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Subject[PATCH v2] mlock: set VM_WRITE in case we don't have read permission.
Date
From: Tao Ma <boyu.mt@taobao.com>

In 5ecfda0, we do some optimization in mlock, but it causes
a very basic test case(attached below) of mlock to fail. So
this patch add another check that if we don't have read permission,
still set FOLL_WRITE flag. Thank KOSAKI for the suggestion.
The test program is attached below.

#include <sys/mman.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/types.h>

int main()
{
char *buf, *testfile = "test_mmap";
int fd, file_len = 40960, ret = -1;
fd = open(testfile, O_RDWR);
if (fd < 0) {
perror("open");
return -1;
}
if (ftruncate(fd, file_len) < 0) {
perror("ftruncate");
goto out;
}
buf = mmap(NULL, file_len, PROT_WRITE, MAP_SHARED, fd, 0);
if (buf == MAP_FAILED) {
perror("mmap");
goto out;
}
if (mlock(buf, file_len) < 0) {
perror("mlock");
goto out;
}
munlock(buf, file_len);
munmap(buf, file_len);
ret = 0;
out:
close(fd);
return ret;
}
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Tao Ma <boyu.mt@taobao.com>
---
mm/mlock.c | 7 +++++++
1 files changed, 7 insertions(+), 0 deletions(-)
diff --git a/mm/mlock.c b/mm/mlock.c
index 13e81ee..8508c5a 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -178,6 +178,13 @@ static long __mlock_vma_pages_range(struct vm_area_struct *vma,
if ((vma->vm_flags & (VM_WRITE | VM_SHARED)) == VM_WRITE)
gup_flags |= FOLL_WRITE;

+ /*
+ * We don't have readable permission. Therefore we can't use read
+ * operation even though it's faster.
+ */
+ if ((vma->vm_flags & (VM_READ|VM_WRITE)) == VM_WRITE)
+ gup_flags |= FOLL_WRITE;
+
if (vma->vm_flags & VM_LOCKED)
gup_flags |= FOLL_MLOCK;

--
1.6.3.GIT


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