vm: Allow in-band metadata for cache blocks

Allow extra space for in-band metadata when allocating cache blocks.

Edited by David van Moolenbroek: since this effectively halves the
potential size of the typical file system cache, do this only when
compiling with instrumentation.

Change-Id: I0840af6420899ede2d5bb7539e79c0a456b5128d

diff --git a/minix/servers/vm/mem_cache.c b/minix/servers/vm/mem_cache.c
index 93421a2592aae9faa47f22895a94cce4e9412ec9..56c596dac7e4009060f11681d20f8241a6dcec6b 100644 (file)
--- a/minix/servers/vm/mem_cache.c
+++ b/minix/servers/vm/mem_cache.c
@@ -30,6 +30,7 @@ static int cache_unreference(struct phys_region *pr);
 static int cache_sanitycheck(struct phys_region *pr, const char *file, int line);
 static int cache_writable(struct phys_region *pr);
 static int cache_resize(struct vmproc *vmp, struct vir_region *vr, vir_bytes l);
+static int cache_lowshrink(struct vir_region *vr, vir_bytes len);
 static int cache_pagefault(struct vmproc *vmp, struct vir_region *region, 
         struct phys_region *ph, int write, vfs_callback_t cb, void *state,
        int len, int *io);
@@ -40,6 +41,7 @@ struct mem_type mem_type_cache = {
        .ev_reference = cache_reference,
        .ev_unreference = cache_unreference,
        .ev_resize = cache_resize,
+       .ev_lowshrink = cache_lowshrink,
        .ev_sanitycheck = cache_sanitycheck,
        .ev_pagefault = cache_pagefault,
        .writable = cache_writable,
@@ -84,6 +86,11 @@ static int cache_resize(struct vmproc *vmp, struct vir_region *vr, vir_bytes l)
        return ENOMEM;
 }
 
+static int cache_lowshrink(struct vir_region *vr, vir_bytes len)
+{
+        return OK;
+}
+
 int
 do_mapcache(message *msg)
 {
@@ -92,6 +99,7 @@ do_mapcache(message *msg)
        off_t ino_off = msg->m_vmmcp.ino_offset;
        int n;
        phys_bytes bytes = msg->m_vmmcp.pages * VM_PAGE_SIZE;
+       phys_bytes alloc_bytes;
        struct vir_region *vr;
        struct vmproc *caller;
        vir_bytes offset;
@@ -107,11 +115,23 @@ do_mapcache(message *msg)
 
        if(bytes < VM_PAGE_SIZE) return EINVAL;
 
-       if(!(vr = map_page_region(caller, VM_PAGE_SIZE, VM_DATATOP, bytes,
-               VR_ANON | VR_WRITABLE, 0, &mem_type_cache))) {
+       alloc_bytes = bytes;
+#ifdef _MINIX_MAGIC
+       /* Make sure there is a 1-page hole available before the region,
+        * in case instrumentation needs to allocate in-band metadata later.
+        * This does effectively halve the usable part of the caller's address
+        * space, though, so only do this if we are instrumenting at all.
+        */
+       alloc_bytes += VM_PAGE_SIZE;
+#endif
+       if (!(vr = map_page_region(caller, VM_PAGE_SIZE, VM_DATATOP,
+           alloc_bytes, VR_ANON | VR_WRITABLE, 0, &mem_type_cache))) {
                printf("VM: map_page_region failed\n");
                return ENOMEM;
        }
+#ifdef _MINIX_MAGIC
+       map_unmap_region(caller, vr, 0, VM_PAGE_SIZE);
+#endif
 
        assert(vr->length == bytes);