[PATCH 0/3][v2] Add comments describing how space reservation works

[PATCH 0/3][v2] Add comments describing how space reservation works

[Josef Bacik]

v1->v2:
- Fixed spelling mistakes, discovered you can use ':set spell' for files other
  than git commit messages.
- Reworked some of the explanations to be more visual to describe the general
  flow.

--------------- Original email ------------------
In talking with Nikolay about the data reservation patches it became clear that
there's way more about the space reservation system that exists soley in my head
than I thought.  I've written three big comments on the different sections of
the space reservation system to hopefully help people understand how the system
works generally.  Again this is from my point of view, and since I'm the one who
wrote it I'm not sure where the gaps in peoples understanding is, so if there's
something that is not clear here now is the time to bring it up.  Hopefully this
will make reviewing patches I submit to this system less scary for reviewers.
Thanks,

Josef

[PATCH 1/3] btrfs: add a comment describing block-rsvs

[Josef Bacik]

This is a giant comment at the top of block-rsv.c describing generally
how block rsvs work.  It is purely about the block rsv's themselves, and
nothing to do with how the actual reservation system works.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/block-rsv.c | 91 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 91 insertions(+)

diff --git a/fs/btrfs/block-rsv.c b/fs/btrfs/block-rsv.c
index d07bd41a7c1e..c3843a0001cb 100644
--- a/fs/btrfs/block-rsv.c
+++ b/fs/btrfs/block-rsv.c
@@ -6,6 +6,97 @@
 #include "space-info.h"
 #include "transaction.h"
 
+/*
+ * HOW DO BLOCK RSVS WORK
+ *
+ *   Think of block_rsv's as buckets for logically grouped metadata
+ *   reservations.  Each block_rsv has a ->size and a ->reserved.  ->size is how
+ *   large we want our block rsv to be, ->reserved is how much space is
+ *   currently reserved for this block reserve.
+ *
+ *   ->failfast exists for the truncate case, and is described below.
+ *
+ * NORMAL OPERATION
+ *
+ *   -> Reserve
+ *     Entrance: btrfs_block_rsv_add, btrfs_block_rsv_refill
+ *
+ *     We call into btrfs_reserve_metadata_bytes() with our bytes, which is
+ *     accounted for in space_info->bytes_may_use, and then add the bytes to
+ *     ->reserved, and ->size in the case of btrfs_block_rsv_add.
+ *
+ *     ->size is an over-estimation of how much we may use for a particular
+ *     operation.
+ *
+ *   -> Use
+ *     Entrance: btrfs_use_block_rsv
+ *
+ *     When we do a btrfs_alloc_tree_block() we call into btrfs_use_block_rsv()
+ *     to determine the appropriate block_rsv to use, and then verify that
+ *     ->reserved has enough space for our tree block allocation.  Once
+ *     successful we subtract fs_info->nodesize from ->reserved.
+ *
+ *   -> Finish
+ *     Entrance: btrfs_block_rsv_release
+ *
+ *     We are finished with our operation, subtract our individual reservation
+ *     from ->size, and then subtract ->size from ->reserved and free up the
+ *     excess if there is any.
+ *
+ *     There is some logic here to refill the delayed refs rsv or the global rsv
+ *     as needed, otherwise the excess is subtracted from
+ *     space_info->bytes_may_use.
+ *
+ * TYPES OF BLOCK RSVS
+ *
+ * BLOCK_RSV_TRANS, BLOCK_RSV_DELOPS, BLOCK_RSV_CHUNK
+ *   These behave normally, as described above, just within the confines of the
+ *   lifetime of their particular operation (transaction for the whole trans
+ *   handle lifetime, for example).
+ *
+ * BLOCK_RSV_GLOBAL
+ *   It is impossible to properly account for all the space that may be required
+ *   to make our extent tree updates.  This block reserve acts as an overflow
+ *   buffer in case our delayed refs rsv does not reserve enough space to update
+ *   the extent tree.
+ *
+ *   We can steal from this in some cases as well, notably on evict() or
+ *   truncate() in order to help users recover from ENOSPC conditions.
+ *
+ * BLOCK_RSV_DELALLOC
+ *   The individual item sizes are determined by the per-inode size
+ *   calculations, which are described with the delalloc code.  This is pretty
+ *   straightforward, it's just the calculation of ->size encodes a lot of
+ *   different items, and thus it gets used when updating inodes, inserting file
+ *   extents, and inserting checksums.
+ *
+ * BLOCK_RSV_DELREFS
+ *   We keep a running tally of how many delayed refs we have on the system.
+ *   We assume each one of these delayed refs are going to use a full
+ *   reservation.  We use the transaction items and pre-reserve space for every
+ *   operation, and use this reservation to refill any gap between ->size and
+ *   ->reserved that may exist.
+ *
+ *   From there it's straightforward, removing a delayed ref means we remove its
+ *   count from ->size and free up reservations as necessary.  Since this is the
+ *   most dynamic block rsv in the system, we will try to refill this block rsv
+ *   first with any excess returned by any other block reserve.
+ *
+ * BLOCK_RSV_EMPTY
+ *   This is the fallback block rsv to make us try to reserve space if we don't
+ *   have a specific bucket for this allocation.  It is mostly used for updating
+ *   the device tree and such, since that is a separate pool we're content to
+ *   just reserve space from the space_info on demand.
+ *
+ * BLOCK_RSV_TEMP
+ *   This is used by things like truncate and iput.  We will temporarily
+ *   allocate a block rsv, set it to some size, and then truncate bytes until we
+ *   have no space left.  With ->failfast set we'll simply return ENOSPC from
+ *   btrfs_use_block_rsv() to signal that we need to unwind and try to make a
+ *   new reservation.  This is because these operations are unbounded, so we
+ *   want to do as much work as we can, and then back off and re-reserve.
+ */
+
 static u64 block_rsv_release_bytes(struct btrfs_fs_info *fs_info,
 				    struct btrfs_block_rsv *block_rsv,
 				    struct btrfs_block_rsv *dest, u64 num_bytes,
-- 
2.24.1

