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bcachefs: Optimize bch2_dirent_name_bytes

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Avoids doing a full strnlen for getting the length of the name of a
dirent entry.

Given the fact that the name of dirents is stored at the end of the
bkey's value, and we know the length of that in u64s, we can find the
last u64 and figure out how many NUL bytes are at the end of the string.

On little endian systems this ends up being the leading zeros of the
last u64, whereas on big endian systems this ends up being the trailing
zeros of the last u64.
We can take that value in bits and divide it by 8 to get the number of
NUL bytes at the end.

There is no endian-fixup or other compatibility here as this is string
data interpreted as a u64.

Signed-off-by: Joshua Ashton <joshua@froggi.es>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
This commit is contained in:
Joshua Ashton 2023-08-12 22:26:30 +01:00 committed by Kent Overstreet
parent 01a7e74fe1
commit 29c336afeb
1 changed files with 16 additions and 3 deletions
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19
fs/bcachefs/dirent.c
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@ -15,10 +15,18 @@
static unsigned bch2_dirent_name_bytes(struct bkey_s_c_dirent d)
{
unsigned len = bkey_val_bytes(d.k) -
offsetof(struct bch_dirent, d_name);
unsigned bkey_u64s = bkey_val_u64s(d.k);
unsigned bkey_bytes = bkey_u64s * sizeof(u64);
u64 last_u64 = ((u64*)d.v)[bkey_u64s - 1];
#if CPU_BIG_ENDIAN
unsigned trailing_nuls = last_u64 ? __builtin_ctzll(last_u64) / 8 : 64 / 8;
#else
unsigned trailing_nuls = last_u64 ? __builtin_clzll(last_u64) / 8 : 64 / 8;
#endif
return strnlen(d.v->d_name, len);
return bkey_bytes -
offsetof(struct bch_dirent, d_name) -
trailing_nuls;
}
struct qstr bch2_dirent_get_name(struct bkey_s_c_dirent d)
@ -113,6 +121,11 @@ int bch2_dirent_invalid(const struct bch_fs *c, struct bkey_s_c k,
return -BCH_ERR_invalid_bkey;
}
if (d_name.len != strnlen(d_name.name, d_name.len)) {
prt_printf(err, "dirent has stray data after name's NUL");
return -BCH_ERR_invalid_bkey;
}
if (d_name.len == 1 && !memcmp(d_name.name, ".", 1)) {
prt_printf(err, "invalid name");
return -BCH_ERR_invalid_bkey;