Amir Goldstein <amir73il@xxxxxxxxx> writes:
> On Mon, Apr 26, 2021 at 9:42 PM Gabriel Krisman Bertazi
> <krisman@xxxxxxxxxxxxx> wrote:
>>
>> In order to support file system health/error reporting over fanotify,
>> fsnotify needs to expose a submission path that doesn't allow sleeping.
>> The only problem I identified with the current submission path is the
>> need to dynamically allocate memory for the event queue.
>>
>> This patch avoids the problem by introducing a new mode in fsnotify,
>> where a ring buffer is used to submit events for a group. Each group
>> has its own ring buffer, and error notifications are submitted
>> exclusively through it.
>>
>> Signed-off-by: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx>
>> ---
>> fs/notify/Makefile | 2 +-
>> fs/notify/group.c | 12 +-
>> fs/notify/notification.c | 10 ++
>> fs/notify/ring.c | 199 +++++++++++++++++++++++++++++++
>> include/linux/fsnotify_backend.h | 37 +++++-
>> 5 files changed, 255 insertions(+), 5 deletions(-)
>> create mode 100644 fs/notify/ring.c
>>
>> diff --git a/fs/notify/Makefile b/fs/notify/Makefile
>> index 63a4b8828df4..61dae1e90f2d 100644
>> --- a/fs/notify/Makefile
>> +++ b/fs/notify/Makefile
>> @@ -1,6 +1,6 @@
>> # SPDX-License-Identifier: GPL-2.0
>> obj-$(CONFIG_FSNOTIFY) += fsnotify.o notification.o group.o mark.o \
>> - fdinfo.o
>> + fdinfo.o ring.o
>>
>> obj-y += dnotify/
>> obj-y += inotify/
>> diff --git a/fs/notify/group.c b/fs/notify/group.c
>> index 08acb1afc0c2..b99b3de36696 100644
>> --- a/fs/notify/group.c
>> +++ b/fs/notify/group.c
>> @@ -81,7 +81,10 @@ void fsnotify_destroy_group(struct fsnotify_group *group)
>> * notification against this group. So clearing the notification queue
>> * of all events is reliable now.
>> */
>> - fsnotify_flush_notify(group);
>> + if (group->flags & FSN_SUBMISSION_RING_BUFFER)
>> + fsnotify_free_ring_buffer(group);
>> + else
>> + fsnotify_flush_notify(group);
>>
>> /*
>> * Destroy overflow event (we cannot use fsnotify_destroy_event() as
>> @@ -136,6 +139,13 @@ static struct fsnotify_group *__fsnotify_alloc_group(
>> group->ops = ops;
>> group->flags = flags;
>>
>> + if (group->flags & FSN_SUBMISSION_RING_BUFFER) {
>> + if (fsnotify_create_ring_buffer(group)) {
>> + kfree(group);
>> + return ERR_PTR(-ENOMEM);
>> + }
>> + }
>> +
>> return group;
>> }
>>
>> diff --git a/fs/notify/notification.c b/fs/notify/notification.c
>> index 75d79d6d3ef0..32f97e7b7a80 100644
>> --- a/fs/notify/notification.c
>> +++ b/fs/notify/notification.c
>> @@ -51,6 +51,10 @@ EXPORT_SYMBOL_GPL(fsnotify_get_cookie);
>> bool fsnotify_notify_queue_is_empty(struct fsnotify_group *group)
>> {
>> assert_spin_locked(&group->notification_lock);
>> +
>> + if (group->flags & FSN_SUBMISSION_RING_BUFFER)
>> + return fsnotify_ring_notify_queue_is_empty(group);
>> +
>> return list_empty(&group->notification_list) ? true : false;
>> }
>>
>> @@ -132,6 +136,9 @@ void fsnotify_remove_queued_event(struct fsnotify_group *group,
>> struct fsnotify_event *event)
>> {
>> assert_spin_locked(&group->notification_lock);
>> +
>> + if (group->flags & FSN_SUBMISSION_RING_BUFFER)
>> + return;
>> /*
>> * We need to init list head for the case of overflow event so that
>> * check in fsnotify_add_event() works
>> @@ -166,6 +173,9 @@ struct fsnotify_event *fsnotify_peek_first_event(struct fsnotify_group *group)
>> {
>> assert_spin_locked(&group->notification_lock);
>>
>> + if (group->flags & FSN_SUBMISSION_RING_BUFFER)
>> + return fsnotify_ring_peek_first_event(group);
>> +
>> return list_first_entry(&group->notification_list,
>> struct fsnotify_event, list);
>> }
>> diff --git a/fs/notify/ring.c b/fs/notify/ring.c
>> new file mode 100644
>> index 000000000000..75e8af1f8d80
>> --- /dev/null
>> +++ b/fs/notify/ring.c
