linux-kernel - [PATCH] iov_iter: call kmap on each page even for lowmem if CONFIG_DEBUG_KMAP_LOCAL_FORCE

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Message-Id: <20240301230908.7507-1-21cnbao@gmail.com>
Date: Sat,  2 Mar 2024 12:09:08 +1300
From: Barry Song <21cnbao@...il.com>
To: viro@...iv.linux.org.uk,
	akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Cc: herbert@...dor.apana.org.au,
	Barry Song <v-songbaohua@...o.com>
Subject: [PATCH] iov_iter: call kmap on each page even for lowmem if CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP is enabled

From: Barry Song <v-songbaohua@...o.com>

copy_page_from_iter_atomic() has the assumption lowmem will only
need one kmap to get start page_address() for all pages. This is
wrong if the debug option CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP is
enabled. This patch fixes it in the same way with skbuff.h by
always applying kmap one by one even for lowmem,

 static inline bool skb_frag_must_loop(struct page *p)
 {
 #if defined(CONFIG_HIGHMEM)
 	if (IS_ENABLED(CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP) || PageHighMem(p))
  		return true;
 #endif
 	return false;
 }

Reported-by: Herbert Xu <herbert@...dor.apana.org.au>
Closes: https://lore.kernel.org/all/ZeGtrvm9WGv5cxQV@gondor.apana.org.au/
Signed-off-by: Barry Song <v-songbaohua@...o.com>
---
 -v1:
 Herbert found and pointed out this problem when he reviewed my patch
 in crypto/scompress.c;
 99.9% of the credit for this patch goes to Herbert.

 lib/iov_iter.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index e0aa6b440ca5..2e8a5b32f152 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -490,7 +490,7 @@ size_t copy_page_from_iter_atomic(struct page *page, size_t offset,
 		char *p;
 
 		n = bytes - copied;
-		if (PageHighMem(page)) {
+		if (IS_ENABLED(CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP) || PageHighMem(page)) {
 			page += offset / PAGE_SIZE;
 			offset %= PAGE_SIZE;
 			n = min_t(size_t, n, PAGE_SIZE - offset);
@@ -501,7 +501,8 @@ 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 ((IS_ENABLED(CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP) || PageHighMem(page))
+		 && copied != bytes && n > 0);
 
 	return copied;
 }
-- 
2.34.1