On 11/7/21 11:42 AM, Jarkko Sakkinen wrote: >>> It should be fairly effecient just to check the pages by using >>> encl->page_tree. >> That sounds more complicated and slower than what I suggested. You >> could even just check the refcount on the page. I _think_ page cache >> pages have a refcount of 2. So, look for the refcount that means "no >> more PCMD in this page", and just free it if so. > Umh, so... there is total 32 PCMD's per one page. When you place PCMD in a page, you do a get_page(). The refcount goes up by one. So, a PCMD page with one PCMD will (I think) have a refcount of 3. If you totally fill it up with 31 *more* PCMD entries, it will have a refcount of 34. You do *not* do a put_page() on the PCMD page at the end of the allocation phase. When the backing storage is freed, you drop the refcount. So, going from 32 PCMD entries to 31 entries in a page, you go from 34->33. When that refcount drops to 2, you know there is no more data in the page that's useful. At that point you can truncate it out of the backing storage. There's no reason to scan the page, or a boatload of other metadata. Just keep a refcount. Just use the *existing* 'struct page' refcount.
