Discussion:
[PATCH] linux-user: Remove extra mapping
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Steve Mcpolin
2018-05-16 14:47:32 UTC
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When a guest mmap()'d a file, a transient MAP_ANONYMOUS mapping was
created, which required the kernel to reserve this memory, then
subsequently released by applying a mapping with just the requested
flags and fd.
This transient mapping causes spurious failures when the available
memory is smaller than the mapping. This patch avoids this transient
mapping.

Signed-off-by: Steve Mcpolin <***@vmware.com>
---
linux-user/mmap.c | 31 +++++++++++++++++++------------
1 file changed, 19 insertions(+), 12 deletions(-)

diff --git a/linux-user/mmap.c b/linux-user/mmap.c
index 9168a20..f91841f 100644
--- a/linux-user/mmap.c
+++ b/linux-user/mmap.c
@@ -453,21 +453,28 @@ abi_long target_mmap(abi_ulong start, abi_ulong len, int prot,
/* Note: we prefer to control the mapping address. It is
especially important if qemu_host_page_size >
qemu_real_host_page_size */
- p = mmap(g2h(start), host_len, prot,
- flags | MAP_FIXED | MAP_ANONYMOUS, -1, 0);
- if (p == MAP_FAILED)
- goto fail;
- /* update start so that it points to the file position at 'offset' */
- host_start = (unsigned long)p;
- if (!(flags & MAP_ANONYMOUS)) {
- p = mmap(g2h(start), len, prot,
+ if (flags & MAP_ANONYMOUS) {
+ offset = 0;
+ host_offset = 0;
+ fd = -1;
+ }
+ p = mmap(g2h(start), len, prot,
flags | MAP_FIXED, fd, host_offset);
- if (p == MAP_FAILED) {
- munmap(g2h(start), host_len);
- goto fail;
+ host_start = (uintptr_t)p;
+ if (p != MAP_FAILED && host_len > HOST_PAGE_ALIGN(len)) {
+ void *q;
+ q = mmap(g2h(start + len), host_len - HOST_PAGE_ALIGN(len),
+ prot, MAP_FIXED | MAP_ANONYMOUS, -1, 0);
+ if (q == MAP_FAILED) {
+ p = MAP_FAILED;
}
- host_start += offset - host_offset;
}
+ if (p == MAP_FAILED) {
+ munmap(g2h(start), host_len);
+ goto fail;
+ }
+ host_start += offset - host_offset;
+ /* update start so that it points to the file position at 'offset' */
start = h2g(host_start);
} else {
if (start & ~TARGET_PAGE_MASK) {
--
2.7.4
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