linux-kernel - [PATCH] iov_iter: fix copy_page_from_iter_atomic() if KMAP_LOCAL_FORCE

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Message-ID: <dd5f0c89-186e-18e1-4f43-19a60f5a9774@google.com>
Date: Sun, 27 Oct 2024 15:23:23 -0700 (PDT)
From: Hugh Dickins <hughd@...gle.com>
To: Alexander Viro <viro@...iv.linux.org.uk>
cc: Andrew Morton <akpm@...ux-foundation.org>, 
    Christian Brauner <brauner@...nel.org>, 
    Matthew Wilcox <willy@...radead.org>, Christoph Hellwig <hch@....de>, 
    Kent Overstreet <kent.overstreet@...ux.dev>, 
    "Darrick J. Wong" <djwong@...nel.org>, 
    Thomas Gleixner <tglx@...utronix.de>, 
    Peter Zijlstra <peterz@...radead.org>, linux-fsdevel@...r.kernel.org, 
    linux-block@...r.kernel.org, linux-kernel@...r.kernel.org, 
    linux-mm@...ck.org
Subject: [PATCH] iov_iter: fix copy_page_from_iter_atomic() if
 KMAP_LOCAL_FORCE_MAP

generic/077 on x86_32 CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP=y with highmem,
on huge=always tmpfs, issues a warning and then hangs (interruptibly):

WARNING: CPU: 5 PID: 3517 at mm/highmem.c:622 kunmap_local_indexed+0x62/0xc9
CPU: 5 UID: 0 PID: 3517 Comm: cp Not tainted 6.12.0-rc4 #2
...
copy_page_from_iter_atomic+0xa6/0x5ec
generic_perform_write+0xf6/0x1b4
shmem_file_write_iter+0x54/0x67

Fix copy_page_from_iter_atomic() by limiting it in that case
(include/linux/skbuff.h skb_frag_must_loop() does similar).

But going forward, perhaps CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP is too
surprising, has outlived its usefulness, and should just be removed?

Fixes: 908a1ad89466 ("iov_iter: Handle compound highmem pages in copy_page_from_iter_atomic()")
Signed-off-by: Hugh Dickins <hughd@...gle.com>
Cc: stable@...r.kernel.org
---
 lib/iov_iter.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 1abb32c0da50..94051b83fdd8 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -461,6 +461,8 @@ size_t copy_page_from_iter_atomic(struct page *page, size_t offset,
 		size_t bytes, struct iov_iter *i)
 {
 	size_t n, copied = 0;
+	bool uses_kmap = IS_ENABLED(CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP) ||
+			 PageHighMem(page);
 
 	if (!page_copy_sane(page, offset, bytes))
 		return 0;
@@ -471,7 +473,7 @@ size_t copy_page_from_iter_atomic(struct page *page, size_t offset,
 		char *p;
 
 		n = bytes - copied;
-		if (PageHighMem(page)) {
+		if (uses_kmap) {
 			page += offset / PAGE_SIZE;
 			offset %= PAGE_SIZE;
 			n = min_t(size_t, n, PAGE_SIZE - offset);
@@ -482,7 +484,7 @@ size_t copy_page_from_iter_atomic(struct page *page, size_t offset,
 		kunmap_atomic(p);
 		copied += n;
 		offset += n;
-	} while (PageHighMem(page) && copied != bytes && n > 0);
+	} while (uses_kmap && copied != bytes && n > 0);
 
 	return copied;
 }
-- 
2.35.3