| Message ID | 20200625113122.7540-2-willy@infradead.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Overhaul memalloc_no* | expand |
On Thu 25-06-20 12:31:17, Matthew Wilcox wrote: > We're short on PF_* flags, so make memalloc_noio its own bit where we > have plenty of space. I do not mind moving that outside of the PF_* space. Unless I misremember all flags in this space were intented to be set only on the current which rules out any RMW races and therefore they can be lockless. I am not sure this holds for the bitfield you are adding this to. At least in_memstall seem to be set on external task as well. But this would require double checking. Maybe that is not really intended or just a bug. > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org> > --- > drivers/block/loop.c | 3 ++- > drivers/md/dm-zoned-metadata.c | 5 ++--- > include/linux/sched.h | 2 +- > include/linux/sched/mm.h | 30 +++++++++++++++++++++++------- > kernel/sys.c | 8 +++----- > 5 files changed, 31 insertions(+), 17 deletions(-) > > diff --git a/drivers/block/loop.c b/drivers/block/loop.c > index 475e1a738560..c8742e25e58a 100644 > --- a/drivers/block/loop.c > +++ b/drivers/block/loop.c > @@ -52,6 +52,7 @@ > #include <linux/module.h> > #include <linux/moduleparam.h> > #include <linux/sched.h> > +#include <linux/sched/mm.h> > #include <linux/fs.h> > #include <linux/file.h> > #include <linux/stat.h> > @@ -929,7 +930,7 @@ static void loop_unprepare_queue(struct loop_device *lo) > > static int loop_kthread_worker_fn(void *worker_ptr) > { > - current->flags |= PF_LOCAL_THROTTLE | PF_MEMALLOC_NOIO; > + set_current_io_flusher(); > return kthread_worker_fn(worker_ptr); > } > > diff --git a/drivers/md/dm-zoned-metadata.c b/drivers/md/dm-zoned-metadata.c > index 130b5a6d9f12..1c5ae674ba20 100644 > --- a/drivers/md/dm-zoned-metadata.c > +++ b/drivers/md/dm-zoned-metadata.c > @@ -1599,9 +1599,8 @@ static int dmz_update_zone(struct dmz_metadata *zmd, struct dm_zone *zone) > > /* > * Get zone information from disk. Since blkdev_report_zones() uses > - * GFP_KERNEL by default for memory allocations, set the per-task > - * PF_MEMALLOC_NOIO flag so that all allocations are done as if > - * GFP_NOIO was specified. > + * GFP_KERNEL by default for memory allocations, use > + * memalloc_noio_save() to prevent recursion into the driver. > */ > noio_flag = memalloc_noio_save(); > ret = blkdev_report_zones(dev->bdev, dmz_start_sect(zmd, zone), 1, > diff --git a/include/linux/sched.h b/include/linux/sched.h > index b62e6aaf28f0..cf18a3d2bc4c 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -801,6 +801,7 @@ struct task_struct { > /* Stalled due to lack of memory */ > unsigned in_memstall:1; > #endif > + unsigned memalloc_noio:1; > > unsigned long atomic_flags; /* Flags requiring atomic access. */ > > @@ -1505,7 +1506,6 @@ extern struct pid *cad_pid; > #define PF_FROZEN 0x00010000 /* Frozen for system suspend */ > #define PF_KSWAPD 0x00020000 /* I am kswapd */ > #define PF_MEMALLOC_NOFS 0x00040000 /* All allocation requests will inherit GFP_NOFS */ > -#define PF_MEMALLOC_NOIO 0x00080000 /* All allocation requests will inherit GFP_NOIO */ > #define PF_LOCAL_THROTTLE 0x00100000 /* Throttle writes only against the bdi I write to, > * I am cleaning dirty pages from some other bdi. */ > #define PF_KTHREAD 0x00200000 /* I am a kernel thread */ > diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h > index 480a4d1b7dd8..1a7e1ab1be85 100644 > --- a/include/linux/sched/mm.h > +++ b/include/linux/sched/mm.h > @@ -175,19 +175,18 @@ static inline bool in_vfork(struct task_struct *tsk) > > /* > * Applies per-task gfp context to the given allocation flags. > - * PF_MEMALLOC_NOIO implies GFP_NOIO > * PF_MEMALLOC_NOFS implies GFP_NOFS > * PF_MEMALLOC_NOCMA implies no allocation from CMA region. > */ > static inline gfp_t current_gfp_context(gfp_t