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7e935898ab
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>