Claude review: Insert instead of copy pages into shmem when shrinking

[Claude Code Review Bot <claude-review@example.com>]

Overall Series Review

Subject: Insert instead of copy pages into shmem when shrinking
Author: =?UTF-8?q?Thomas=20Hellstr=C3=B6m?= <thomas.hellstrom@linux.intel.com>
Patches: 19
Reviewed: 2026-05-16T13:43:39.916433

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This 2-patch series from Thomas Hellström introduces a zero-copy path for TTM buffer object shrinking. Instead of allocating shmem pages and copying data into them (the current approach), TTM pages are inserted directly into the shmem page cache, avoiding both the memory overhead and the fragmentation caused by splitting high-order pages at shrink time. The motivation is sound: under memory pressure, having to allocate a full high-order page just to copy into it before freeing the original defeats the purpose of shrinking.

The architecture is clean: Patch 1 adds a general `shmem_insert_folio()` interface in mm/shmem with appropriate preconditions and assertions, and Patch 2 rewires TTM to use it. The series touches core mm code (`mm/page_alloc.c`, `mm/shmem.c`) which will require mm maintainer sign-off (Andrew Morton and/or the relevant mm reviewers are CC'd).

Both patches carry `Assisted-by: GitHub_Copilot:claude-sonnet-4.6`, indicating AI assistance in development. Given the complexity of the mm interactions and the subtlety of compound page lifecycle management, this warrants careful scrutiny — several of the issues below may reflect areas where AI-generated code follows patterns without fully accounting for the writeback protocol.

**Key concerns:**
1. The writeback path in `shmem_insert_folio()` skips `folio_clear_dirty_for_io()` and `folio_set_reclaim()` before calling `shmem_writeout()`, deviating from the protocol used both by the writeback infrastructure and by the old TTM code.
2. `undo_compound_page()` is exported globally but is a very sharp tool — it's only safe under narrow preconditions that aren't enforced at runtime.
3. The EEXIST handling in `shmem_insert_folio()` silently truncates existing content, which may mask logic bugs in callers.

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