flags) > { > - if (unlikely(current->flags & > - (PF_MEMALLOC_NOIO | PF_MEMALLOC_NOFS | PF_MEMALLOC_NOCMA))) { > + if (unlikely(current->flags & (PF_MEMALLOC_NOFS | PF_MEMALLOC_NOCMA) || > + current->memalloc_noio)) { > /* > * NOIO implies both NOIO and NOFS and it is a weaker context > * so always make sure it makes precedence > */ > - if (current->flags & PF_MEMALLOC_NOIO) > + if (current->memalloc_noio) > flags &= ~(__GFP_IO | __GFP_FS); > else if (current->flags & PF_MEMALLOC_NOFS) > flags &= ~__GFP_FS; > @@ -224,8 +223,8 @@ static inline void fs_reclaim_release(gfp_t gfp_mask) { } > */ > static inline unsigned int memalloc_noio_save(void) > { > - unsigned int flags = current->flags & PF_MEMALLOC_NOIO; > - current->flags |= PF_MEMALLOC_NOIO; > + unsigned int flags = current->memalloc_noio; > + current->memalloc_noio = 1; > return flags; > } > > @@ -239,7 +238,7 @@ static inline unsigned int memalloc_noio_save(void) > */ > static inline void memalloc_noio_restore(unsigned int flags) > { > - current->flags = (current->flags & ~PF_MEMALLOC_NOIO) | flags; > + current->memalloc_noio = flags ? 1 : 0; > } > > /** > @@ -309,6 +308,23 @@ static inline void memalloc_nocma_restore(unsigned int flags) > } > #endif > > +static inline void set_current_io_flusher(void) > +{ > + current->flags |= PF_LOCAL_THROTTLE; > + current->memalloc_noio = 1; > +} > + > +static inline void clear_current_io_flusher(void) > +{ > + current->flags &= ~PF_LOCAL_THROTTLE; > + current->memalloc_noio = 0; > +} > + > +static inline bool get_current_io_flusher(void) > +{ > + return current->flags & PF_LOCAL_THROTTLE; > +} > + > #ifdef CONFIG_MEMCG > /** > * memalloc_use_memcg - Starts the remote memcg charging scope. > diff --git a/kernel/sys.c b/kernel/sys.c > index 00a96746e28a..78c90d1e92f4 100644 > --- a/kernel/sys.c > +++ b/kernel/sys.c > @@ -2275,8 +2275,6 @@ int __weak arch_prctl_spec_ctrl_set(struct task_struct *t, unsigned long which, > return -EINVAL; > } > > -#define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LOCAL_THROTTLE) > - > SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, > unsigned long, arg4, unsigned long, arg5) > { > @@ -2512,9 +2510,9 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, > return -EINVAL; > > if (arg2 == 1) > - current->flags |= PR_IO_FLUSHER; > + set_current_io_flusher(); > else if (!arg2) > - current->flags &= ~PR_IO_FLUSHER; > + clear_current_io_flusher(); > else > return -EINVAL; > break; > @@ -2525,7 +2523,7 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, > if (arg2 || arg3 || arg4 || arg5) > return -EINVAL; > > - error = (current->flags & PR_IO_FLUSHER) == PR_IO_FLUSHER; > + error = get_current_io_flusher(); > break; > default: > error = -EINVAL; > -- > 2.27.0 >
On Thu, Jun 25, 2020 at 02:22:39PM +0200, Michal Hocko wrote: > On Thu 25-06-20 12:31:17, Matthew Wilcox wrote: > > We're short on PF_* flags, so make memalloc_noio its own bit where we > > have plenty of space. > > I do not mind moving that outside of the PF_* space. Unless I > misremember all flags in this space were intented to be set only on the > current which rules out any RMW races and therefore they can be > lockless. I am not sure this holds for the bitfield you are adding this > to. At least in_memstall seem to be set on external task as well. But > this would require double checking. Maybe that is not really intended or > just a bug. I was going from the comment: /* Unserialized, strictly 'current' */ (which you can't see from the context of the diff, but is above the block) The situation with ->flags is a little more ambiguous: /* * Only the _current_ task can read/write to tsk->flags, but other * tasks can access tsk->flags in readonly mode for example * with tsk_used_math (like during threaded core dumping). * There is however an exception to this rule during ptrace * or during fork: the ptracer task is allowed to write to the * child->flags of its traced child (same goes for fork, the parent * can write to the child->flags), because we're guaranteed the * child is not running and in turn not changing child->flags * at the same time the parent does it. */ but it wasn't unsafe to use the PF_ flags in the way that you were. It's just crowded. If in_memstall is set on other tasks, then it should be moved to the PFA flags, which there are plenty of. But a quick grep shows it only being read on other tasks and always set on current: kernel/sched/psi.c: *flags = current->in_memstall; kernel/sched/psi.c: * in_memstall setting & accounting needs to be atomic wrt kernel/sched/psi.c: current->in_memstall = 1; kernel/sched/psi.c: * in_memstall clearing & accounting needs to be atomic wrt kernel/sched/psi.c: current->in_memstall = 0; kernel/sched/psi.c: if (task->in_memstall) kernel/sched/stats.h: if (p->in_memstall) kernel/sched/stats.h: if (p->in_memstall) kernel/sched/stats.h: if (unlikely(p->in_iowait || p->in_memstall)) { kernel/sched/stats.h: if (p->in_memstall) kernel/sched/stats.h: if (unlikely(rq->curr->in_memstall)) so I think everything is fine.
On Thu 25-06-20 13:34:18, Matthew Wilcox wrote: > On Thu, Jun 25, 2020 at 02:22:39PM +0200, Michal Hocko wrote: > > On Thu 25-06-20 12:31:17, Matthew Wilcox wrote: > > > We're short on PF_* flags, so make memalloc_noio its own bit where we > > > have plenty of space. > > > > I do not mind moving that outside of the PF_* space. Unless I > > misremember all flags in this space were intented to be set only on the > > current which rules out any RMW races and therefore they can be > > lockless. I am not sure this holds for the bitfield you are adding this > > to. At least in_memstall seem to be set on external task as well. But > > this would require double checking. Maybe that is not really intended or > > just a bug. > > I was going from the comment: > > /* Unserialized, strictly 'current' */ > (which you can't see from the context of the diff, but is above the block) > > The situation with ->flags is a little more ambiguous: > > /* > * Only the _current_ task can read/write to tsk->flags, but other > * tasks can access tsk->flags in readonly mode for example > * with tsk_used_math (like during threaded core dumping). > * There is however an exception to this rule during ptrace > * or during fork: the ptracer task is allowed to write to the > * child->flags of its traced child (same goes for fork, the parent > * can write to the child->flags), because we're guaranteed the > * child is not running and in turn not changing child->flags > * at the same time the parent does it. > */ OK, I have obviously missed that. > but it wasn't unsafe to use the PF_ flags in the way that you were. > It's just crowded. > > If in_memstall is set on other tasks, then it should be moved to the > PFA flags, which there are plenty of. > > But a quick grep shows it only being read on other tasks and always > set on current: > > kernel/sched/psi.c: *flags = current->in_memstall; > kernel/sched/psi.c: * in_memstall setting & accounting needs to be atomic wrt > kernel/sched/psi.c: current->in_memstall = 1; > kernel/sched/psi.c: * in_memstall clearing & accounting needs to be atomic wrt > kernel/sched/psi.c: current->in_memstall = 0; > kernel/sched/psi.c: if (task->in_memstall) Have a look at cgroup_move_task. So I believe this is something to be fixed but independent on your change. Feel free to add Acked-by: Michal Hocko <mhocko@suse.com> > kernel/sched/stats.h: if (p->in_memstall) > kernel/sched/stats.h: if (p->in_memstall) > kernel/sched/stats.h: if (unlikely(p->in_iowait || p->in_memstall)) { > kernel/sched/stats.h: if (p->in_memstall) > kernel/sched/stats.h: if (unlikely(rq->curr->in_memstall)) > > so I think everything is fine.