>> @@ -0,0 +1,199 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +#include <linux/types.h>
>> +#include <linux/fsnotify.h>
>> +#include <linux/memcontrol.h>
>> +
>> +#define INVALID_RING_SLOT -1
>> +
>> +#define FSNOTIFY_RING_PAGES 16
>> +
>> +#define NEXT_SLOT(cur, len, ring_size) ((cur + len) & (ring_size-1))
>> +#define NEXT_PAGE(cur, ring_size) (round_up(cur, PAGE_SIZE) & (ring_size-1))
>> +
>> +bool fsnotify_ring_notify_queue_is_empty(struct fsnotify_group *group)
>> +{
>> + assert_spin_locked(&group->notification_lock);
>> +
>> + if (group->ring_buffer.tail == group->ring_buffer.head)
>> + return true;
>> + return false;
>> +}
>> +
>> +struct fsnotify_event *fsnotify_ring_peek_first_event(struct fsnotify_group *group)
>> +{
>> + u64 ring_size = group->ring_buffer.nr_pages << PAGE_SHIFT;
>> + struct fsnotify_event *fsn;
>> + char *kaddr;
>> + u64 tail;
>> +
>> + assert_spin_locked(&group->notification_lock);
>> +
>> +again:
>> + tail = group->ring_buffer.tail;
>> +
>> + if ((PAGE_SIZE - (tail & (PAGE_SIZE-1))) < sizeof(struct fsnotify_event)) {
>> + group->ring_buffer.tail = NEXT_PAGE(tail, ring_size);
>> + goto again;
>> + }
>> +
>> + kaddr = kmap_atomic(group->ring_buffer.pages[tail / PAGE_SIZE]);
>> + if (!kaddr)
>> + return NULL;
>> + fsn = (struct fsnotify_event *) (kaddr + (tail & (PAGE_SIZE-1)));
>> +
>> + if (fsn->slot_len == INVALID_RING_SLOT) {
>> + group->ring_buffer.tail = NEXT_PAGE(tail, ring_size);
>> + kunmap_atomic(kaddr);
>> + goto again;
>> + }
>> +
>> + /* will be unmapped when entry is consumed. */
>> + return fsn;
>> +}
>> +
>> +void fsnotify_ring_buffer_consume_event(struct fsnotify_group *group,
>> + struct fsnotify_event *event)
>> +{
>> + u64 ring_size = group->ring_buffer.nr_pages << PAGE_SHIFT;
>> + u64 new_tail = NEXT_SLOT(group->ring_buffer.tail, event->slot_len, ring_size);
>> +
>> + kunmap_atomic(event);
>> +
>> + pr_debug("%s: group=%p tail=%llx->%llx ring_size=%llu\n", __func__,
>> + group, group->ring_buffer.tail, new_tail, ring_size);
>> +
>> + WRITE_ONCE(group->ring_buffer.tail, new_tail);
>> +}
>> +
>> +struct fsnotify_event *fsnotify_ring_alloc_event_slot(struct fsnotify_group *group,
>> + size_t size)
>> + __acquires(&group->notification_lock)
>> +{
>> + struct fsnotify_event *fsn;
>> + u64 head, tail;
>> + u64 ring_size = group->ring_buffer.nr_pages << PAGE_SHIFT;
>> + u64 new_head;
>> + void *kaddr;
>> +
>> + if (WARN_ON(!(group->flags & FSN_SUBMISSION_RING_BUFFER) || size > PAGE_SIZE))
>> + return ERR_PTR(-EINVAL);
>> +
>> + pr_debug("%s: start group=%p ring_size=%llu, requested=%lu\n", __func__, group,
>> + ring_size, size);
>> +
>> + spin_lock(&group->notification_lock);
>> +again:
>> + head = group->ring_buffer.head;
>> + tail = group->ring_buffer.tail;
>> + new_head = NEXT_SLOT(head, size, ring_size);
>> +
>> + /* head would catch up to tail, corrupting an entry. */
>> + if ((head < tail && new_head > tail) || (head > new_head && new_head > tail)) {
>> + fsn = ERR_PTR(-ENOMEM);
>> + goto err;
>> + }
>> +
>> + /*
>> + * Not event a skip message fits in the page. We can detect the
>> + * lack of space. Move on to the next page.
>> + */
>> + if ((PAGE_SIZE - (head & (PAGE_SIZE-1))) < sizeof(struct fsnotify_event)) {
>> + /* Start again on next page */
>> + group->ring_buffer.head = NEXT_PAGE(head, ring_size);
>> + goto again;
>> + }
>> +
>> + kaddr = kmap_atomic(group->ring_buffer.pages[head / PAGE_SIZE]);
>> + if (!kaddr) {
>> + fsn = ERR_PTR(-EFAULT);
>> + goto err;
>> + }
>> +
>> + fsn = (struct fsnotify_event *) (kaddr + (head & (PAGE_SIZE-1)));
>> +
>> + if ((head >> PAGE_SHIFT) != (new_head >> PAGE_SHIFT)) {
>> + /*
>> + * No room in the current page. Add a fake entry
>> + * consuming the end the page to avoid splitting event
>> + * structure.
