Re: [RFC PATCH v2 02/13] epoll: introduce user structures for polling from userspace
On 2019-01-21 22:34, Linus Torvalds wrote:
So I'm not entirely convinced, but I guess actual numbers and users
might convince me otherwise.
However, a quick comment:
On Tue, Jan 22, 2019 at 9:15 AM Roman Penyaev wrote:
+struct epoll_uitem {
+ __poll_t ready_events;
+ struct epoll_event event;
+};
This really ends up being a horrible data structure.
struct epoll_event is declared as
struct epoll_event {
__poll_t events;
__u64 data;
} EPOLL_PACKED;
and __poll_t is "unsigned". So on pretty much all 64-bit architectures
except for x86-64 (which sets that packed attribute), you have a
packing hole there in between the events and the data, and "struct
epoll_event" has 8-byte alignment.
Now, in "struct epoll_uitem", you end up having *another* packing hold
in between "ready_events" and "struct epoll_event".
So this data structure that has 16 bytes of actual data, ends up being
24 bytes in size.
Again, x86-64 happens to be the exception to this, but that's a random
small implementation detail, not a design thing.
I think "struct epoll_event" was badly designed to begin with to have
this issue, but it shouldn't then be an excuse to make things even
worse with this array of "struct epoll_uitem" things.
Hmm?
Ha! Yes, you are right. Eyes see "packed" and brain responds
"ok, this is 12 bytes, + 4 for ready_events = 16, perfect".
I have not paid any attention to how actually this EPOLL_PACKED is
defined. Not nice at all. I will unfold the structure like this:
/*
* Item, shared with userspace. Unfortunately we can't embed
epoll_event
* structure, because it is badly aligned on all 64-bit archs, except
* x86-64 (see EPOLL_PACKED). sizeof(epoll_uitem) == 16
*/
struct epoll_uitem {
__poll_t ready_events;
__poll_t events;
__u64 data;
};
Also BUILD_BUG_ON(sizeof(epoll_uitem) != 16) somewhere in alloc won't
hurt.
--
Roman
Re: [RFC PATCH v2 02/13] epoll: introduce user structures for polling from userspace
So I'm not entirely convinced, but I guess actual numbers and users
might convince me otherwise.
However, a quick comment:
On Tue, Jan 22, 2019 at 9:15 AM Roman Penyaev wrote:
>
> +struct epoll_uitem {
> + __poll_t ready_events;
> + struct epoll_event event;
> +};
This really ends up being a horrible data structure.
struct epoll_event is declared as
struct epoll_event {
__poll_t events;
__u64 data;
} EPOLL_PACKED;
and __poll_t is "unsigned". So on pretty much all 64-bit architectures
except for x86-64 (which sets that packed attribute), you have a
packing hole there in between the events and the data, and "struct
epoll_event" has 8-byte alignment.
Now, in "struct epoll_uitem", you end up having *another* packing hold
in between "ready_events" and "struct epoll_event".
So this data structure that has 16 bytes of actual data, ends up being
24 bytes in size.
Again, x86-64 happens to be the exception to this, but that's a random
small implementation detail, not a design thing.
I think "struct epoll_event" was badly designed to begin with to have
this issue, but it shouldn't then be an excuse to make things even
worse with this array of "struct epoll_uitem" things.
Hmm?
Linus
[RFC PATCH v2 02/13] epoll: introduce user structures for polling from userspace
This one introduces structures of user items array:
struct user_epheader -
describes inserted epoll items.
struct user_epitem -
single epoll item, visible to userspace.
Signed-off-by: Roman Penyaev
Cc: Andrew Morton
Cc: Davidlohr Bueso
Cc: Jason Baron
Cc: Al Viro
Cc: "Paul E. McKenney"
Cc: Linus Torvalds
Cc: Andrea Parri
Cc: linux-fsde...@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
fs/eventpoll.c | 9 +
include/uapi/linux/eventpoll.h | 19 +++
2 files changed, 28 insertions(+)
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 2cc183e86a29..f598442512f3 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -9,6 +9,8 @@
*
* Davide Libenzi
*
+ * Polling from userspace support by Roman Penyaev
+ * (C) Copyright 2019 SUSE, All Rights Reserved
*/
#include
@@ -109,6 +111,13 @@
#define EP_ITEM_COST (sizeof(struct epitem) + sizeof(struct eppoll_entry))
+/*
+ * That is around 1.3mb of allocated memory for one epfd. What is more
+ * important is ->index_length, which should be ^2, so do not increase
+ * max items number to avoid size doubling of user index.
+ */
+#define EP_USERPOLL_MAX_ITEMS_NR 65536
+
struct epoll_filefd {
struct file *file;
int fd;
diff --git a/include/uapi/linux/eventpoll.h b/include/uapi/linux/eventpoll.h
index 39dfc29f0f52..690a625ddeb2 100644
--- a/include/uapi/linux/eventpoll.h
+++ b/include/uapi/linux/eventpoll.h
@@ -79,4 +79,23 @@ struct epoll_event {
__u64 data;
} EPOLL_PACKED;
+struct epoll_uitem {
+ __poll_t ready_events;
+ struct epoll_event event;
+};
+
+#define EPOLL_USERPOLL_HEADER_SIZE 128
+#define EPOLL_USERPOLL_HEADER_MAGIC 0xeb01eb01
+
+struct epoll_uheader {
+ u32 magic; /* epoll user header magic */
+ u32 header_length; /* length of the header + items */
+ u32 index_length; /* length of the index ring, always pow2 */
+ u32 max_items_nr; /* max number of items */
+ u32 head; /* updated by userland */
+ u32 tail; /* updated by kernel */
+
+ struct epoll_uitem items[] __aligned(EPOLL_USERPOLL_HEADER_SIZE);
+};
+
#endif /* _UAPI_LINUX_EVENTPOLL_H */
--
2.19.1
