On Thu, Mar 03, 2022 at 08:21:19PM -0800, Hugh Dickins wrote: > The page_mapcount_reset() when folio_mapped() while mapping_exiting() > was devised long before there were huge or compound pages in the cache. > It is still valid for small pages, but not at all clear what's right to > check and reset on large pages. Just don't try when folio_test_large(). Thanks for bringing this up! I was really unsure about this chunk of code when converting unaccount_page_cache_page() to filemap_unaccount_folio(). Part of me wants to just delete the whole thing. I'm unconvinced by the argument; surely it's better to leak memory than perhaps reuse a page which should not have been freed yet? Also, the code doesn't take into account that folio_mapped() is freaking expensive for THP (512 cache lines, blowing away 32kB of your L1 cache!), and we may as well calculate folio_mapcount() while we're doing it. Do you see this report often on machines that don't have VM_BUG_ON_FOLIO() enabled?
