linux-kernel - [PATCH] mm/hugetlb: Move vmf_anon_prepare upfront in hugetlb

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Date: Tue, 21 May 2024 09:34:46 +0200
From: Oscar Salvador <osalvador@...e.de>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	Muchun Song <muchun.song@...ux.dev>,
	Vishal Moola <vishal.moola@...il.com>,
	Oscar Salvador <osalvador@...e.de>
Subject: [PATCH] mm/hugetlb: Move vmf_anon_prepare upfront in hugetlb_wp

hugetlb_wp calls vmf_anon_prepare() after having allocated a page, which
means that we might need to call restore_reserve_on_error() upon error.
vmf_anon_prepare() releases the vma lock before returning, but
restore_reserve_on_error() expects the vma lock to be held by the caller.

Fix it by calling vmf_anon_prepare() before allocating the page.

Signed-off-by: Oscar Salvador <osalvador@...e.de>
Fixes: 9acad7ba3e25 ("hugetlb: use vmf_anon_prepare() instead of anon_vma_prepare()")
---
I did not hit this bug, I just spotted this because I was looking at hugetlb_wp
for some other reason. And I did not want to get creative to see if I could
trigger this so I could get a backtrace.
My assumption is that we could trigger this if 1) this was a shared mapping,
so no anon_vma and 2) we call in GUP code with FOLL_WRITE, which would cause
the FLAG_UNSHARE to be passed, so we will end up in hugetlb_wp().

 mm/hugetlb.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 6be78e7d4f6e..eb0d8a45505e 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6005,6 +6005,15 @@ static vm_fault_t hugetlb_wp(struct folio *pagecache_folio,
 	 * be acquired again before returning to the caller, as expected.
 	 */
 	spin_unlock(vmf->ptl);
+
+	/*
+	 * When the original hugepage is shared one, it does not have
+	 * anon_vma prepared.
+	 */
+	ret = vmf_anon_prepare(vmf);
+	if (unlikely(ret))
+		goto out_release_old;
+
 	new_folio = alloc_hugetlb_folio(vma, vmf->address, outside_reserve);
 
 	if (IS_ERR(new_folio)) {
@@ -6058,14 +6067,6 @@ static vm_fault_t hugetlb_wp(struct folio *pagecache_folio,
 		goto out_release_old;
 	}
 
-	/*
-	 * When the original hugepage is shared one, it does not have
-	 * anon_vma prepared.
-	 */
-	ret = vmf_anon_prepare(vmf);
-	if (unlikely(ret))
-		goto out_release_all;
-
 	if (copy_user_large_folio(new_folio, old_folio, vmf->real_address, vma)) {
 		ret = VM_FAULT_HWPOISON_LARGE | VM_FAULT_SET_HINDEX(hstate_index(h));
 		goto out_release_all;
-- 
2.45.1