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bcachefs: Add a comment for bucket helper types

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We've had bugs in the past with incorrect integer conversions in disk
accounting code, which is why bucket helpers now always return s64s; add
a comment explaining this.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
This commit is contained in:
Kent Overstreet 2024-07-30 20:35:59 -04:00
parent 7442b5cdf2
commit 90b211fa2d
1 changed files with 8 additions and 0 deletions
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8
fs/bcachefs/alloc_background.h
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@ -82,6 +82,14 @@ static inline bool bucket_data_type_mismatch(enum bch_data_type bucket,
bucket_data_type(bucket) != bucket_data_type(ptr);
}
/*
* It is my general preference to use unsigned types for unsigned quantities -
* however, these helpers are used in disk accounting calculations run by
* triggers where the output will be negated and added to an s64. unsigned is
* right out even though all these quantities will fit in 32 bits, since it
* won't be sign extended correctly; u64 will negate "correctly", but s64 is the
* simpler option here.
*/
static inline s64 bch2_bucket_sectors_total(struct bch_alloc_v4 a)
{
return a.stripe_sectors + a.dirty_sectors + a.cached_sectors;