feat(laguna): pager-blob warm-restore survives eviction#451
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…ter eviction LagunaCacheSnapshot gains is_pooled + kvflash_blob fields mirroring PrefixSnapshot in qwen35. snapshot_save() serializes the pager blob when the pool has relocated chunks instead of refusing. restore_and_generate_impl() deserializes the blob and prefills only the suffix [snap_pos, prompt_len) — the restore-consume path that survives eviction. Non-pooled (identity) saves use the existing GPU-tensor copy unchanged. kvflash_pager.h: forward-port serialize/deserialize/ledger/pinning from the perf/qwen35-decode-fuse-elementwise branch so laguna can call them. The header is arch-agnostic; the impl is shared with qwen35 once merged to main. Validated: -fsyntax-only clean (laguna_backend.cpp, single-TU). Full CUDA build + NIAH recall gate deferred (resource contention).
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Port map (Step 1)
qwen35 source of truth → laguna gap:
PrefixSnapshot.is_pooled+kvflash_blobLagunaCacheSnapshothad neithersnapshot_save_pooled_at/snapshot_save(pooled branch):kvflash_pager_.serialize()→snap.kvflash_blobsnapshot_saverefused with!is_identity()guardrestore_and_generate_implpooled branch:kvflash_pager_.deserialize()+ suffix-only prefillrestore_and_generate_implalways did full diff re-prefill over identity-restored GPU tensorscommon/kvflash_pager.h:serialize(max_chunks)/deserialize()/ ledger / pinningkvflash_pager.hlacked these (added inperf/qwen35-decode-fuse-elementwise)QK scorer (
KvFlashQkPool) is phase 2 — laguna has noKvFlashQkPoolmember and the attention layer path (laguna_target_graph.cpp) does not capture Q/K vectors. This PR is the prerequisite snapshot/restore infra only.What is implemented
LagunaCacheSnapshot:is_pooled+kvflash_blobfields.LagunaBackend::snapshot_save: whenpool_relocated(i.e.cur_pos > pool_tokensor!is_identity()), serialize blob intosnap.kvflash_bloband setis_pooled=true. Non-pooled (identity) path unchanged.generate_implinline snap: same pool-relocated branch — blob save with chunk-alignedmax_chunks.restore_and_generate_impl: pooled branch useskvflash_pager_.deserialize()+ suffix-onlylaguna_stepfor[snap_pos, N); exact-hit re-embeds last token. Non-pooled branch untouched.common/kvflash_pager.h: forward-portserialize(max_chunks),deserialize(), ledger section (per-chunkwas_resident+score), critical-chunk pinning, andchunk_host_ptr/k_seg_bytes/v_seg_bytesaccessors.Validated
-fsyntax-onlyclean onlaguna_backend.cpp(single-TU, g++-11, all production flags).TODO (deferred — resource contention)
laguna_internal.hsnapshot free: clearkvflash_blobinlaguna_snapshot_freeto avoid holding stale host bytes after free (2-line follow-up).Composition note
This PR is a prerequisite for adding the QK scorer (phase 2). The QK
pool_chunk_hostfix from PR #446 does not apply to laguna directly — laguna has noKvFlashQkPool; that is the phase-2 addition once this blob infra is validated.