On Thu, Nov 14, 2019 at 9:43 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Thu, Nov 14, 2019 at 9:08 PM Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> wrote:
> >
> > On Thu, Nov 14, 2019 at 09:05:26PM -0800, Andrii Nakryiko wrote:
> > > On Thu, Nov 14, 2019 at 8:45 PM Alexei Starovoitov
> > > <alexei.starovoitov@xxxxxxxxx> wrote:
> > > >
> > > > On Thu, Nov 14, 2019 at 08:02:23PM -0800, Andrii Nakryiko wrote:
> > > > > Add ability to memory-map contents of BPF array map. This is extremely useful
> > > > > for working with BPF global data from userspace programs. It allows to avoid
> > > > > typical bpf_map_{lookup,update}_elem operations, improving both performance
> > > > > and usability.
> > > > >
> > > > > There had to be special considerations for map freezing, to avoid having
> > > > > writable memory view into a frozen map. To solve this issue, map freezing and
> > > > > mmap-ing is happening under mutex now:
> > > > > - if map is already frozen, no writable mapping is allowed;
> > > > > - if map has writable memory mappings active (accounted in map->writecnt),
> > > > > map freezing will keep failing with -EBUSY;
> > > > > - once number of writable memory mappings drops to zero, map freezing can be
> > > > > performed again.
> > > > >
> > > > > Only non-per-CPU plain arrays are supported right now. Maps with spinlocks
> > > > > can't be memory mapped either.
> > > > >
> > > > > For BPF_F_MMAPABLE array, memory allocation has to be done through vmalloc()
> > > > > to be mmap()'able. We also need to make sure that array data memory is
> > > > > page-sized and page-aligned, so we over-allocate memory in such a way that
> > > > > struct bpf_array is at the end of a single page of memory with array->value
> > > > > being aligned with the start of the second page. On deallocation we need to
> > > > > accomodate this memory arrangement to free vmalloc()'ed memory correctly.
> > > > >
> > > > > One important consideration regarding how memory-mapping subsystem functions.
> > > > > Memory-mapping subsystem provides few optional callbacks, among them open()
> > > > > and close(). close() is called for each memory region that is unmapped, so
> > > > > that users can decrease their reference counters and free up resources, if
> > > > > necessary. open() is *almost* symmetrical: it's called for each memory region
> > > > > that is being mapped, **except** the very first one. So bpf_map_mmap does
> > > > > initial refcnt bump, while open() will do any extra ones after that. Thus
> > > > > number of close() calls is equal to number of open() calls plus one more.
> > > > >
> > > > > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> > > > > Cc: Rik van Riel <riel@xxxxxxxxxxx>
> > > > > Acked-by: Song Liu <songliubraving@xxxxxx>
> > > > > Acked-by: John Fastabend <john.fastabend@xxxxxxxxx>
> > > > > Signed-off-by: Andrii Nakryiko <andriin@xxxxxx>
> > > > > ---
> > > > > include/linux/bpf.h | 11 ++--
> > > > > include/linux/vmalloc.h | 1 +
> > > > > include/uapi/linux/bpf.h | 3 ++
> > > > > kernel/bpf/arraymap.c | 59 +++++++++++++++++---
> > > > > kernel/bpf/syscall.c | 99 ++++++++++++++++++++++++++++++++--
> > > > > mm/vmalloc.c | 20 +++++++
> > > > > tools/include/uapi/linux/bpf.h | 3 ++
> > > > > 7 files changed, 184 insertions(+), 12 deletions(-)
> > > > >
> > > > > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > > > > index 6fbe599fb977..8021fce98868 100644
> > > > > --- a/include/linux/bpf.h
> > > > > +++ b/include/linux/bpf.h
> > > > > @@ -12,6 +12,7 @@
> > > > > #include <linux/err.h>
> > > > > #include <linux/rbtree_latch.h>
> > > > > #include <linux/numa.h>
> > > > > +#include <linux/mm_types.h>
> > > > > #include <linux/wait.h>
> > > > > #include <linux/u64_stats_sync.h>
> > > > >
> > > > > @@ -66,6 +67,7 @@ struct bpf_map_ops {
> > > > > u64 *imm, u32 off);
> > > > > int (*map_direct_value_meta)(const struct bpf_map *map,
> > > > > u64 imm, u32 *off);
> > > > > + int (*map_mmap)(struct bpf_map *map, struct vm_area_struct *vma);
> > > > > };
> > > > >
> > > > > struct bpf_map_memory {
> > > > > @@ -94,9 +96,10 @@ struct bpf_map {
> > > > > u32 btf_value_type_id;
> > > > > struct btf *btf;
> > > > > struct bpf_map_memory memory;
> > > > > + char name[BPF_OBJ_NAME_LEN];
> > > > > bool unpriv_array;
> > > > > - bool frozen; /* write-once */
> > > > > - /* 48 bytes hole */
> > > > > + bool frozen; /* write-once; write-protected by freeze_mutex */
> > > > > + /* 22 bytes hole */
> > > > >
> > > > > /* The 3rd and 4th cacheline with misc members to avoid false sharing
> > > > > * particularly with refcounting.
