[uml-devel] madvise(DONTNEED) on tmpfs pages instead of /dev/anon

[Blaisorblade <blaisorblade@yahoo.it>]

UML would need to free file-backed pages anonymously mapped from the host 
memory. In all relevant cases, those pages are file-backed from tmpfs (or 
ramfs, but that can also be excluded)

About this, I was thinking back to what Rik said in Ottawa at the OLS. He said 
"just use madvise(DONTNEED) on it", but I remarked that the pages were 
file-backed.

Actually, however, since backing store for tmpfs is just pagecache/swapcache 
memory,

I have the doubt that madvise(DONTNEED) already does what we need, and if not 
that it might be easily fixed (since zap_page_range->unmap_vmas -> 
unmap_page_range accept a range of pages to be flushed - that was a recent 
change from Hugh Dickins).

For UML, the patch to use this support should be trivial (given in 
pseudocode):

+//remember virt, is an address in the (guest) kernel page range
int physmem_remove_mapping(void *virt)
{
        struct phys_desc *desc;

        virt = (void *) ((unsigned long) virt & PAGE_MASK);
        desc = find_phys_mapping(virt);
-        if(desc == NULL)
-                return(0);
+        if(desc == NULL) {
+                madvise(virt, PAGE_SIZE, MADV_DONTNEED);
+                return 0;
+       }

        remove_mapping(desc);
        return(1);
}

-- 
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
http://www.user-mode-linux.org/~blaisorblade


	

	
		
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