lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
Open Source and information security mailing list archives
| ||
|
Date: Tue, 22 Nov 2022 01:33:27 -0800 (PST) From: Hugh Dickins <hughd@...gle.com> To: "Kirill A. Shutemov" <kirill@...temov.name> cc: Hugh Dickins <hughd@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Johannes Weiner <hannes@...xchg.org>, Matthew Wilcox <willy@...radead.org>, David Hildenbrand <david@...hat.com>, Vlastimil Babka <vbabka@...e.cz>, Peter Xu <peterx@...hat.com>, Yang Shi <shy828301@...il.com>, John Hubbard <jhubbard@...dia.com>, Mike Kravetz <mike.kravetz@...cle.com>, Sidhartha Kumar <sidhartha.kumar@...cle.com>, Muchun Song <songmuchun@...edance.com>, Miaohe Lin <linmiaohe@...wei.com>, Naoya Horiguchi <naoya.horiguchi@...ux.dev>, Mina Almasry <almasrymina@...gle.com>, James Houghton <jthoughton@...gle.com>, Zach O'Keefe <zokeefe@...gle.com>, linux-kernel@...r.kernel.org, linux-mm@...ck.org Subject: Re: [PATCH 2/3] mm,thp,rmap: subpages_mapcount COMPOUND_MAPPED if PMD-mapped On Mon, 21 Nov 2022, Kirill A. Shutemov wrote: > On Fri, Nov 18, 2022 at 01:14:17AM -0800, Hugh Dickins wrote: > > Can the lock_compound_mapcount() bit_spin_lock apparatus be removed now? > > Yes. Not by atomic64_t or cmpxchg games, those get difficult on 32-bit; > > but if we slightly abuse subpages_mapcount by additionally demanding that > > one bit be set there when the compound page is PMD-mapped, then a cascade > > of two atomic ops is able to maintain the stats without bit_spin_lock. > > Yay! New home for PageDoubleMap()! :P :) You only asked for one bit for PageDoubleMap, I've been greedier; so it's not surprising if it has worked out better now. ... > Jokes aside, looks neat. > > Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com> Thanks; but I'm very glad that Linus expressed his dissatisfaction with the first implementation, this one does feel much better. > > As always few minor nits below. ... > > @@ -893,8 +902,12 @@ static inline int total_mapcount(struct page *page) > > > > static inline bool folio_large_is_mapped(struct folio *folio) > > { > > - return atomic_read(folio_mapcount_ptr(folio)) + > > - atomic_read(folio_subpages_mapcount_ptr(folio)) >= 0; > > + /* > > + * Reading folio_mapcount_ptr() below could be omitted if hugetlb > > + * participated in incrementing subpages_mapcount when compound mapped. > > + */ > > + return atomic_read(folio_mapcount_ptr(folio)) >= 0 || > > + atomic_read(folio_subpages_mapcount_ptr(folio)) > 0; > > Maybe check folio_subpages_mapcount_ptr() first? It would avoid > folio_mapcount_ptr() read for everything, but hugetlb. Okay: I'm not convinced, but don't mind switching those around: done. > > --- a/mm/debug.c > > +++ b/mm/debug.c > > @@ -97,7 +97,7 @@ static void __dump_page(struct page *page) > > pr_warn("head:%p order:%u compound_mapcount:%d subpages_mapcount:%d compound_pincount:%d\n", > > head, compound_order(head), > > head_compound_mapcount(head), > > - head_subpages_mapcount(head), > > + head_subpages_mapcount(head) & SUBPAGES_MAPPED, > > Looks like applying the SUBPAGES_MAPPED mask belong to the > head_subpages_mapcount() helper, not to the caller. Yes, that would be more consistent, helper function doing the massage. Done. __dump_page() then remains unchanged, but free_tail_pages_check() uses subpages_mapcount_ptr(head_page) to check the whole field is zero. v2 coming up - thanks. Hugh
Powered by blists - more mailing lists