[LU-19939] Optimize tiny write fast path in do_file_write_iter

• Type: Improvement
• Resolution: Unresolved
• Priority: Medium
• Severity: 3
• Assignee: WC Triage
• Reporter: Patrick Farrell

Description

The tiny write path in do_file_write_iter pays unnecessary overhead from PCC file_write_iter, hybrid DIO switch checks, and ktime_get calls, even though tiny writes never use PCC or hybrid DIO.

Move the tiny write attempt before the PCC and hybrid checks so that successful tiny writes (page already dirty in cache) return immediately without touching any of that code.

Defer ktime_get() to just before the CLIO path, since tiny writes do not need write latency timing. Guard ll_stats_ops_tally with a check so the tiny write fast path avoids two ktime_get calls.