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Avoid premature metadata enospc

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server_get_log_trees() sets the low flag in a mount's meta_avail
allocator, triggering enospc for any space consuming allocatins in the
mount, if the server's global meta_vail pool falls below the reserved
block count.  Before each server transaction opens we swap the global
meta_avail and meta_freed allocators to ensure that the transaction has
at least the reserved count of blocks available.

This creates a risk of premature enospc as the global meta_avail pool
drains and swaps to the larger meta_freed.  The pool can be close to the
reserved count, perhaps at it exactly.  _get_log_trees can fill the
client's mount, even a little, and drop the global meta_avail total
under the reserved count, triggering enospc, even though meta_Freed
could have had quite a lot of blocks.

The fix is to ensure that the global meta_avail has 2x the reserved
count and swapping if it falls under that.  This ensures that a server
transaction can consume an entire reserved count and still have enough
to avoid triggering enospc.

This fixes a scattering of rare premature enospc returns that were
hitting during tests.  It was rare for meta_avail to fall just at the
reserved count and for get_log_trees to have to refill the client
allocator, but it happened.

Signed-off-by: Zach Brown <zab@versity.com>
This commit is contained in:
Zach Brown
2021-07-30 13:00:15 -07:00
parent 6d0694f1b0
commit 7e935898ab
1 changed files with 12 additions and 10 deletions
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22
kmod/src/server.c
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@@ -323,7 +323,6 @@ static void scoutfs_server_commit_func(struct work_struct *work)
struct commit_waiter *cw;
struct commit_waiter *pos;
struct llist_node *node;
u64 reserved;
int ret;
trace_scoutfs_server_commit_work_enter(sb, 0, 0);
@@ -389,16 +388,19 @@ static void scoutfs_server_commit_func(struct work_struct *work)
server->other_freed = &super->server_meta_freed[server->other_ind];
/*
* The reserved metadata blocks includes the max size of
* outstanding allocators and a server transaction could be
* asked to refill all those allocators from meta_avail. If our
* meta_avail falls below the reserved count, and freed is still
* above it, then swap so that we don't start returning enospc
* until we're truly low.
* get_log_trees sets ALLOC_LOW when its allocator drops below
* the reserved blocks after having filled the log trees's avail
* allocator during its transaction. To avoid prematurely
* setting the low flag and causing enospc we make sure that the
* next transaction's meta_avail has 2x the reserved blocks so
* that it can consume a full reserved amount and still have
* enough to avoid enospc. We swap to freed if avail is under
* the buffer and freed is larger.
*/
reserved = scoutfs_server_reserved_meta_blocks(sb);
if (le64_to_cpu(server->meta_avail->total_len) <= reserved &&
le64_to_cpu(server->meta_freed->total_len) > reserved)
if ((le64_to_cpu(server->meta_avail->total_len) <
(scoutfs_server_reserved_meta_blocks(sb) * 2)) &&
(le64_to_cpu(server->meta_freed->total_len) >
le64_to_cpu(server->meta_avail->total_len)))
swap(server->meta_avail, server->meta_freed);
ret = 0;