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bits are superseded by commit 26bab757d41b ("linux-user: Introduce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>

diff --git a/linux-user/mmap.c b/linux-user/mmap.c
index 48e13737962ab430e3b52c37941bde4324f53d17..4e7a6be6ee19d023162756db4791b8dd3ff091c2 100644 (file)
--- 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;
+}

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index f55cdebee5abb4fda5313298a0fdf74df69feb41..8f68f255c0d96cbcdb03e389aaf60233cca91818 100644 (file)
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -538,7 +538,7 @@ static inline int target_to_host_errno(int target_errno)
     }
 }
 
-static inline abi_long get_errno(abi_long ret)
+abi_long get_errno(abi_long ret)
 {
     if (ret == -1)
         return -host_to_target_errno(errno);
@@ -11807,11 +11807,7 @@ static abi_long do_syscall1(CPUArchState *cpu_env, int num, abi_long arg1,
 
 #ifdef TARGET_NR_madvise
     case TARGET_NR_madvise:
-        /* A straight passthrough may not be safe because qemu sometimes
-           turns private file-backed mappings into anonymous mappings.
-           This will break MADV_DONTNEED.
-           This is a hint, so ignoring and returning success is ok.  */
-        return 0;
+        return target_madvise(arg1, arg2, arg3);
 #endif
 #ifdef TARGET_NR_fcntl64
     case TARGET_NR_fcntl64:

diff --git a/linux-user/user-internals.h b/linux-user/user-internals.h
index 6175ce53dbd6cb3906e07e37906ce6e25193b9b7..0280e76addda504ea9745a17324ba9d6923335bc 100644 (file)
--- a/linux-user/user-internals.h
+++ b/linux-user/user-internals.h
@@ -65,6 +65,7 @@ abi_long do_syscall(CPUArchState *cpu_env, int num, abi_long arg1,
                     abi_long arg8);
 extern __thread CPUState *thread_cpu;
 G_NORETURN void cpu_loop(CPUArchState *env);
+abi_long get_errno(abi_long ret);
 const char *target_strerror(int err);
 int get_osversion(void);
 void init_qemu_uname_release(void);

diff --git a/linux-user/user-mmap.h b/linux-user/user-mmap.h
index d1dec99c02499e2449c3286a211ccd795c5401e5..480ce1c11487b74d8b2b409fb0653f18b36e2d3b 100644 (file)
--- a/linux-user/user-mmap.h
+++ b/linux-user/user-mmap.h
@@ -25,6 +25,7 @@ int target_munmap(abi_ulong start, abi_ulong len);
 abi_long target_mremap(abi_ulong old_addr, abi_ulong old_size,
                        abi_ulong new_size, unsigned long flags,
                        abi_ulong new_addr);
+abi_long target_madvise(abi_ulong start, abi_ulong len_in, int advice);
 extern unsigned long last_brk;
 extern abi_ulong mmap_next_start;
 abi_ulong mmap_find_vma(abi_ulong, abi_ulong, abi_ulong);