The patch titled Subject: util_macros.h: fix/rework find_closest() macros has been added to the -mm mm-nonmm-unstable branch. Its filename is util_macrosh-fix-rework-find_closest-macros.patch This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/util_macrosh-fix-rework-find_closest-macros.patch This patch will later appear in the mm-nonmm-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next via the mm-everything branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there every 2-3 working days ------------------------------------------------------ From: Alexandru Ardelean <aardelean@xxxxxxxxxxxx> Subject: util_macros.h: fix/rework find_closest() macros Date: Thu, 31 Oct 2024 08:37:06 +0200 A bug was found in the find_closest() (find_closest_descending() is also affected after some testing), where for certain values with small progressions, the rounding (done by averaging 2 values) causes an incorrect index to be returned. The rounding issues occur for progressions of 1, 2 and 3. It goes away when the progression/interval between two values is 4 or larger. It's particularly bad for progressions of 1. For example if there's an array of 'a = { 1, 2, 3 }', using 'find_closest(2, a ...)' would return 0 (the index of '1'), rather than returning 1 (the index of '2'). This means that for exact values (with a progression of 1), find_closest() will misbehave and return the index of the value smaller than the one we're searching for. For progressions of 2 and 3, the exact values are obtained correctly; but values aren't approximated correctly (as one would expect). Starting with progressions of 4, all seems to be good. This change reworks the find_closest(x,) macros to also check the difference between the left and right elements when 'x'. If the distance to the right is smaller (than the distance to the left), the index is incremented by 1. This also makes redundant the need for using the DIV_ROUND_CLOSEST() macro. For find_closest_descending(), the operator was changed from '>=' to '>'. Since the iteration is happening from the highest-to-lowest values, the '>=' comparison would (for small progressions) prefer higher values (as closer to the given values). For example: Given array 'a[] = { 10, 7, 4, 1 };' find_closest_descending(2, a,...) returns the index[2] for 4 find_closest_descending(5, a,...) returns the index[1] for 7 find_closest_descending(8, a,...) returns the index[0] for 10 Link: https://lkml.kernel.org/r/20241031063707.795842-1-aardelean@xxxxxxxxxxxx Signed-off-by: Alexandru Ardelean <aardelean@xxxxxxxxxxxx> Cc: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- include/linux/util_macros.h | 14 ++++++++++---- 1 file changed, 10 insertions(+), 4 deletions(-) --- a/include/linux/util_macros.h~util_macrosh-fix-rework-find_closest-macros +++ a/include/linux/util_macros.h @@ -7,12 +7,18 @@ #define __find_closest(x, a, as, op) \ ({ \ typeof(as) __fc_i, __fc_as = (as) - 1; \ - typeof(x) __fc_x = (x); \ + typeof(x) __fc_mid_x, __fc_x = (x); \ + typeof(x) __fc_left, __fc_right; \ typeof(*a) const *__fc_a = (a); \ for (__fc_i = 0; __fc_i < __fc_as; __fc_i++) { \ - if (__fc_x op DIV_ROUND_CLOSEST(__fc_a[__fc_i] + \ - __fc_a[__fc_i + 1], 2)) \ + __fc_mid_x = (__fc_a[__fc_i] + __fc_a[__fc_i + 1]) / 2; \ + if (__fc_x op __fc_mid_x) { \ + __fc_left = __fc_mid_x - __fc_a[__fc_i]; \ + __fc_right = __fc_a[__fc_i + 1] - __fc_mid_x; \ + if (__fc_right < __fc_left) \ + __fc_i++; \ break; \ + } \ } \ (__fc_i); \ }) @@ -38,7 +44,7 @@ * Similar to find_closest() but 'a' is expected to be sorted in descending * order. */ -#define find_closest_descending(x, a, as) __find_closest(x, a, as, >=) +#define find_closest_descending(x, a, as) __find_closest(x, a, as, >) /** * is_insidevar - check if the @ptr points inside the @var memory range. _ Patches currently in -mm which might be from aardelean@xxxxxxxxxxxx are util_macrosh-fix-rework-find_closest-macros.patch lib-util_macros_kunit-add-kunit-test-for-util_macrosh.patch