Fix critical KV cache crash bug #771 #777
Open
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Resolves issue mozilla-ai#771 where server crashes with std::length_error when KV cache context shifting attempts to resize cache_tokens vector with integer underflow. The bug occurs in update_slots() when n_discard >= cache_tokens.size(), causing cache_tokens.resize(size - n_discard) to underflow and request massive memory allocation, triggering std::length_error exception. Changes: - Add bounds checking before cache_tokens.resize() in server.cpp:1714 - Clear cache_tokens when n_discard would cause underflow - Prevent negative n_discard values from causing issues This fix prevents production server crashes reported with Chinese text translation workloads and high memory pressure scenarios.
mofosyne
approved these changes
Aug 1, 2025
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
2 participants
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
Problem
Issue #771: Production servers crash with
std::length_error: vectorduring KV cache management.Root Cause
Integer underflow in
update_slots()at line 1714:When
n_discard >= cache_tokens.size(), the subtraction underflows, requesting massive memory allocation.Fix
Added bounds checking before resize:
Testing
Fixes production crashes during high memory pressure scenarios.