* mount: tolerance-window write pattern detection for concurrent writeback
WriterPattern decides whether each dirty chunk is buffered in RAM
(NewMemChunk) or spilled to an on-disk swap file (NewSwapFileChunk), so a
sequential stream misclassified as random is pushed through disk-backed
swap.
The old detector compared each write's start to the previous write's
exact stop offset (atomic.Swap of lastWriteStopOffset, then
lastOffset == offset). That is brittle under concurrent FUSE writeback:
MonitorWriteAt runs before the per-handle write lock, so parallel Write
upcalls interleave the swap, and writeback flushes dirty pages slightly
out of offset order. A genuinely sequential stream then repeatedly reads
as random and spills to swap.
Replace the exact match with the same frontier + tolerance approach used
on the read side: track a never-regressing max frontier (offset+size) via
a CAS loop and treat a write whose start is within SeqTolerance (8 MiB) of
that frontier as sequential. The existing +/-ModeChangeLimit hysteresis is
kept, so a single outlier write can't flip the mode.
Adds page_writer_pattern_test.go covering sequentiality, hysteresis,
reorder tolerance, the inclusive tolerance boundary, frontier
non-regression (the CAS invariant), and recovery back to sequential mode.
* mount: read write frontier inside the CAS loop
Capture the frontier from the CAS loop's own load rather than a separate
up-front snapshot, so the sequentiality diff is judged against the freshest
pre-image even if a concurrent writeback upcall advances the frontier while
we loop. Also drops the now-redundant load. Behavior is unchanged for the
single-threaded tests; addresses review feedback on #9984.
Chris Lu