> +static void update_ksm_flags(struct mmap_state *map)
> +{
> + map->flags = ksm_vma_flags(map->mm, map->file, map->flags);
> +}
> +
> +/*
> + * Are we guaranteed no driver can change state such as to preclude KSM merging?
> + * If so, let's set the KSM mergeable flag early so we don't break VMA merging.
> + *
> + * This is applicable when PR_SET_MEMORY_MERGE has been set on the mm_struct via
> + * prctl() causing newly mapped VMAs to have the KSM mergeable VMA flag set.
> + *
> + * If this is not the case, then we set the flag after considering mergeability,
> + * which will prevent mergeability as, when PR_SET_MEMORY_MERGE is set, a new
> + * VMA will not have the KSM mergeability VMA flag set, but all other VMAs will,
> + * preventing any merge.
> + */
> +static bool can_set_ksm_flags_early(struct mmap_state *map)
> +{
> + struct file *file = map->file;
> +
> + /* Anonymous mappings have no driver which can change them. */
> + if (!file)
> + return true;
> +
> + /* shmem is safe. */
Excuse me, why it's safe here? Does KSM support shmem?
> + if (shmem_file(file))
> + return true;
> +
> + /*
> + * If .mmap_prepare() is specified, then the driver will have already
> + * manipulated state prior to updating KSM flags.
> + */
Recommend expanding the comments here with slightly more verbose explanations to improve
code comprehension. Consider adding the following note (even though your commit log is
already sufficiently clear. :)
/*
* If .mmap_prepare() is specified, then the driver will have already
* manipulated state prior to updating KSM flags. So no need to worry
* about mmap callbacks modifying vm_flags after the KSM flag has been
* updated here, which could otherwise affect KSM eligibility.
*/
> + if (file->f_op->mmap_prepare)
> + return true;
> +
> + return false;
> +}
> +
