Claude review: dma-buf: system_heap: Optimize sg_table-to-pages conversion in vmap

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Overall Series Review

Subject: dma-buf: system_heap: Optimize sg_table-to-pages conversion in vmap
Author: "Barry Song (Xiaomi)" <baohua@kernel.org>
Patches: 3
Reviewed: 2026-04-12T14:23:03.509679

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This is a single-patch optimization for `system_heap_do_vmap()` in the DMA-buf system heap. The patch replaces the `for_each_sgtable_page()` iterator (which calls into `__sg_page_iter_next()` for every single page) with a nested loop that iterates over scatterlist entries and then fills pages using simple pointer arithmetic within each entry. The approach is sound and the performance improvement is plausible -- `for_each_sgtable_page()` involves non-trivial per-page overhead (function call to `__sg_page_iter_next()`, sg boundary checking, etc.) that is completely unnecessary when the caller knows each sg entry's pages are contiguous.

The optimization is **correct** for the system heap because every sg entry is created with offset=0 and length=page_size(page) (see `system_heap_allocate()` at line 394 of the current source), guaranteeing page-aligned lengths and contiguous pages within each entry.

The change to move the `WARN_ON` out of the loop and convert it from `>=` (overflow guard) to `!=` (exact match) is also a good improvement -- it catches both over-counting *and* under-counting.

**Minor issues noted below, but overall this is a clean, well-motivated optimization.**

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