linux-user: Add partial support for MADV_DONTNEED

Currently QEMU ignores madvise(MADV_DONTNEED), which break apps that
rely on this for zeroing out memory [1]. Improve the situation by doing
a passthrough when the range in question is a host-page-aligned
anonymous mapping.

This is based on the patches from Simon Hausmann [2] and Chris Fallin
[3]. The structure is taken from Simon's patch. The PAGE_MAP_ANONYMOUS
bits are superseded by commit 26bab757d41b ("linux-user: Introduce
PAGE_ANON"). In the end the patch acts like the one from Chris: we
either pass-through the entire syscall, or do nothing, since doing this
only partially would not help the affected applications much. Finally,
add some extra checks to match the behavior of the Linux kernel [4].

[1] https://gitlab.com/qemu-project/qemu/-/issues/326
[2] https://patchew.org/QEMU/20180827084037.25316-1-simon.hausmann@qt.io/
[3] https://github.com/bytecodealliance/wasmtime/blob/v0.37.0/ci/qemu-madvise.patch
[4] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/madvise.c?h=v5.19-rc3#n1368

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20220621144205.158452-1-iii@linux.ibm.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

1 files changed, 64 insertions, 0 deletions

diff --git a/linux-user/mmap.c b/linux-user/mmap.c
index 48e1373796..4e7a6be6ee 100644
--- a/linux-user/mmap.c
+++ b/linux-user/mmap.c
@@ -835,3 +835,67 @@ abi_long target_mremap(abi_ulong old_addr, abi_ulong old_size,
     mmap_unlock();
     return new_addr;
 }
+
+static bool can_passthrough_madv_dontneed(abi_ulong start, abi_ulong end)
+{
+    ulong addr;
+
+    if ((start | end) & ~qemu_host_page_mask) {
+        return false;
+    }
+
+    for (addr = start; addr < end; addr += TARGET_PAGE_SIZE) {
+        if (!(page_get_flags(addr) & PAGE_ANON)) {
+            return false;
+        }
+    }
+
+    return true;
+}
+
+abi_long target_madvise(abi_ulong start, abi_ulong len_in, int advice)
+{
+    abi_ulong len, end;
+    int ret = 0;
+
+    if (start & ~TARGET_PAGE_MASK) {
+        return -TARGET_EINVAL;
+    }
+    len = TARGET_PAGE_ALIGN(len_in);
+
+    if (len_in && !len) {
+        return -TARGET_EINVAL;
+    }
+
+    end = start + len;
+    if (end < start) {
+        return -TARGET_EINVAL;
+    }
+
+    if (end == start) {
+        return 0;
+    }
+
+    if (!guest_range_valid_untagged(start, len)) {
+        return -TARGET_EINVAL;
+    }
+
+    /*
+     * A straight passthrough may not be safe because qemu sometimes turns
+     * private file-backed mappings into anonymous mappings.
+     *
+     * This is a hint, so ignoring and returning success is ok.
+     *
+     * This breaks MADV_DONTNEED, completely implementing which is quite
+     * complicated. However, there is one low-hanging fruit: host-page-aligned
+     * anonymous mappings. In this case passthrough is safe, so do it.
+     */
+    mmap_lock();
+    if ((advice & MADV_DONTNEED) &&
+        can_passthrough_madv_dontneed(start, end)) {
+        ret = get_errno(madvise(g2h_untagged(start), len, MADV_DONTNEED));
+    }
+    mmap_unlock();
+
+    return ret;
+}