[PATCH] vmcore: convert mmap_vmcore_fault() to use folios

[PATCH] vmcore: convert mmap_vmcore_fault() to use folios

[Tal Zussman]

Use a folio for the page cache page the s390 fault handler reads the
old kernel's memory into. This removes four compound_head() calls and
one of the last callers of find_or_create_page().

The vmcore mapping only has order-0 folios, so the logic is unchanged.

Compile tested for s390.

Signed-off-by: Tal Zussman <tz2294@columbia.edu>
---
This is based on the kexec-next branch of the liveupdate tree.
---
 fs/proc/vmcore.c | 27 ++++++++++++++-------------
 1 file changed, 14 insertions(+), 13 deletions(-)

diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
index 44d15436439f..2dc3803aa11d 100644
--- a/fs/proc/vmcore.c
+++ b/fs/proc/vmcore.c
@@ -474,29 +474,30 @@ static vm_fault_t mmap_vmcore_fault(struct vm_fault *vmf)
 	pgoff_t index = vmf->pgoff;
 	struct iov_iter iter;
 	struct kvec kvec;
-	struct page *page;
+	struct folio *folio;
 	loff_t offset;
 	int rc;
 
-	page = find_or_create_page(mapping, index, GFP_KERNEL);
-	if (!page)
+	folio = __filemap_get_folio(mapping, index,
+			FGP_LOCK | FGP_ACCESSED | FGP_CREAT, GFP_KERNEL);
+	if (IS_ERR(folio))
 		return VM_FAULT_OOM;
-	if (!PageUptodate(page)) {
-		offset = (loff_t) index << PAGE_SHIFT;
-		kvec.iov_base = page_address(page);
-		kvec.iov_len = PAGE_SIZE;
-		iov_iter_kvec(&iter, ITER_DEST, &kvec, 1, PAGE_SIZE);
+	if (!folio_test_uptodate(folio)) {
+		offset = folio_pos(folio);
+		kvec.iov_base = folio_address(folio);
+		kvec.iov_len = folio_size(folio);
+		iov_iter_kvec(&iter, ITER_DEST, &kvec, 1, folio_size(folio));
 
 		rc = __read_vmcore(&iter, &offset);
 		if (rc < 0) {
-			unlock_page(page);
-			put_page(page);
+			folio_unlock(folio);
+			folio_put(folio);
 			return vmf_error(rc);
 		}
-		SetPageUptodate(page);
+		folio_mark_uptodate(folio);
 	}
-	unlock_page(page);
-	vmf->page = page;
+	folio_unlock(folio);
+	vmf->page = folio_file_page(folio, index);
 	return 0;
 #else
 	return VM_FAULT_SIGBUS;

---
base-commit: 4d17a004cfd5489f88ad0a7a151dea11a533338b
change-id: 20260907-vmcore-fault-folio-4b382eeff262

Best regards,
--  
Tal Zussman <tz2294@columbia.edu>

Re: [PATCH] vmcore: convert mmap_vmcore_fault() to use folios

[sashiko-bot]

> Use a folio for the page cache page the s390 fault handler reads the
> old kernel's memory into. This removes four compound_head() calls and
> one of the last callers of find_or_create_page().
> 
> The vmcore mapping only has order-0 folios, so the logic is unchanged.
> 
> Compile tested for s390.
> 
> Signed-off-by: Tal Zussman <tz2294@columbia.edu>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260915-vmcore-fault-folio-v1-1-a0cb6278670f@columbia.edu?part=1