Re: [PATCH][RFC] move close_range(2) into fs/file.c, fold __close_range() into it

[Al Viro <viro@zeniv.linux.org.uk>]

On Sun, Jun 02, 2024 at 09:42:38PM +0100, Al Viro wrote:
> 	We never had callers for __close_range() except for close_range(2)
> itself.  Nothing of that sort has appeared in four years and if any users
> do show up, we can always separate those suckers again.

BTW, looking through close_range()...  We have

static unsigned int sane_fdtable_size(struct fdtable *fdt, unsigned int max_fds)
{
	unsigned int count;

	count = count_open_files(fdt);
	if (max_fds < NR_OPEN_DEFAULT)
		max_fds = NR_OPEN_DEFAULT;
	return ALIGN(min(count, max_fds), BITS_PER_LONG);
}

which decides how large a table would be needed for descriptors below max_fds
that are opened in fdt.  And we start with finding the last opened descriptor
in fdt (well, rounded up to BITS_PER_LONG, if you look at count_open_files()).

Why do we bother to look at _anything_ past the max_fds?  Does anybody have
objections to the following?

diff --git a/fs/file.c b/fs/file.c
index f9fcebc7c838..4976ede108e0 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -276,20 +276,6 @@ static inline bool fd_is_open(unsigned int fd, const struct fdtable *fdt)
 	return test_bit(fd, fdt->open_fds);
 }
 
-static unsigned int count_open_files(struct fdtable *fdt)
-{
-	unsigned int size = fdt->max_fds;
-	unsigned int i;
-
-	/* Find the last open fd */
-	for (i = size / BITS_PER_LONG; i > 0; ) {
-		if (fdt->open_fds[--i])
-			break;
-	}
-	i = (i + 1) * BITS_PER_LONG;
-	return i;
-}
-
 /*
  * Note that a sane fdtable size always has to be a multiple of
  * BITS_PER_LONG, since we have bitmaps that are sized by this.
@@ -305,12 +291,18 @@ static unsigned int count_open_files(struct fdtable *fdt)
  */
 static unsigned int sane_fdtable_size(struct fdtable *fdt, unsigned int max_fds)
 {
-	unsigned int count;
+	const unsigned int min_words = BITS_TO_LONGS(NR_OPEN_DEFAULT);  // 1
+	unsigned int words;
+
+	if (max_fds <= NR_OPEN_DEFAULT)
+		return NR_OPEN_DEFAULT;
+
+	words = BITS_TO_LONGS(min(max_fds, fdt->max_fds)); // >= min_words
+
+	while (words > min_words && !fdt->open_fds[words - 1])
+		words--;
 
-	count = count_open_files(fdt);
-	if (max_fds < NR_OPEN_DEFAULT)
-		max_fds = NR_OPEN_DEFAULT;
-	return ALIGN(min(count, max_fds), BITS_PER_LONG);
+	return words * BITS_PER_LONG;
 }
 
 /*