linux-kernel - [PATCH v3 2/3] s390/crash: use static swap buffer for copy_to_user

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Date:   Mon, 18 Jul 2022 15:32:40 +0200
From:   Alexander Gordeev <agordeev@...ux.ibm.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Alexander Egorenkov <egorenar@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>
Cc:     Baoquan He <bhe@...hat.com>, Christoph Hellwig <hch@....de>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org
Subject: [PATCH v3 2/3] s390/crash: use static swap buffer for copy_to_user_real()

Currently a temporary page-size buffer is allocated for copying
oldmem to user space. That limits copy_to_user_real() operation
only to stages when virtual memory is available and still makes
it possible to fail while the system is being dumped.

Instead of reallocating single page on each copy_oldmem_page()
iteration use a statically allocated buffer.

This also paves the way for a further memcpy_real() rework where
no swap buffer is needed altogether.

Signed-off-by: Alexander Gordeev <agordeev@...ux.ibm.com>
---
 arch/s390/kernel/crash_dump.c | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/arch/s390/kernel/crash_dump.c b/arch/s390/kernel/crash_dump.c
index 8d7332d4444c..fd9fe93e48b6 100644
--- a/arch/s390/kernel/crash_dump.c
+++ b/arch/s390/kernel/crash_dump.c
@@ -53,6 +53,7 @@ struct save_area {
 };
 
 static LIST_HEAD(dump_save_areas);
+static char memcpy_real_buf[PAGE_SIZE];
 
 /*
  * Allocate a save area
@@ -179,23 +180,18 @@ int copy_oldmem_kernel(void *dst, unsigned long src, size_t count)
 static int copy_to_user_real(void __user *dest, unsigned long src, unsigned long count)
 {
 	int offs = 0, size, rc;
-	char *buf;
 
-	buf = (char *)__get_free_page(GFP_KERNEL);
-	if (!buf)
-		return -ENOMEM;
 	rc = -EFAULT;
 	while (offs < count) {
 		size = min(PAGE_SIZE, count - offs);
-		if (memcpy_real(buf, src + offs, size))
+		if (memcpy_real(memcpy_real_buf, src + offs, size))
 			goto out;
-		if (copy_to_user(dest + offs, buf, size))
+		if (copy_to_user(dest + offs, memcpy_real_buf, size))
 			goto out;
 		offs += size;
 	}
 	rc = 0;
 out:
-	free_page((unsigned long)buf);
 	return rc;
 }
 
-- 
2.34.1