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Date: Wed, 11 Apr 2018 01:03:43 +0000
From: Greg Thelen <gthelen@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Wang Long <wanglong19@...tuan.com>,
Michal Hocko <mhocko@...nel.org>,
Johannes Weiner <hannes@...xchg.org>,
Tejun Heo <tj@...nel.org>, npiggin@...il.com,
LKML <linux-kernel@...r.kernel.org>,
Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH v3] writeback: safer lock nesting
On Tue, Apr 10, 2018 at 1:38 PM Andrew Morton <akpm@...ux-foundation.org>
wrote:
> On Mon, 9 Apr 2018 17:59:08 -0700 Greg Thelen <gthelen@...gle.com> wrote:
> > lock_page_memcg()/unlock_page_memcg() use spin_lock_irqsave/restore() if
> > the page's memcg is undergoing move accounting, which occurs when a
> > process leaves its memcg for a new one that has
> > memory.move_charge_at_immigrate set.
> >
> > unlocked_inode_to_wb_begin,end() use spin_lock_irq/spin_unlock_irq() if
the
> > given inode is switching writeback domains. Switches occur when enough
> > writes are issued from a new domain.
> >
> > This existing pattern is thus suspicious:
> > lock_page_memcg(page);
> > unlocked_inode_to_wb_begin(inode, &locked);
> > ...
> > unlocked_inode_to_wb_end(inode, locked);
> > unlock_page_memcg(page);
> >
> > If both inode switch and process memcg migration are both in-flight then
> > unlocked_inode_to_wb_end() will unconditionally enable interrupts while
> > still holding the lock_page_memcg() irq spinlock. This suggests the
> > possibility of deadlock if an interrupt occurs before
> > unlock_page_memcg().
> >
> > truncate
> > __cancel_dirty_page
> > lock_page_memcg
> > unlocked_inode_to_wb_begin
> > unlocked_inode_to_wb_end
> > <interrupts mistakenly enabled>
> > <interrupt>
> > end_page_writeback
> > test_clear_page_writeback
> > lock_page_memcg
> > <deadlock>
> > unlock_page_memcg
> >
> > Due to configuration limitations this deadlock is not currently possible
> > because we don't mix cgroup writeback (a cgroupv2 feature) and
> > memory.move_charge_at_immigrate (a cgroupv1 feature).
> >
> > If the kernel is hacked to always claim inode switching and memcg
> > moving_account, then this script triggers lockup in less than a minute:
> > cd /mnt/cgroup/memory
> > mkdir a b
> > echo 1 > a/memory.move_charge_at_immigrate
> > echo 1 > b/memory.move_charge_at_immigrate
> > (
> > echo $BASHPID > a/cgroup.procs
> > while true; do
> > dd if=/dev/zero of=/mnt/big bs=1M count=256
> > done
> > ) &
> > while true; do
> > sync
> > done &
> > sleep 1h &
> > SLEEP=$!
> > while true; do
> > echo $SLEEP > a/cgroup.procs
> > echo $SLEEP > b/cgroup.procs
> > done
> >
> > Given the deadlock is not currently possible, it's debatable if there's
> > any reason to modify the kernel. I suggest we should to prevent future
> > surprises.
> >
> > ...
> >
> > Changelog since v2:
> > - explicitly initialize wb_lock_cookie to silence compiler warnings.
> But only in some places. What's up with that?
I annotated it in places where my compiler was complaining about. But
you're right. It's better to init all 4.
> >
> > ...
> >
> > --- a/include/linux/backing-dev.h
> > +++ b/include/linux/backing-dev.h
> > @@ -346,7 +346,7 @@ static inline struct bdi_writeback
*inode_to_wb(const struct inode *inode)
> > /**
> > * unlocked_inode_to_wb_begin - begin unlocked inode wb access
transaction
> > * @inode: target inode
> > - * @lockedp: temp bool output param, to be passed to the end function
> > + * @cookie: output param, to be passed to the end function
> > *
> > * The caller wants to access the wb associated with @inode but isn't
> > * holding inode->i_lock, mapping->tree_lock or wb->list_lock. This
> > @@ -354,12 +354,11 @@ static inline struct bdi_writeback
*inode_to_wb(const struct inode *inode)
> > * association doesn't change until the transaction is finished with
> > * unlocked_inode_to_wb_end().
> > *
> > - * The caller must call unlocked_inode_to_wb_end() with *@...kdep
> > - * afterwards and can't sleep during transaction. IRQ may or may not
be
> > - * disabled on return.
> > + * The caller must call unlocked_inode_to_wb_end() with *@...kie
afterwards and
> > + * can't sleep during transaction. IRQ may or may not be disabled on
return.
> > */
> Grammar is a bit awkward here,
> >
> > ...
> >
> > --- a/mm/page-writeback.c
> > +++ b/mm/page-writeback.c
> > @@ -2501,13 +2501,13 @@ void account_page_redirty(struct page *page)
> > if (mapping && mapping_cap_account_dirty(mapping)) {
> > struct inode *inode = mapping->host;
> > struct bdi_writeback *wb;
> > - bool locked;
> > + struct wb_lock_cookie cookie = {0};
> Trivia: it's better to use "= {}" here. That has the same effect and
> it doesn't assume that the first field is a scalar. And indeed, the
> first field is a bool so it should be {false}!
Nod. Thanks for the tip.
> So...
> --- a/include/linux/backing-dev.h~writeback-safer-lock-nesting-fix
> +++ a/include/linux/backing-dev.h
> @@ -355,7 +355,8 @@ static inline struct bdi_writeback *inod
> * unlocked_inode_to_wb_end().
> *
> * The caller must call unlocked_inode_to_wb_end() with *@...kie
afterwards and
> - * can't sleep during transaction. IRQ may or may not be disabled on
return.
> + * can't sleep during the transaction. IRQs may or may not be disabled
on
> + * return.
> */
> static inline struct bdi_writeback *
> unlocked_inode_to_wb_begin(struct inode *inode, struct wb_lock_cookie
*cookie)
> --- a/mm/page-writeback.c~writeback-safer-lock-nesting-fix
> +++ a/mm/page-writeback.c
> @@ -2501,7 +2501,7 @@ void account_page_redirty(struct page *p
> if (mapping && mapping_cap_account_dirty(mapping)) {
> struct inode *inode = mapping->host;
> struct bdi_writeback *wb;
> - struct wb_lock_cookie cookie = {0};
> + struct wb_lock_cookie cookie = {};
> wb = unlocked_inode_to_wb_begin(inode, &cookie);
> current->nr_dirtied--;
> @@ -2613,7 +2613,7 @@ void __cancel_dirty_page(struct page *pa
> if (mapping_cap_account_dirty(mapping)) {
> struct inode *inode = mapping->host;
> struct bdi_writeback *wb;
> - struct wb_lock_cookie cookie = {0};
> + struct wb_lock_cookie cookie = {};
> lock_page_memcg(page);
> wb = unlocked_inode_to_wb_begin(inode, &cookie);
> @@ -2653,7 +2653,7 @@ int clear_page_dirty_for_io(struct page
> if (mapping && mapping_cap_account_dirty(mapping)) {
> struct inode *inode = mapping->host;
> struct bdi_writeback *wb;
> - struct wb_lock_cookie cookie = {0};
> + struct wb_lock_cookie cookie = {};
> /*
> * Yes, Virginia, this is indeed insane.
> But I wonder about the remaining uninitialized wb_lock_cookies?
Yeah, I'll post an v4 with everything folded in.
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