FAILED: patch "[PATCH] mm/compaction: fix bug in hugetlb handling pathway" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x a84edd52f0a0fa193f0f685769939cf84510755b
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025042101-algorithm-simply-9ce6@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From a84edd52f0a0fa193f0f685769939cf84510755b Mon Sep 17 00:00:00 2001
From: "Vishal Moola (Oracle)" <[email protected]>
Date: Mon, 31 Mar 2025 19:10:24 -0700
Subject: [PATCH] mm/compaction: fix bug in hugetlb handling pathway

The compaction code doesn't take references on pages until we're certain we should attempt to handle it.

In the hugetlb case, isolate_or_dissolve_huge_page() may return -EBUSY without taking a reference to the folio associated with our pfn. If our folio's refcount drops to 0, compound_nr() becomes unpredictable, making low_pfn and nr_scanned unreliable. The user-visible effect is minimal - this should rarely happen (if ever).

Fix this by storing the folio statistics earlier on the stack (just like the THP and Buddy cases).

Also revert commit 66fe1cf7f581 ("mm: compaction: use helper compound_nr in isolate_migratepages_block") to make backporting easier.

index 139f00c0308a..ca71fd3c3181 100644
@@ -981,13 +981,13 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 		}
 
 		if (PageHuge(page)) {
+			const unsigned int order = compound_order(page);
 			/*
 			 * skip hugetlbfs if we are not compacting for pages
 			 * bigger than its order. THPs and other compound pages
 			 * are handled below.
 			 */
 			if (!cc->alloc_contig) {
-				const unsigned int order = compound_order(page);
 
 				if (order <= MAX_PAGE_ORDER) {
 					low_pfn += (1UL << order) - 1;

@@ -1011,8 +1011,8 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 				 /* Do not report -EBUSY down the chain */
 				if (ret == -EBUSY)
 					ret = 0;
-				low_pfn += compound_nr(page) - 1;
-				nr_scanned += compound_nr(page) - 1;
+				low_pfn += (1UL << order) - 1;
+				nr_scanned += (1UL << order) - 1;
 				goto isolate_fail;
 			}