linux-kernel - [PATCH v2 2/7] mm/hugetlb: Stop using avoid

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Message-ID: <20250107204002.2683356-3-peterx@redhat.com>
Date: Tue,  7 Jan 2025 15:39:57 -0500
From: Peter Xu <peterx@...hat.com>
To: linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Cc: Breno Leitao <leitao@...ian.org>,
	Rik van Riel <riel@...riel.com>,
	Muchun Song <muchun.song@...ux.dev>,
	Naoya Horiguchi <nao.horiguchi@...il.com>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Ackerley Tng <ackerleytng@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	peterx@...hat.com,
	Oscar Salvador <osalvador@...e.de>
Subject: [PATCH v2 2/7] mm/hugetlb: Stop using avoid_reserve flag in fork()

When fork() and stumble on top of a dma-pinned hugetlb private page, CoW
must happen during fork() to guarantee dma coherency.

In this specific path, hugetlb pages need to be allocated for the child
process.  Stop using avoid_reserve=1 flag here: it's not required to be
used here, as dest_vma (which is destined to be a MAP_PRIVATE hugetlb vma)
will have no private vma resv map, and that will make sure it won't be able
to use a vma reservation later.

No functional change intended with this change.  Said that, it's still
wanted to do this, so as to reduce the usage of avoid_reserve to the only
one user, which is also why this flag was introduced initially in commit
04f2cbe35699 ("hugetlb: guarantee that COW faults for a process that called
mmap(MAP_PRIVATE) on hugetlbfs will succeed").  I don't see whoever else
should set it at all.

Further patch will clean up resv accounting based on this.

Signed-off-by: Peter Xu <peterx@...hat.com>
---
 mm/hugetlb.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 2bf971f77553..7be8c35d2a83 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5369,7 +5369,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
 				spin_unlock(src_ptl);
 				spin_unlock(dst_ptl);
 				/* Do not use reserve as it's private owned */
-				new_folio = alloc_hugetlb_folio(dst_vma, addr, 1);
+				new_folio = alloc_hugetlb_folio(dst_vma, addr, 0);
 				if (IS_ERR(new_folio)) {
 					folio_put(pte_folio);
 					ret = PTR_ERR(new_folio);
-- 
2.47.0