[PATCH 2/3] btrfs: add a comment describing delalloc space reservation

[Josef Bacik]

delalloc space reservation is tricky because it encompasses both data
and metadata.  Make it clear what each side does, the general flow of
how space is moved throughout the lifetime of a write, and what goes
into the calculations.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/delalloc-space.c | 101 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 101 insertions(+)

diff --git a/fs/btrfs/delalloc-space.c b/fs/btrfs/delalloc-space.c
index c13d8609cc99..9366509917fc 100644
--- a/fs/btrfs/delalloc-space.c
+++ b/fs/btrfs/delalloc-space.c
@@ -9,6 +9,107 @@
 #include "qgroup.h"
 #include "block-group.h"
 
+/*
+ * HOW DOES THIS WORK
+ *
+ * There are two stages to data reservations, one for data and one for metadata
+ * to handle the new extents and checksums generated by writing data.
+ *
+ *
+ * DATA RESERVATION
+ *   The general flow of the data reservation is as follows
+ *
+ *   -> Reserve
+ *     We call into btrfs_reserve_data_bytes() for the user request bytes that
+ *     they wish to write.  We make this reservation and add it to
+ *     space_info->bytes_may_use.  We set EXTENT_DELALLOC on the inode io_tree
+ *     for the range and carry on if this is buffered, or follow up trying to
+ *     make a real allocation if we are pre-allocating or doing O_DIRECT.
+ *
+ *   -> Use
+ *     At writepages()/prealloc/O_DIRECT time we will call into
+ *     btrfs_reserve_extent() for some part or all of this range of bytes.  We
+ *     will make the allocation and subtract space_info->bytes_may_use by the
+ *     original requested length and increase the space_info->bytes_reserved by
+ *     the allocated length.  This distinction is important because compression
+ *     may allocate a smaller on disk extent than we previously reserved.
+ *
+ *   -> Allocation
+ *     finish_ordered_io() will insert the new file extent item for this range,
+ *     and then add a delayed ref update for the extent tree.  Once that delayed
+ *     ref is written the extent size is subtracted from
+ *     space_info->bytes_reserved and added to space_info->bytes_used.
+ *
+ *   Error handling
+ *
+ *   -> By the reservation maker
+ *     This is the simplest case, we haven't completed our operation and we know
+ *     how much we reserved, we can simply call
+ *     btrfs_free_reserved_data_space*() and it will be removed from
+ *     space_info->bytes_may_use.
+ *
+ *   -> After the reservation has been made, but before cow_file_range()
+ *     This is specifically for the delalloc case.  You must clear
+ *     EXTENT_DELALLOC with the EXTENT_CLEAR_DATA_RESV bit, and the range will
+ *     be subtracted from space_info->bytes_may_use.
+ *
+ * METADATA RESERVATION
+ *   The general metadata reservation lifetimes are discussed elsewhere, this
+ *   will just focus on how it is used for delalloc space.
+ *
+ *   We keep track of two things on a per inode bases
+ *
+ *   ->outstanding_extents
+ *     This is the number of file extent items we'll need to handle all of the
+ *     outstanding DELALLOC space we have in this inode.  We limit the maximum
+ *     size of an extent, so a large contiguous dirty area may require more than
+ *     one outstanding_extent, which is why count_max_extents() is used to
+ *     determine how many outstanding_extents get added.
+ *
+ *   ->csum_bytes
+ *     This is essentially how many dirty bytes we have for this inode, so we
+ *     can calculate the number of checksum items we would have to add in order
+ *     to checksum our outstanding data.
+ *
+ *   We keep a per-inode block_rsv in order to make it easier to keep track of
+ *   our reservation.  We use btrfs_calculate_inode_block_rsv_size() to
+ *   calculate the current theoretical maximum reservation we would need for the
+ *   metadata for this inode.  We call this and then adjust our reservation as
+ *   necessary, either by attempting to reserve more space, or freeing up excess
+ *   space.
+ *
+ * OUTSTANDING_EXTENTS HANDLING
+ *  ->outstanding_extents is used for keeping track of how many extents we will
+ *  need to use for this inode, and it will fluctuate depending on where you are
+ *  in the life cycle of the dirty data.  Consider the following normal case for
+ *  a completely clean inode, with a num_bytes < our maximum allowed extent size
+ *
+ *  -> reserve
+ *    ->outstanding_extents += 1 (current value is 1)
+ *
+ *  -> set_delalloc
+ *    ->outstanding_extents += 1 (currrent value is 2)
+ *
+ *  -> btrfs_delalloc_release_extents()
+ *    ->outstanding_extents -= 1 (current value is 1)
+ *
+ *    We must call this once we are done, as we hold our reservation for the
+ *    duration of our operation, and then assume set_delalloc will update the
+ *    counter appropriately.
+ *
+ *  -> add ordered extent
+ *    ->outstanding_extents += 1 (current value is 2)
+ *
+ *  -> btrfs_clear_delalloc_extent
+ *    ->outstanding_extents -= 1 (current value is 1)
+ *
+ *  -> finish_ordered_io/btrfs_remove_ordered_extent
+ *    ->outstanding_extents -= 1 (current value is 0)
+ *
+ *  Each stage is responsible for their own accounting of the extent, thus
+ *  making error handling and cleanup easier.
+ */
+
 int btrfs_alloc_data_chunk_ondemand(struct btrfs_inode *inode, u64 bytes)
 {
 	struct btrfs_root *root = inode->root;
-- 
2.24.1

[PATCH 3/3] btrfs: describe the space reservation system in general

[Josef Bacik]

Add another comment to cover how the space reservation system works
generally.  This covers the actual reservation flow, as well as how
flushing is handled.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/space-info.c | 146 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 146 insertions(+)

diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c
index 56425674e940..db387816455c 100644
--- a/fs/btrfs/space-info.c
+++ b/fs/btrfs/space-info.c
@@ -10,6 +10,152 @@
 #include "transaction.h"
 #include "block-group.h"
 
