Re: [Qemu-trivial] [Qemu-devel] [PATCH] linux-user: Remove extra mapping

[Laurent Vivier]

Le 16/05/2018 à 16:47, Steve Mcpolin a écrit :
> 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 <address@hidden>
> ---
>  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),

I think you should use REAL_HOST_PAGE_ALIGN(len) above.

This is the page size used by the host mmap() (it comes from
getpagesize()), whereas HOST_PAGE_ALIGN() can come from the command line
arguments.

Thanks,
Laurent