeing memcg css
@ 2021-04-01 9:33 Muchun Song
2021-04-01 10:26 ` Michal Hocko
0 siblings, 1 reply; 5+ messages in thread
From: Muchun Song @ 2021-04-01 9:33 UTC (permalink / raw)
To: viro, tj, axboe, willy; +Cc: linux-fsdevel, linux-kernel, Muchun Song
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.
Fixes: 682aa8e1a6a1 ("writeback: implement unlocked_inode_to_wb transaction and use it for stat updates")
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
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
^ permalink raw reply related [flat|nested] 5+ messages in thread
* Re: [PATCH v2] writeback: fix obtain a reference to a freeing memcg css
2021-04-01 9:33 [PATCH v2] writeback: fix obtain a reference to a freeing memcg css Muchun Song
@ 2021-04-01 10:26 ` Michal Hocko
2021-04-01 13:59 ` [External] " Muchun Song
0 siblings, 1 reply; 5+ messages in thread
From: Michal Hocko @ 2021-04-01 10:26 UTC (permalink / raw)
To: Muchun Song; +Cc: viro, tj, axboe, willy, linux-fsdevel, linux-kernel
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@bytedance.com>
For the css part feel free to add
Acked-by: Michal Hocko <mhocko@suse.com>
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
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [External] Re: [PATCH v2] writeback: fix obtain a reference to a freeing memcg css
2021-04-01 10:26 ` Michal Hocko
@ 2021-04-01 13:59 ` Muchun Song
2021-04-01 14:37 ` Michal Hocko
0 siblings, 1 reply; 5+ messages in thread
From: Muchun Song @ 2021-04-01 13:59 UTC (permalink / raw)
To: Michal Hocko
Cc: Alexander Viro, Tejun Heo, axboe, Matthew Wilcox, linux-fsdevel,
LKML
On Thu, Apr 1, 2021 at 6:26 PM Michal Hocko <mhocko@suse.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@bytedance.com>
>
> For the css part feel free to add
> Acked-by: Michal Hocko <mhocko@suse.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
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [External] Re: [PATCH v2] writeback: fix obtain a reference to a freeing memcg css
2021-04-01 13:59 ` [External] " Muchun Song
@ 2021-04-01 14:37 ` Michal Hocko
2021-04-01 14:39 ` Muchun Song
0 siblings, 1 reply; 5+ messages in thread
From: Michal Hocko @ 2021-04-01 14:37 UTC (permalink / raw)
To: Muchun Song
Cc: Alexander Viro, Tejun Heo, axboe, Matthew Wilcox, linux-fsdevel,
LKML
On Thu 01-04-21 21:59:13, Muchun Song wrote:
> On Thu, Apr 1, 2021 at 6:26 PM Michal Hocko <mhocko@suse.com> wrote:
[...]
> > 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.
I do not think this is necessary. Taking the reference is a standard
way and I am not sure it requires a comment. I meant to say that not
having a reference should really have a comment explaining why.
Thanks!
--
Michal Hocko
SUSE Labs
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [External] Re: [PATCH v2] writeback: fix obtain a reference to a freeing memcg css
2021-04-01 14:37 ` Michal Hocko
@ 2021-04-01 14:39 ` Muchun Song
0 siblings, 0 replies; 5+ messages in thread
From: Muchun Song @ 2021-04-01 14:39 UTC (permalink / raw)
To: Michal Hocko
Cc: Alexander Viro, Tejun Heo, axboe, Matthew Wilcox, linux-fsdevel,
LKML
On Thu, Apr 1, 2021 at 10:37 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Thu 01-04-21 21:59:13, Muchun Song wrote:
> > On Thu, Apr 1, 2021 at 6:26 PM Michal Hocko <mhocko@suse.com> wrote:
> [...]
> > > 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.
>
> I do not think this is necessary. Taking the reference is a standard
> way and I am not sure it requires a comment. I meant to say that not
> having a reference should really have a comment explaining why.
Very Sorry. I got it wrong, thank you for further explanation.
>
> Thanks!
> --
> Michal Hocko
> SUSE Labs
^ permalink raw reply [flat|nested] 5+ messages in thread
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2021-04-01 9:33 [PATCH v2] writeback: fix obtain a reference to a freeing memcg css Muchun Song
2021-04-01 10:26 ` Michal Hocko
2021-04-01 13:59 ` [External] " Muchun Song
2021-04-01 14:37 ` Michal Hocko
2021-04-01 14:39 ` Muchun Song
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