+/*
+ * HOW DOES SPACE RESERVATION WORK
+ *
+ * If you want to know about delalloc specifically, there is a separate comment
+ * for that with the delalloc code.  This comment is about how the whole system
+ * works generally.
+ *
+ * BASIC CONCEPTS
+ *
+ *   1) space_info.  This is the ultimate arbiter of how much space we can use.
+ *   There's a description of the bytes_ fields with the struct declaration,
+ *   refer to that for specifics on each field.  Suffice it to say that for
+ *   reservations we care about total_bytes - SUM(space_info->bytes_) when
+ *   determining if there is space to make an allocation.  There is a space_info
+ *   for METADATA, SYSTEM, and DATA areas.
+ *
+ *   2) block_rsv's.  These are basically buckets for every different type of
+ *   metadata reservation we have.  You can see the comment in the block_rsv
+ *   code on the rules for each type, but generally block_rsv->reserved is how
+ *   much space is accounted for in space_info->bytes_may_use.
+ *
+ *   3) btrfs_calc*_size.  These are the worst case calculations we used based
+ *   on the number of items we will want to modify.  We have one for changing
+ *   items, and one for inserting new items.  Generally we use these helpers to
+ *   determine the size of the block reserves, and then use the actual bytes
+ *   values to adjust the space_info counters.
+ *
+ * MAKING RESERVATIONS, THE NORMAL CASE
+ *   We call into either btrfs_reserve_data_bytes() or
+ *   btrfs_reserve_metadata_bytes(), depending on which we're looking for, with
+ *   num_bytes we want to reserve.
+ *
+ *   ->reserve
+ *     space_info->bytes_may_reserve += num_bytes
+ *
+ *   ->extent allocation
+ *     Call btrfs_add_reserved_bytes() which does
+ *     space_info->bytes_may_reserve -= num_bytes
+ *     space_info->bytes_reserved += extent_bytes
+ *
+ *   ->insert reference
+ *     Call btrfs_update_block_group() which does
+ *     space_info->bytes_reserved -= extent_bytes
+ *     space_info->bytes_used += extent_bytes
+ *
+ * MAKING RESERVATIONS, FLUSHING NORMALLY (non-priority)
+ *
+ *   Assume we are unable to simply make the reservation because we do not have
+ *   enough space
+ *
+ *   -> __reserve_bytes
+ *     create a reserve_ticket with ->bytes set to our reservation, add it to
+ *     the tail of space_info->tickets, kick async flush thread
+ *
+ *   ->handle_reserve_ticket
+ *     wait on ticket->wait for ->bytes to be reduced to 0, or ->error to be set
+ *     on the ticket.
+ *
+ *   -> btrfs_async_reclaim_metadata_space/btrfs_async_reclaim_data_space
+ *     Flushes various things attempting to free up space.
+ *
+ *   -> btrfs_try_granting_tickets()
+ *     This is called by anything that either subtracts space from
+ *     space_info->bytes_may_use, ->bytes_pinned, etc, or adds to the
+ *     space_info->total_bytes.  This loops through the ->priority_tickets and
+ *     then the ->tickets list checking to see if the reservation can be
+ *     completed.  If it can the space is added to space_info->bytes_may_use and
+ *     the ticket is woken up.
+ *
+ *   -> ticket wakeup
+ *     Check if ->bytes == 0, if it does we got our reservation and we can carry
+ *     on, if not return the appropriate error (ENOSPC, but can be EINTR if we
+ *     were interrupted.)
+ *
+ * MAKING RESERVATIONS, FLUSHING HIGH PRIORITY
+ *
+ *   Same as the above, except we add ourselves to the
+ *   space_info->priority_tickets, and we do not use ticket->wait, we simply
+ *   call flush_space() ourselves for the states that are safe for us to call
