linux-kernel - Re: [PATCH v4 00/14] Introduce Copy-On-Write to Page Table

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

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <Y+veStmKQa5o3A6j@strix-laptop>
Date:   Wed, 15 Feb 2023 03:17:30 +0800
From:   Chih-En Lin <shiyn.lin@...il.com>
To:     Pasha Tatashin <pasha.tatashin@...een.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        John Hubbard <jhubbard@...dia.com>,
        Nadav Amit <namit@...are.com>, Barry Song <baohua@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Yang Shi <shy828301@...il.com>, Peter Xu <peterx@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Zach O'Keefe <zokeefe@...gle.com>,
        Yun Zhou <yun.zhou@...driver.com>,
        Hugh Dickins <hughd@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Yu Zhao <yuzhao@...gle.com>, Juergen Gross <jgross@...e.com>,
        Tong Tiangen <tongtiangen@...wei.com>,
        Liu Shixin <liushixin2@...wei.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Li kunyu <kunyu@...china.com>,
        Minchan Kim <minchan@...nel.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Gautam Menghani <gautammenghani201@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Brown <broonie@...nel.org>, Will Deacon <will@...nel.org>,
        Vincenzo Frascino <Vincenzo.Frascino@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Andrei Vagin <avagin@...il.com>,
        Barret Rhoden <brho@...gle.com>,
        Michal Hocko <mhocko@...e.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Alexey Gladkov <legion@...nel.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-trace-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org,
        Dinglan Peng <peng301@...due.edu>,
        Pedro Fonseca <pfonseca@...due.edu>,
        Jim Huang <jserv@...s.ncku.edu.tw>,
        Huichun Feng <foxhoundsk.tw@...il.com>
Subject: Re: [PATCH v4 00/14] Introduce Copy-On-Write to Page Table

On Tue, Feb 14, 2023 at 01:52:16PM -0500, Pasha Tatashin wrote:
> On Tue, Feb 14, 2023 at 1:42 PM Chih-En Lin <shiyn.lin@...il.com> wrote:
> >
> > On Tue, Feb 14, 2023 at 11:30:26AM -0500, Pasha Tatashin wrote:
> > > > > The thing with THP is, that during fork(), we always allocate a backup PTE
> > > > > table, to be able to PTE-map the THP whenever we have to. Otherwise we'd
> > > > > have to eventually fail some operations we don't want to fail -- similar to
> > > > > the case where break_cow_pte() could fail now due to -ENOMEM although we
> > > > > really don't want to fail (e.g., change_pte_range() ).
> > > > >
> > > > > I always considered that wasteful, because in many scenarios, we'll never
> > > > > ever split a THP and possibly waste memory.
> > > > >
> > > > > Optimizing that for THP (e.g., don't always allocate backup THP, have some
> > > > > global allocation backup pool for splits + refill when close-to-empty) might
> > > > > provide similar fork() improvements, both in speed and memory consumption
> > > > > when it comes to anonymous memory.
> > > >
> > > > When collapsing huge pages, do/can they reuse those PTEs for backup?
> > > > So, we don't have to allocate the PTE or maintain the pool.
> > >
> > > It might not work for all pages, as collapsing pages might have had
> > > holes in the user page table, and there were no PTE tables.
> >
> > So if there have holes in the user page table, after we doing the
> > collapsing and then splitting. Do those holes be filled? Assume it is,
> > then, I think it's the reason why it's not work for all the pages.
> >
> > But, after those operations, Will the user get the additional and
> > unexpected memory (which is from the huge page filling)?
> 
> Yes, more memory is going to be allocated for a process in such THP
> collapse case. This is similar to madvise huge pages, and touching the
> first byte may allocate 2M.

Thanks for the explanation.
Yeah, It seems like the reuse case can't work for all the pages.

Thanks,
Chih-En Lin