Re: [PATCH RFC 05/15] fsnotify: Support event submission through ring buffer

[Gabriel Krisman Bertazi <krisman@collabora.com>]

Amir Goldstein <amir73il@gmail.com> writes:

> On Mon, Apr 26, 2021 at 9:42 PM Gabriel Krisman Bertazi
> <krisman@collabora.com> 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@collabora.com>
>> ---
>>  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