> > > > > @@ -104,7 +107,8 @@ struct bpf_map {
> > > > > atomic64_t refcnt ____cacheline_aligned;
> > > > > atomic64_t usercnt;
> > > > > struct work_struct work;
> > > > > - char name[BPF_OBJ_NAME_LEN];
> > > > > + struct mutex freeze_mutex;
> > > > > + u64 writecnt; /* writable mmap cnt; protected by freeze_mutex */
> > > > > };
> > > >
> > > > Can the mutex be moved into bpf_array instead of being in bpf_map that is
> > > > shared across all map types?
> > >
> > > No, freezing logic is common to all maps. Same for writecnt and
> > > mmap()-ing overall.
> >
> > How mmap is going to work for hash map ? and for prog_array?
> >
>
> It probably won't. But one day we might have hash map using open
> adressing, which will be more prone to memory-mapping. Or, say, some
> sort of non-per-CPU ring buffer would be a good candidate as well. It
> doesn't seem like a good idea to restrict mmap()-ability to just array
> for no good reason.
>
> But speaking about freeze_mutex specifically. It's to coordinate
> writable memory-mapping and frozen flag. Even if we make
> memory-mapping bpf_array specific, map->frozen is generic flag and
> handled in syscall.c generically, so I just can't protect it only from
> bpf_array side.
Alternatively we can use spinlock. I don't think it's too ugly, tbh. See below.
>From 0da495b911adad495857f1c0fc3596f1d06a705f Mon Sep 17 00:00:00 2001
From: Andrii Nakryiko <andriin@xxxxxx>
Date: Fri, 15 Nov 2019 08:32:43 -0800
Subject: [PATCH bpf-next] bpf: switch freeze locking to use spin_lock and save
space
Switch to spin_lock in favor of mutex. Due to mmap-ing itself happening not
under spinlock, there needs to be an extra "correction" step for writecnt, if
mapping fails.
