[PATCH 2/3] shmem: update memory reservation on truncate

Konstantin Khlebnikov
2014-06-24T20:16:18Z

Shared anonymous mapping created without MAP_NORESERVE holds memory
reservation for whole range of shmem segment. Usually there is no way to
change its size, but /proc/<pid>/map_files/...
(available if CONFIG_CHECKPOINT_RESTORE=y) allows to do that.

This patch adjust memory reservation in shmem_setattr().

Signed-off-by: Konstantin Khlebnikov <koct9i@gmail.com>

---

exploit:

#include <sys/mman.h>
#include <unistd.h>
#include <stdio.h>

int main(int argc, char **argv)
{
unsigned long addr;
char path[100];

/* charge 4KiB */
addr = (unsigned long)mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0);
sprintf(path, "/proc/self/map_files/%lx-%lx", addr, addr + 4096);
truncate(path, 1 << 30);
/* uncharge 1GiB */
}
---
mm/shmem.c | 17 +++++++++++++++++
1 file changed, 17 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index 0aabcbd..a3c49d6 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -149,6 +149,19 @@ static inline void shmem_unacct_size(unsigned long flags, loff_t size)
vm_unacct_memory(VM_ACCT(size));
}

+static inline int shmem_reacct_size(unsigned long flags,
+ loff_t oldsize, loff_t newsize)
+{
+ if (!(flags & VM_NORESERVE)) {
+ if (VM_ACCT(newsize) > VM_ACCT(oldsize))
+ return security_vm_enough_memory_mm(current->mm,
+ VM_ACCT(newsize) - VM_ACCT(oldsize));
+ else if (VM_ACCT(newsize) < VM_ACCT(oldsize))
+ vm_unacct_memory(VM_ACCT(oldsize) - VM_ACCT(newsize));
+ }
+ return 0;
+}
+
/*
* ... whereas tmpfs objects are accounted incrementally as
* pages are allocated, in order to allow huge sparse files.
@@ -543,6 +556,10 @@ static int shmem_setattr(struct dentry *dentry, struct iattr *attr)
loff_t newsize = attr->ia_size;

if (newsize != oldsize) {
+ error = shmem_reacct_size(SHMEM_I(inode)->flags,
+ oldsize, newsize);
+ if (error)
+ return error;
i_size_write(inode, newsize);
inode->i_ctime = inode->i_mtime = CURRENT_TIME;
}