+ *   without deadlocking and hope for the best.
+ *
+ * THE FLUSHING STATES
+ *
+ *   Generally speaking we will have two cases for each state, a "nice" state
+ *   and a "ALL THE THINGS" state.  In btrfs we delay a lot of work in order to
+ *   reduce the locking over head on the various trees, and even to keep from
+ *   doing any work at all in the case of delayed refs.  Each of these delayed
+ *   things however hold reservations, and so letting them run allows us to
+ *   reclaim space so we can make new reservations.
+ *
+ *   FLUSH_DELAYED_ITEMS
+ *     Every inode has a delayed item to update the inode.  Take a simple write
+ *     for example, we would update the inode item at write time to update the
+ *     mtime, and then again at finish_ordered_io() time in order to update the
+ *     isize or bytes.  We keep these delayed items to coalesce these operations
+ *     into a single operation done on demand.  These are an easy way to reclaim
+ *     metadata space.
+ *
+ *   FLUSH_DELALLOC
+ *     Look at the delalloc comment to get an idea of how much space is reserved
+ *     for delayed allocation.  We can reclaim some of this space simply by
+ *     running delalloc, but usually we need to wait for ordered extents to
+ *     reclaim the bulk of this space.
+ *
+ *   FLUSH_DELAYED_REFS
+ *     We have a block reserve for the outstanding delayed refs space, and every
+ *     delayed ref operation holds a reservation.  Running these is a quick way
+ *     to reclaim space, but we want to hold this until the end because COW can
+ *     churn a lot and we can avoid making some extent tree modifications if we
+ *     are able to delay for as long as possible.
+ *
+ *   ALLOC_CHUNK
+ *     We will skip this the first time through space reservation, because of
+ *     overcommit and we don't want to have a lot of useless metadata space when
+ *     our worst case reservations will likely never come true.
+ *
+ *   RUN_DELAYED_IPUTS
+ *     If we're freeing inodes we're likely freeing checksums, file extent
+ *     items, and extent tree items.  Loads of space could be freed up by these
+ *     operations, however they won't be usable until the transaction commits.
+ *
+ *   COMMIT_TRANS
+ *     may_commit_transaction() is the ultimate arbiter on whether we commit the
+ *     transaction or not.  In order to avoid constantly churning we do all the
+ *     above flushing first and then commit the transaction as the last resort.
+ *     However we need to take into account things like pinned space that would
+ *     be freed, plus any delayed work we may not have gotten rid of in the case
+ *     of metadata.
+ *
+ * OVERCOMMIT
+ *
+ *   Because we hold so many reservations for metadata we will allow you to
+ *   reserve more space than is currently free in the currently allocate
+ *   metadata space.  This only happens with metadata, data does not allow
+ *   overcommitting.
+ *
+ *   You can see the current logic for when we allow overcommit in
+ *   btrfs_can_overcommit(), but it only applies to unallocated space.  If there
+ *   is no unallocated space to be had, all reservations are kept within the
+ *   free space in the allocated metadata chunks.
+ *
+ *   Because of overcommitting, you generally want to use the
+ *   btrfs_can_overcommit() logic for metadata allocations, as it does the right
+ *   thing with or without extra unallocated space.
+ */
+
 u64 __pure btrfs_space_info_used(struct btrfs_space_info *s_info,
 			  bool may_use_included)
 {
-- 
2.24.1

