Merge: Revert GUP commit that introduces potential bug

MR: https://gitlab.com/redhat/centos-stream/src/kernel/centos-stream-10/-/merge_requests/1156

JIRA: https://issues.redhat.com/browse/RHEL-101826

```
commit 517f496
Author: David Hildenbrand <david@redhat.com>
Date:   Wed Jun 11 15:13:14 2025 +0200

    mm/gup: revert "mm: gup: fix infinite loop within __get_longterm_locked"

    After commit 1aaf8c1 ("mm: gup: fix infinite loop within
    __get_longterm_locked") we are able to longterm pin folios that are not
    supposed to get longterm pinned, simply because they temporarily have the
    LRU flag cleared (esp.  temporarily isolated).

    For example, two __get_longterm_locked() callers can race, or
    __get_longterm_locked() can race with anything else that temporarily
    isolates folios.

    The introducing commit mentions the use case of a driver that uses
    vm_ops->fault to insert pages allocated through cma_alloc() into the page
    tables, assuming they can later get longterm pinned.  These pages/ folios
    would never have the LRU flag set and consequently cannot get isolated.
    There is no known in-tree user making use of that so far, fortunately.

    To handle that in the future -- and avoid retrying forever to
    isolate/migrate them -- we will need a different mechanism for the CMA
    area *owner* to indicate that it actually already allocated the page and
    is fine with longterm pinning it.  The LRU flag is not suitable for that.

    Probably we can lookup the relevant CMA area and query the bitmap; we only
    have have to care about some races, probably.  If already allocated, we
    could just allow longterm pinning)

    Anyhow, let's fix the "must not be longterm pinned" problem first by
    reverting the original commit.

    Link: https://lkml.kernel.org/r/20250611131314.594529-1-david@redhat.com
    Fixes: 1aaf8c1 ("mm: gup: fix infinite loop within __get_longterm_locked")
    Signed-off-by: David Hildenbrand <david@redhat.com>
    Closes: https://lore.kernel.org/all/20250522092755.GA3277597@tiffany/
    Reported-by: Hyesoo Yu <hyesoo.yu@samsung.com>
    Reviewed-by: John Hubbard <jhubbard@nvidia.com>
    Cc: Jason Gunthorpe <jgg@ziepe.ca>
    Cc: Peter Xu <peterx@redhat.com>
    Cc: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
    Cc: Aijun Sun <aijun.sun@unisoc.com>
    Cc: Alistair Popple <apopple@nvidia.com>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
```

Signed-off-by: CKI Backport Bot <cki-ci-bot+cki-gitlab-backport-bot@redhat.com>

---

<small>Created 2025-07-03 23:14 UTC by backporter - [KWF FAQ](https://red.ht/kernel_workflow_doc) - [Slack #team-kernel-workflow](https://redhat-internal.slack.com/archives/C04LRUPMJQ5) - [Source](https://gitlab.com/cki-project/kernel-workflow/-/blob/main/webhook/utils/backporter.py) - [Documentation](https://gitlab.com/cki-project/kernel-workflow/-/blob/main/docs/README.backporter.md) - [Report an issue](https://issues.redhat.com/secure/CreateIssueDetails!init.jspa?pid=12334433&issuetype=1&priority=4&summary=backporter+webhook+issue&components=kernel-workflow+/+backporter)</small>

Approved-by: Jarod Wilson <jarod@redhat.com>
Approved-by: Waiman Long <longman@redhat.com>
Approved-by: Donald Dutile <ddutile@redhat.com>
Approved-by: Rafael Aquini <raquini@redhat.com>
Approved-by: CKI KWF Bot <cki-ci-bot+kwf-gitlab-com@redhat.com>

Merged-by: CKI GitLab Kmaint Pipeline Bot <26919896-cki-kmaint-pipeline-bot@users.noreply.gitlab.com>

Author: CKI KWF Bot

mm/gup.c

@@ -2323,13 +2323,13 @@ static void pofs_unpin(struct pages_or_folios *pofs)
 /*
  * Returns the number of collected folios. Return value is always >= 0.
  */
-static void collect_longterm_unpinnable_folios(
+static unsigned long collect_longterm_unpinnable_folios(
 		struct list_head *movable_folio_list,
 		struct pages_or_folios *pofs)
 {
+	unsigned long i, collected = 0;
 	struct folio *prev_folio = NULL;
 	bool drain_allow = true;
-	unsigned long i;
 
 	for (i = 0; i < pofs->nr_entries; i++) {
 		struct folio *folio = pofs_get_folio(pofs, i);
@@ -2341,6 +2341,8 @@ static void collect_longterm_unpinnable_folios(
 		if (folio_is_longterm_pinnable(folio))
 			continue;
 
+		collected++;
+
 		if (folio_is_device_coherent(folio))
 			continue;
 
@@ -2362,6 +2364,8 @@ static void collect_longterm_unpinnable_folios(
 				    NR_ISOLATED_ANON + folio_is_file_lru(folio),
 				    folio_nr_pages(folio));
 	}
+
+	return collected;
 }
 
 /*
@@ -2438,9 +2442,11 @@ static long
 check_and_migrate_movable_pages_or_folios(struct pages_or_folios *pofs)
 {
 	LIST_HEAD(movable_folio_list);
+	unsigned long collected;
 
-	collect_longterm_unpinnable_folios(&movable_folio_list, pofs);
-	if (list_empty(&movable_folio_list))
+	collected = collect_longterm_unpinnable_folios(&movable_folio_list,
+						       pofs);
+	if (!collected)
 		return 0;
 
 	return migrate_longterm_unpinnable_folios(&movable_folio_list, pofs);