Revert "mm/ksm: convert break_ksm() from walk_page_range_vma() to folio_walk"

Revert "mm/ksm: convert break_ksm() from walk_page_range_vma() to folio_walk"

Patch series "ksm: perform a range-walk to jump over holes in break_ksm",
v4.

When unmerging an address range, unmerge_ksm_pages function walks every
page address in the specified range to locate ksm pages.  This becomes
highly inefficient when scanning large virtual memory areas that contain
mostly unmapped regions, causing the process to get blocked for several
minutes.

This patch makes break_ksm, function called by unmerge_ksm_pages for every
page in an address range, perform a range walk, allowing it to skip over
entire unmapped holes in a VMA, avoiding unnecessary lookups.

As pointed out by David Hildenbrand in [1], unmerge_ksm_pages() is called
from:

* ksm_madvise() through madvise(MADV_UNMERGEABLE).  There are not a lot
  of users of that function.

* __ksm_del_vma() through ksm_del_vmas().  Effectively called when
  disabling KSM for a process either through the sysctl or from s390x gmap
  code when enabling storage keys for a VM.

Consider the following test program which creates a 32 TiB mapping in the
virtual address space but only populates a single page:

#include <unistd.h>
#include <stdio.h>
#include <sys/mman.h>

/* 32 TiB */
const size_t size = 32ul * 1024 * 1024 * 1024 * 1024;

int main() {
        char *area = mmap(NULL, size, PROT_READ | PROT_WRITE,
                          MAP_NORESERVE | MAP_PRIVATE | MAP_ANON, -1, 0);

        if (area == MAP_FAILED) {
                perror("mmap() failed\n");
                return -1;
        }

        /* Populate a single page such that we get an anon_vma. */
        *area = 0;

        /* Enable KSM. */
        madvise(area, size, MADV_MERGEABLE);
        madvise(area, size, MADV_UNMERGEABLE);
        return 0;
}

Without this patch, this program takes 9 minutes to finish, while with
this patch it finishes in less then 5 seconds.

This patch (of 3):

This reverts commit e317a8d8b4f600fc7ec9725e26417030ee594f52 and changes
function break_ksm_pmd_entry() to use folios.

This reverts break_ksm() to use walk_page_range_vma() instead of
folio_walk_start().

Change break_ksm_pmd_entry() to call is_ksm_zero_pte() only if we know the
folio is present, and also rename variable ret to found.  This will make
it easier to later modify break_ksm() to perform a proper range walk.

Link: https://lkml.kernel.org/r/20251105184912.186329-1-pedrodemargomes@gmail.com
Link: https://lkml.kernel.org/r/20251105184912.186329-2-pedrodemargomes@gmail.com
Link: https://lore.kernel.org/linux-mm/e0886fdf-d198-4130-bd9a-be276c59da37@redhat.com/
Signed-off-by: Pedro Demarchi Gomes <pedrodemargomes@gmail.com>
Suggested-by: David Hildenbrand (Red Hat) <david@kernel.org>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>