Mask and zero bits in metadata bytes for bulk zeroing #707
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Merged
wks
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Dec 1, 2022
| let zero = |data_start: Address, data_end: Address| { | ||
| let meta_start = address_to_meta_address(self, data_start); | ||
| let meta_start_shift = meta_byte_lshift(self, data_start); | ||
| let meta_end = address_to_meta_address(self, data_end); |
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It is OK if the metadata is contiguous. But if it is discontiguous, then meta_start and meta_end may be in two unrelated chunks.
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I changed zero to zero_contiguous which uses the same implementation as what you reviewed, and added zero_discontiguous which computes the total bits that need to be zeroed without computing the end address.
| meta_start, | ||
| address_to_meta_address(self, second_data_chunk) - meta_start, | ||
| ); | ||
| zero(start, second_data_chunk); |
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Two problems:
- If
startis already aligned toBYTES_IN_CHUNK, thenstart.align_up(BYTES_IN_CHUNK)will be equal tostart, and it will end up callingzero(start, start). - If the metadata is discontiguous, passing
second_data_chunkto thezerofunction will result inmeta_endbeing in a different metadata chunk, and unrelated metadata in between will be overwritten.
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- I added a check, if the size to be zeroed is 0, then the method simply returns.
- See my reply for the above comments.
| address_to_meta_address(self, next_data_chunk), | ||
| metadata_bytes_per_chunk(self.log_bytes_in_region, self.log_num_of_bits), | ||
| ); | ||
| zero(next_data_chunk, next_data_chunk + BYTES_IN_CHUNK); |
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Same problem here. next_data_chunk + BYTES_IN_CHUNK will be mapped to the next metadata chunk.
discontiguous/chunked metadata.
wks
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Dec 2, 2022
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| let meta_total_bits = (data_bytes >> self.log_bytes_in_region) << self.log_num_of_bits; | ||
| let meta_delta_bytes = meta_total_bits >> LOG_BITS_IN_BYTE; | ||
| let meta_delta_bits: u8 = (meta_total_bits % BITS_IN_BYTE) as u8; |
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Nicely done. I think these lines of code are even clearer than those in address_to_contiguous_meta_address and meta_byte_lshift.
udesou
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Dec 5, 2022
This PR fixes a bug in `bzero_metadata()` discussed in mmtk#643 (comment). When the data address range to be bulk zeroed cannot be mapped into whole bytes in the side metadata, the other bits in the bytes which should not be updated will get zeroed unexpectedly.
wenyuzhao
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to wenyuzhao/mmtk-core
that referenced
this pull request
Mar 20, 2023
This PR fixes a bug in `bzero_metadata()` discussed in mmtk#643 (comment). When the data address range to be bulk zeroed cannot be mapped into whole bytes in the side metadata, the other bits in the bytes which should not be updated will get zeroed unexpectedly.
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This PR fixes a bug in
bzero_metadata()discussed in #643 (comment). When the data address range to be bulk zeroed cannot be mapped into whole bytes in the side metadata, the other bits in the bytes which should not be updated will get zeroed unexpectedly.