Daniel Phillips [off-list ref] writes:

On August 16, 2001 12:26 pm, Stephen C. Tweedie wrote:

quoted

Hi,

On Thu, Aug 16, 2001 at 10:30:35AM +0200, Daniel Phillips wrote:

quoted

because the use count is overloaded.  So how about adding a PG_pinned
flag, and users need to set it for any page they intend to pin.

It needs to be a count, not a flag (consider multiple mlock() calls
from different processes, or multiple direct IO writeouts from the
same memory to disk.)  

Yes, the question is how to do this without adding a yet another field
to struct page.

atomic_add(&page->count, 65536);  Basically you can add the high bits.  
But we only need the count seperate so that when a page becomes
demand freeable we can remove it from the global unfreeable page count.
But please let's not call a non-freeable page pinned.  We already use that
term for pages that are temporarily pinned for I/O.  And pinning in my mind
is not a permanent situation.

Actually except for mlock on a user space page we can use only a single bit,
so it might make more sense on the munlock case to walk the list of vma's
and see if the page is still mlocked somewhere else.

Something like:
if (test_bit(&page->flags, PG_Unfreeable)) {
        if (page->mapping && (page->mapping->i_mmap || page->mapping->i_mmap_shared)) {
                /* walk page->mapping->i_mmap & page->mapping->i_mmap->i_mmap_shared */
                /* if the page is no longer mlocked clear PG_Unfreeable */
	} else {
                clear_bit(&page->flags, PG_Unfreeable);
        }
	if (!test_bit(&page->flags, PG_Unfreeable)) {
		atomic_dec(&unfreeable_pages);
		/* Actually because of the limited range of the atomic
                 * types we probably need a spinlock...
		 */
        }       
}


kmalloc, the slab cache, and the inode cache are where we get most of
the pages that aren't freeable.  And since those cases don't mmap the
pages it shouldn't be too much overhead in the common cases.

Additionally if we do have a variant on free_page that only does the
tests for unlocking when we know we are freeing something from a
locked vma, we should be able to keep the overhead down quite nicely.

Eric
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