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[Bugfix][Hardware][AMD] Fix last_page_len calculation in AITER MLA decode #31282

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Merged
vllm-bot merged 3 commits into from
Jan 2, 2026
Merged

[Bugfix][Hardware][AMD] Fix last_page_len calculation in AITER MLA decode #31282

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8a20ea8
[Bugfix][Hardware][AMD] Fix last_page_len calculation in AITER MLA de…
c0de128 Dec 24, 2025
b2268c5
Use pre-initialized buffer for paged_kv_last_page_len
c0de128 Dec 25, 2025
c52c29f
Optimize: reuse paged_kv_last_page_len buffer slice
c0de128 Dec 26, 2025
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21 changes: 12 additions & 9 deletions vllm/v1/attention/backends/mla/rocm_aiter_mla.py
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Original file line number Diff line number Diff line change
Expand Up @@ -88,16 +88,20 @@ def __init__(
# TODO: we can disambiguate between decode and mixed-prefill decode here
# so we can only use the persistent buffer if a cudagraph is actually
# being used.

# paged_kv_last_page_len is always 1s (kernel block size is always 1),
# so we create it once and reuse slices in both eager and cudagraph modes.
self.paged_kv_last_page_len = torch.ones(
max_num_reqs, dtype=torch.int32, device=device
)

if self.compilation_config.cudagraph_mode.has_full_cudagraphs():
self.paged_kv_indptr = torch.zeros(
max_num_reqs + 1, dtype=torch.int32, device=device
)
self.paged_kv_indices = torch.zeros(
max_num_pages, dtype=torch.int32, device=device
)
self.paged_kv_last_page_len = torch.zeros(
max_num_reqs, dtype=torch.int32, device=device
)

self.qo_indptr = torch.zeros(
max_num_reqs + 1, dtype=torch.int32, device=device
Expand All @@ -122,7 +126,9 @@ def _build_decode(
).unsqueeze(0) < seq_lens_device.unsqueeze(1)
paged_kv_indices = block_table_tensor[mask]

paged_kv_last_page_len = torch.where(seq_lens_device == 0, 1, seq_lens_device)
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@ganyi1996ppo ganyi1996ppo Dec 25, 2025

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This line indeed looks incorrect to me. @zq1997 please double confirm it, is this a mistake?

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@zq1997 zq1997 Dec 25, 2025

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After confirmation, I am very sorry that this was indeed a mistake I made.

# kernel block size is always 1, so each page has exactly 1 token.
# last_page_len is always 1 - just slice the pre-initialized buffer.
paged_kv_last_page_len = self.paged_kv_last_page_len[:num_reqs]

paged_kv_indptr = torch.cat(
[
Expand All @@ -148,11 +154,8 @@ def _build_decode(
self.paged_kv_indptr[1 + num_reqs :].fill_(paged_kv_indptr[-1])
paged_kv_indptr = self.paged_kv_indptr[: 1 + num_reqs]

self.paged_kv_last_page_len[:num_reqs].copy_(
paged_kv_last_page_len, non_blocking=True
)
self.paged_kv_last_page_len[num_reqs:].fill_(1)
paged_kv_last_page_len = self.paged_kv_last_page_len[:num_reqs]
# paged_kv_last_page_len already uses the pre-initialized buffer slice
# (set above), so no copy needed - buffer is always 1s.

self.qo_indptr[: 1 + num_reqs].copy_(
query_start_loc_device, non_blocking=True
Expand Down