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fix: promote index to type longlong to avoid numerical overflow #1500

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Merged
tridao merged 1 commit into from
Mar 2, 2025
Merged

fix: promote index to type longlong to avoid numerical overflow #1500

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2 changes: 1 addition & 1 deletion csrc/flash_attn/src/flash_bwd_kernel.h
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Original file line number Diff line number Diff line change
Expand Up @@ -118,7 +118,7 @@ inline __device__ void compute_dq_dk_dv_1colblock(const Params &params, const in
const index_t row_offset_dq = binfo.q_offset(params.dq_batch_stride, params.dq_row_stride, bidb)
+ (m_block_max - 1) * kBlockM * params.dq_row_stride + bidh * params.dq_head_stride;
const index_t row_offset_dq_accum = binfo.q_offset(params.seqlen_q_rounded * params.h * params.d_rounded, params.h * params.d_rounded, bidb)
+ ((m_block_max - 1) * kBlockM + (params.cu_seqlens_q == nullptr ? 0 : 128 * bidb)) * params.h * params.d_rounded + bidh * params.d_rounded
+ ((m_block_max - 1) * kBlockM + (params.cu_seqlens_q == nullptr ? 0 : 128ll * bidb)) * params.h * params.d_rounded + bidh * params.d_rounded
// If deterministic, each thread block will do atomicAdd to a different dQ_accum buffer.
+ (!params.deterministic ? 0 : blockIdx.x * params.dq_accum_split_stride);
const index_t row_offset_lse = (params.unpadded_lse? bidh * params.total_q + binfo.q_offset(params.seqlen_q, 1, bidb): (bidb * params.h + bidh) * params.seqlen_q) + (m_block_max - 1) * kBlockM;
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4 changes: 2 additions & 2 deletions csrc/flash_attn/src/flash_bwd_preprocess_kernel.h
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Expand Up @@ -79,7 +79,7 @@ inline __device__ void compute_dot_do_o(const Params &params) {
const index_t row_offset_o = binfo.q_offset(params.o_batch_stride, params.o_row_stride, bidb)
+ m_block * kBlockM * params.o_row_stride + bidh * params.o_head_stride;
const index_t row_offset_dq_accum = binfo.q_offset(params.seqlen_q_rounded * params.h * params.d_rounded, params.h * params.d_rounded, bidb)
+ (m_block * kBlockM + (params.cu_seqlens_q == nullptr ? 0 : 128 * bidb)) * params.h * params.d_rounded + bidh * params.d_rounded;
+ (m_block * kBlockM + (params.cu_seqlens_q == nullptr ? 0 : 128ll * bidb)) * params.h * params.d_rounded + bidh * params.d_rounded;
// Regarding 128 * params.b see a comment in mha_varlen_bwd about padding of dq_accum and softmax_d
const index_t row_offset_dpsum = (params.unpadded_lse ? (bidh * (params.total_q + 128 * params.b) + binfo.q_offset(params.seqlen_q_rounded, 1, bidb) + 128 * bidb): (bidb * params.h + bidh) * params.seqlen_q_rounded) + m_block * kBlockM;

Expand Down Expand Up @@ -205,7 +205,7 @@ inline __device__ void convert_dQ(const Params &params, const int nsplits) {
const index_t row_offset_dq = binfo.q_offset(params.dq_batch_stride, params.dq_row_stride, bidb)
+ m_block * kBlockM * params.dq_row_stride + bidh * params.dq_head_stride;
const index_t row_offset_dq_accum = binfo.q_offset(params.seqlen_q_rounded * params.h * params.d_rounded, params.h * params.d_rounded, bidb)
+ (m_block * kBlockM + (params.cu_seqlens_q == nullptr ? 0 : 128 * bidb)) * params.h * params.d_rounded + bidh * params.d_rounded;
+ (m_block * kBlockM + (params.cu_seqlens_q == nullptr ? 0 : 128ll * bidb)) * params.h * params.d_rounded + bidh * params.d_rounded;

Tensor gdQ = make_tensor(make_gmem_ptr(reinterpret_cast<Element *>(params.dq_ptr) + row_offset_dq),
Shape<Int<kBlockM>, Int<kHeadDim>>{},
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