Re: [PATCH 0/3][v2] Add comments describing how space reservation works

[David Sterba]

On Tue, Feb 04, 2020 at 01:18:53PM -0500, Josef Bacik wrote:
> v1->v2:
> - Fixed spelling mistakes, discovered you can use ':set spell' for files other
>   than git commit messages.
> - Reworked some of the explanations to be more visual to describe the general
>   flow.
> 
> --------------- Original email ------------------
> In talking with Nikolay about the data reservation patches it became clear that
> there's way more about the space reservation system that exists soley in my head
> than I thought.  I've written three big comments on the different sections of
> the space reservation system to hopefully help people understand how the system
> works generally.  Again this is from my point of view, and since I'm the one who
> wrote it I'm not sure where the gaps in peoples understanding is, so if there's
> something that is not clear here now is the time to bring it up.  Hopefully this
> will make reviewing patches I submit to this system less scary for reviewers.
> Thanks,
> 

Thanks, added to misc-next.

Re: [PATCH 1/3] btrfs: add a comment describing block-rsvs

[David Sterba]

On Tue, Feb 04, 2020 at 01:18:54PM -0500, Josef Bacik wrote:
> This is a giant comment at the top of block-rsv.c describing generally
> how block rsvs work.  It is purely about the block rsv's themselves, and
> nothing to do with how the actual reservation system works.

> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  fs/btrfs/block-rsv.c | 91 ++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 91 insertions(+)
> 
> diff --git a/fs/btrfs/block-rsv.c b/fs/btrfs/block-rsv.c
> index d07bd41a7c1e..c3843a0001cb 100644
> --- a/fs/btrfs/block-rsv.c
> +++ b/fs/btrfs/block-rsv.c
> @@ -6,6 +6,97 @@
>  #include "space-info.h"
>  #include "transaction.h"
>  
> +/*
> + * HOW DO BLOCK RSVS WORK

For documentation please try to avoid abbreviations unless they're at
least somehow explained or can be infered from the previous.

RSVS = RESERVES