diff --git a/drivers/block/loop.c b/drivers/block/loop.c index 475e1a738560..c8742e25e58a 100644 --- a/drivers/block/loop.c +++ b/drivers/block/loop.c @@ -52,6 +52,7 @@ #include <linux/module.h> #include <linux/moduleparam.h> #include <linux/sched.h> +#include <linux/sched/mm.h> #include <linux/fs.h> #include <linux/file.h> #include <linux/stat.h> @@ -929,7 +930,7 @@ static void loop_unprepare_queue(struct loop_device *lo) static int loop_kthread_worker_fn(void *worker_ptr) { - current->flags |= PF_LOCAL_THROTTLE | PF_MEMALLOC_NOIO; + set_current_io_flusher(); return kthread_worker_fn(worker_ptr); } diff --git a/drivers/md/dm-zoned-metadata.c b/drivers/md/dm-zoned-metadata.c index 130b5a6d9f12..1c5ae674ba20 100644 --- a/drivers/md/dm-zoned-metadata.c +++ b/drivers/md/dm-zoned-metadata.c @@ -1599,9 +1599,8 @@ static int dmz_update_zone(struct dmz_metadata *zmd, struct dm_zone *zone) /* * Get zone information from disk. Since blkdev_report_zones() uses - * GFP_KERNEL by default for memory allocations, set the per-task - * PF_MEMALLOC_NOIO flag so that all allocations are done as if - * GFP_NOIO was specified. + * GFP_KERNEL by default for memory allocations, use + * memalloc_noio_save() to prevent recursion into the driver. */ noio_flag = memalloc_noio_save(); ret = blkdev_report_zones(dev->bdev, dmz_start_sect(zmd, zone), 1, diff --git a/include/linux/sched.h b/include/linux/sched.h index b62e6aaf28f0..cf18a3d2bc4c 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -801,6 +801,7 @@ struct task_struct { /* Stalled due to lack of memory */ unsigned in_memstall:1; #endif + unsigned memalloc_noio:1; unsigned long atomic_flags; /* Flags requiring atomic access. */ @@ -1505,7 +1506,6 @@ extern struct pid *cad_pid; #define PF_FROZEN 0x00010000 /* Frozen for system suspend */ #define PF_KSWAPD 0x00020000 /* I am kswapd */ #define PF_MEMALLOC_NOFS 0x00040000 /* All allocation requests will inherit GFP_NOFS */ -#define PF_MEMALLOC_NOIO 0x00080000 /* All allocation requests will inherit GFP_NOIO */ #define PF_LOCAL_THROTTLE 0x00100000 /* Throttle writes only against the bdi I write to, * I am cleaning dirty pages from some other bdi. */ #define PF_KTHREAD 0x00200000 /* I am a kernel thread */ diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h index 480a4d1b7dd8..1a7e1ab1be85 100644 --- a/include/linux/sched/mm.h +++ b/include/linux/sched/mm.h @@ -175,19 +175,18 @@ static inline bool in_vfork(struct task_struct *tsk) /* * Applies per-task gfp context to the given allocation flags. - * PF_MEMALLOC_NOIO implies GFP_NOIO * PF_MEMALLOC_NOFS implies GFP_NOFS * PF_MEMALLOC_NOCMA implies no allocation from CMA region. */ static inline gfp_t current_gfp_context(gfp_t flags) { - if (unlikely(current->flags & - (PF_MEMALLOC_NOIO | PF_MEMALLOC_NOFS | PF_MEMALLOC_NOCMA))) { + if (unlikely(current->flags & (PF_MEMALLOC_NOFS | PF_MEMALLOC_NOCMA) || + current->memalloc_noio)) { /* * NOIO implies both NOIO and NOFS and it is a weaker context * so always make sure it makes precedence */ - if (current->flags & PF_MEMALLOC_NOIO) + if (current->memalloc_noio) flags &= ~(__GFP_IO | __GFP_FS); else if (current->flags & PF_MEMALLOC_NOFS) flags &= ~__GFP_FS; @@ -224,8 +223,8 @@ static inline void fs_reclaim_release(gfp_t gfp_mask) { } */ static inline unsigned int memalloc_noio_save(void) { - unsigned int flags = current->flags & PF_MEMALLOC_NOIO; - current->flags |= PF_MEMALLOC_NOIO; + unsigned int flags = current->memalloc_noio; + current->memalloc_noio = 1; return flags; } @@ -239,7 +238,7 @@ static inline unsigned int memalloc_noio_save(void) */ static inline void memalloc_noio_restore(unsigned int flags) { - current->flags = (current->flags & ~PF_MEMALLOC_NOIO) | flags; + current->memalloc_noio = flags ? 1 : 0; } /** @@ -309,6 +308,23 @@ static inline void memalloc_nocma_restore(unsigned int flags) } #endif +static inline void set_current_io_flusher(void) +{ + current->flags |= PF_LOCAL_THROTTLE; + current->memalloc_noio = 1; +} + +static inline void clear_current_io_flusher(void) +{ + current->flags &= ~PF_LOCAL_THROTTLE; + current->memalloc_noio = 0; +} + +static inline bool get_current_io_flusher(void) +{ + return current->flags & PF_LOCAL_THROTTLE; +} + #ifdef CONFIG_MEMCG /** * memalloc_use_memcg - Starts the remote memcg charging scope. diff --git a/kernel/sys.c b/kernel/sys.c index 00a96746e28a..78c90d1e92f4 100644 --- a/kernel/sys.c +++ b/kernel/sys.c @@ -2275,8 +2275,6 @@ int __weak arch_prctl_spec_ctrl_set(struct task_struct *t, unsigned long which, return -EINVAL; } -#define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LOCAL_THROTTLE) - SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, unsigned long, arg4, unsigned long, arg5) { @@ -2512,9 +2510,9 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, return -EINVAL; if (arg2 == 1) - current->flags |= PR_IO_FLUSHER; + set_current_io_flusher(); else if (!arg2) - current->flags &= ~PR_IO_FLUSHER; + clear_current_io_flusher(); else return -EINVAL; break; @@ -2525,7 +2523,7 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, if (arg2 || arg3 || arg4 || arg5) return -EINVAL; - error = (current->flags & PR_IO_FLUSHER) == PR_IO_FLUSHER; + error = get_current_io_flusher(); break; default: error = -EINVAL;
We're short on PF_* flags, so make memalloc_noio its own bit where we have plenty of space. Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org> --- drivers/block/loop.c | 3 ++- drivers/md/dm-zoned-metadata.c | 5 ++--- include/linux/sched.h | 2 +- include/linux/sched/mm.h | 30 +++++++++++++++++++++++------- kernel/sys.c | 8 +++----- 5 files changed, 31 insertions(+), 17 deletions(-)