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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