Signed-off-by: Andrii Nakryiko <andriin@xxxxxx>
---
include/linux/bpf.h | 6 ++---
kernel/bpf/syscall.c | 52 ++++++++++++++++++++++++--------------------
2 files changed, 31 insertions(+), 27 deletions(-)
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 8021fce98868..ceaefd69c6bb 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -98,7 +98,7 @@ struct bpf_map {
struct bpf_map_memory memory;
char name[BPF_OBJ_NAME_LEN];
bool unpriv_array;
- bool frozen; /* write-once; write-protected by freeze_mutex */
+ bool frozen; /* write-once; write-protected by freeze_lock */
/* 22 bytes hole */
/* The 3rd and 4th cacheline with misc members to avoid false sharing
@@ -107,8 +107,8 @@ struct bpf_map {
atomic64_t refcnt ____cacheline_aligned;
atomic64_t usercnt;
struct work_struct work;
- struct mutex freeze_mutex;
- u64 writecnt; /* writable mmap cnt; protected by freeze_mutex */
+ u64 writecnt; /* writable mmap cnt; protected by freeze_lock */
+ spinlock_t freeze_lock;
};
static inline bool map_value_has_spin_lock(const struct bpf_map *map)
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 810e53990641..fc782ab06d57 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -448,9 +448,9 @@ static void bpf_map_mmap_open(struct vm_area_struct *vma)
bpf_map_inc(map);
if (vma->vm_flags & VM_WRITE) {
- mutex_lock(&map->freeze_mutex);
+ spin_lock(&map->freeze_lock);
map->writecnt++;
- mutex_unlock(&map->freeze_mutex);
+ spin_unlock(&map->freeze_lock);
}
}
@@ -460,9 +460,9 @@ static void bpf_map_mmap_close(struct vm_area_struct *vma)
struct bpf_map *map = vma->vm_file->private_data;
if (vma->vm_flags & VM_WRITE) {
- mutex_lock(&map->freeze_mutex);
+ spin_lock(&map->freeze_lock);
map->writecnt--;
- mutex_unlock(&map->freeze_mutex);
+ spin_unlock(&map->freeze_lock);
}
bpf_map_put(map);
@@ -484,28 +484,34 @@ static int bpf_map_mmap(struct file *filp,
struct vm_area_struct *vma)
if (!(vma->vm_flags & VM_SHARED))
return -EINVAL;
- mutex_lock(&map->freeze_mutex);
-
+ spin_lock(&map->freeze_lock);
if ((vma->vm_flags & VM_WRITE) && map->frozen) {
- err = -EPERM;
- goto out;
+ spin_unlock(&map->freeze_lock);
+ return -EPERM;
}
-
+ /* speculatively increase writecnt assuming mmap will succeed */
+ if (vma->vm_flags & VM_WRITE)
+ map->writecnt++;
+ spin_unlock(&map->freeze_lock);
+
/* set default open/close callbacks */
vma->vm_ops = &bpf_map_default_vmops;
vma->vm_private_data = map;
err = map->ops->map_mmap(map, vma);
- if (err)
- goto out;
+ if (err) {
+ /* mmap failed, undo speculative writecnt increment */
+ if (vma->vm_flags & VM_WRITE) {
+ spin_lock(&map->freeze_lock);
+ map->writecnt--;
+ spin_unlock(&map->freeze_lock);
+ }
+ return err;
+ }
bpf_map_inc(map);
- if (vma->vm_flags & VM_WRITE)
- map->writecnt++;
-out:
- mutex_unlock(&map->freeze_mutex);
- return err;
+ return 0;
}
const struct file_operations bpf_map_fops = {
@@ -661,7 +667,7 @@ static int map_create(union bpf_attr *attr)
atomic64_set(&map->refcnt, 1);
atomic64_set(&map->usercnt, 1);
- mutex_init(&map->freeze_mutex);
+ spin_lock_init(&map->freeze_lock);
if (attr->btf_key_type_id || attr->btf_value_type_id) {
struct btf *btf;
@@ -1244,12 +1250,15 @@ static int map_freeze(const union bpf_attr *attr)
if (CHECK_ATTR(BPF_MAP_FREEZE))
return -EINVAL;
+ if (!capable(CAP_SYS_ADMIN))
+ return -EPERM;
+
f = fdget(ufd);
map = __bpf_map_get(f);
if (IS_ERR(map))
return PTR_ERR(map);
- mutex_lock(&map->freeze_mutex);
+ spin_lock(&map->freeze_lock);
if (map->writecnt) {
err = -EBUSY;
@@ -1259,14 +1268,9 @@ static int map_freeze(const union bpf_attr *attr)
err = -EBUSY;
goto err_put;
}
- if (!capable(CAP_SYS_ADMIN)) {
- err = -EPERM;
- goto err_put;
- }
-
WRITE_ONCE(map->frozen, true);
err_put:
- mutex_unlock(&map->freeze_mutex);
+ spin_unlock(&map->freeze_lock);
fdput(f);
return err;
}
--
2.17.1