>> + */
>> + fsn->slot_len = INVALID_RING_SLOT;
>> + kunmap_atomic(kaddr);
>> + /* Start again on the next page */
>> + group->ring_buffer.head = NEXT_PAGE(head, ring_size);
>> +
>> + goto again;
>> + }
>> + fsn->slot_len = size;
>> +
>> + return fsn;
>> +
>> +err:
>> + spin_unlock(&group->notification_lock);
>> + return fsn;
>> +}
>> +
>> +void fsnotify_ring_commit_slot(struct fsnotify_group *group, struct fsnotify_event *fsn)
>> + __releases(&group->notification_lock)
>> +{
>> + u64 ring_size = group->ring_buffer.nr_pages << PAGE_SHIFT;
>> + u64 head = group->ring_buffer.head;
>> + u64 new_head = NEXT_SLOT(head, fsn->slot_len, ring_size);
>> +
>> + pr_debug("%s: group=%p head=%llx->%llx ring_size=%llu\n", __func__,
>> + group, head, new_head, ring_size);
>> +
>> + kunmap_atomic(fsn);
>> + group->ring_buffer.head = new_head;
>> +
>> + spin_unlock(&group->notification_lock);
>> +
>> + wake_up(&group->notification_waitq);
>> + kill_fasync(&group->fsn_fa, SIGIO, POLL_IN);
>> +
>> +}
>> +
>> +void fsnotify_free_ring_buffer(struct fsnotify_group *group)
>> +{
>> + int i;
>> +
>> + for (i = 0; i < group->ring_buffer.nr_pages; i++)
>> + __free_page(group->ring_buffer.pages[i]);
>> + kfree(group->ring_buffer.pages);
>> + group->ring_buffer.nr_pages = 0;
>> +}
>> +
>> +int fsnotify_create_ring_buffer(struct fsnotify_group *group)
>> +{
>> + int nr_pages = FSNOTIFY_RING_PAGES;
>> + int i;
>> +
>> + pr_debug("%s: group=%p pages=%d\n", __func__, group, nr_pages);
>> +
>> + group->ring_buffer.pages = kmalloc_array(nr_pages, sizeof(struct pages *),
>> + GFP_KERNEL);
>> + if (!group->ring_buffer.pages)
>> + return -ENOMEM;
>> +
>> + group->ring_buffer.head = 0;
>> + group->ring_buffer.tail = 0;
>> +
>> + for (i = 0; i < nr_pages; i++) {
>> + group->ring_buffer.pages[i] = alloc_pages(GFP_KERNEL, 1);
>> + if (!group->ring_buffer.pages)
>> + goto err_dealloc;
>> + }
>> +
>> + group->ring_buffer.nr_pages = nr_pages;
>> +
>> + return 0;
>> +
>> +err_dealloc:
>> + for (--i; i >= 0; i--)
>> + __free_page(group->ring_buffer.pages[i]);
>> + kfree(group->ring_buffer.pages);
>> + group->ring_buffer.nr_pages = 0;
>> + return -ENOMEM;
>> +}
>> +
>> +
>
> Nothing in this file is fsnotify specific.
> Is there no kernel lib implementation for this already?
> If there isn't (I'd be very surprised) please put this in lib/ and post it
> for wider review including self tests.
About the implementation, the only generic code I could find is
include/linux/circ_buf.h, but it doesn't really do much or fit well
here. For instance, it doesn't deal well with non-contiguous pages.
There are other smarter implementations around the kernel like
Documentation/trace/ring-buffer-design.txt, that would be a better
candidate to be a generic ring buffer in lib/, but I admit to haven't
checked them well enough to see if they would solve the problem for
fsnotify, which has a very simple ring buffer anyway.
>> diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
>> index 190c6a402e98..a1a4dd69e5ed 100644
>> --- a/include/linux/fsnotify_backend.h
>> +++ b/include/linux/fsnotify_backend.h
>> @@ -74,6 +74,8 @@
>> #define ALL_FSNOTIFY_PERM_EVENTS (FS_OPEN_PERM | FS_ACCESS_PERM | \
>> FS_OPEN_EXEC_PERM)
>>
>> +#define FSN_SUBMISSION_RING_BUFFER 0x00000080
>
> FSNOTIFY_GROUP_FLAG_RING_BUFFER please (or FSN_GROUP_ if you must)
> and please define this above struct fsnotify_group, even right above the flags
> field like FSNOTIFY_CONN_FLAG_HAS_FSID
>
> *IF* we go this way :)
>
> Thanks,
> Amir.
--
Gabriel Krisman Bertazi
