vfs: remove always taken if-branch in find_next_fd()

find_next_fd() finds the next free fd slot in the passed fdtable's
bitmap. It does so in two steps: first it checks whether the bitmap
has a free entry in the word containing start. If not, it looks at
second level bitmap that registers which words in the first level bitmap
are full and then looks at the first level bitmap at the first non-full
word.

In the current code the second level lookup is done by:

  bitbit = find_next_zero_bit(fdt->full_fds_bits, maxbit, bitbit) *
    BITS_PER_LONG;

where bitbit = start / BITS_PER_LONG. However, in the fast path (first
step) we already checked the word at bitbit, so we can skip that word bit
and start at bitbit+1. This also means that we can get rid of the branch

  if (bitbit > start)
    start = bitbit;

since if we set

  bitbit = find_next_zero_bit(fdt->full_fds_bits, maxbit, bitbit+1) *
    BITS_PER_LONG;

the reassigned bitbit can never be less than

  ((start/BITS_PER_LONG)+1) * BITS_PER_LONG > start

So the branch is always taken.

Obviously the reuse of the variable name bitbit (and the name itself) is
quite confusing, so change that as well.

Signed-off-by: Jori Koolstra <jkoolstra@xs4all.nl>
Link: https://patch.msgid.link/20260420101801.806785-1-jkoolstra@xs4all.nl
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/file.c b/fs/file.c
index 2c81c0b162d0571af71bb5ae57163f1b6ceb8cfa..e5c75b22e0c7c3e5fcee007b4c200b5019e3745d 100644 (file)
--- a/fs/file.c
+++ b/fs/file.c
@@ -544,24 +544,23 @@ struct files_struct init_files = {
 static unsigned int find_next_fd(struct fdtable *fdt, unsigned int start)
 {
        unsigned int maxfd = fdt->max_fds; /* always multiple of BITS_PER_LONG */
-       unsigned int maxbit = maxfd / BITS_PER_LONG;
-       unsigned int bitbit = start / BITS_PER_LONG;
+       unsigned int max_fds_words = maxfd / BITS_PER_LONG;
+       unsigned int fds_word_idx = start / BITS_PER_LONG;
        unsigned int bit;
 
        /*
         * Try to avoid looking at the second level bitmap
         */
-       bit = find_next_zero_bit(&fdt->open_fds[bitbit], BITS_PER_LONG,
+       bit = find_next_zero_bit(&fdt->open_fds[fds_word_idx], BITS_PER_LONG,
                                 start & (BITS_PER_LONG - 1));
        if (bit < BITS_PER_LONG)
-               return bit + bitbit * BITS_PER_LONG;
+               return bit + (fds_word_idx * BITS_PER_LONG);
 
-       bitbit = find_next_zero_bit(fdt->full_fds_bits, maxbit, bitbit) * BITS_PER_LONG;
-       if (bitbit >= maxfd)
+       bit = BITS_PER_LONG *
+               find_next_zero_bit(fdt->full_fds_bits, max_fds_words, fds_word_idx + 1);
+       if (bit >= maxfd)
                return maxfd;
-       if (bitbit > start)
-               start = bitbit;
-       return find_next_zero_bit(fdt->open_fds, maxfd, start);
+       return find_next_zero_bit(fdt->open_fds, maxfd, bit);
 }
 
 /*