On Mon, 2007-10-15 at 18:11 -0500, Matt Mackall wrote:
> > Could we just have /proc/kpagereferenced? Is there a legitimate need
> > for other flags to be visible?
>
> Referenced, dirty, uptodate, lru, active, slab, writeback, reclaim,
> and buddy all look like they might be interesting to me from the point
> of view of watching what's happening in the VM graphically in
> real-time.
This is true, but it forces a lot of logic from the kernel to be run in
userspace to figure out what is going on. Looking at mainline today:
#define PG_reclaim 17 /* To be reclaimed asap */
...
#define PG_readahead PG_reclaim /* Reminder to do async read-ahead */
All of a sudden, to figure out which flag it actually is, we need to
have all of the logic that the kernel does.
Does this establish a fixed user<->kernel ABI that will keep us from
doing this in the future:
-#define PG_slab 7 /* slab debug (Suparna wants this) */
+#define PG_slab 14 /* slab debug (Suparna wants this) */
Or, even something like this:
-#define PageSlab(page) test_bit(PG_slab, &(page)->flags)
+#define PageSlab(page) (!PageLRU(page) && !PageHighmem(page))
If we actually had several (or even still one file) that exposed this
state, independent of the actual content of page->flags, I think we'd be
better off. I think that's the difference between a fun, super-useful
debugging feature and one that can stay in mainline and have
applications stay using it (without breaking) for a long time.
The flags you listed are things that I would imagine will always exist,
logically. But, we might not always have a specific page flag for pages
under writeback or in the buddy list for that matter. PG_buddy isn't
that old. Perhaps that would be better abstracted to something like
page_in_main_allocator().
-- Dave
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