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: Mon, 20 Jan 2020 08:22:37 +0100 From: Michal Hocko <mhocko@...nel.org> To: David Rientjes <rientjes@...gle.com> Cc: Andrew Morton <akpm@...ux-foundation.org>, Wei Yang <richardw.yang@...ux.intel.com>, hannes@...xchg.org, vdavydov.dev@...il.com, ktkhai@...tuozzo.com, kirill.shutemov@...ux.intel.com, yang.shi@...ux.alibaba.com, cgroups@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org, alexander.duyck@...il.com, stable@...r.kernel.org Subject: Re: [Patch v4] mm: thp: remove the defer list related code since this will not happen On Sat 18-01-20 15:36:06, David Rientjes wrote: > On Sat, 18 Jan 2020, Andrew Morton wrote: > > > On Sat, 18 Jan 2020 07:38:36 +0800 Wei Yang <richardw.yang@...ux.intel.com> wrote: > > > > > If compound is true, this means it is a PMD mapped THP. Which implies > > > the page is not linked to any defer list. So the first code chunk will > > > not be executed. > > > > > > Also with this reason, it would not be proper to add this page to a > > > defer list. So the second code chunk is not correct. > > > > > > Based on this, we should remove the defer list related code. > > > > > > Fixes: 87eaceb3faa5 ("mm: thp: make deferred split shrinker memcg aware") > > > > > > Signed-off-by: Wei Yang <richardw.yang@...ux.intel.com> > > > Suggested-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com> > > > Cc: <stable@...r.kernel.org> [5.4+] > > > > This patch is identical to "mm: thp: grab the lock before manipulating > > defer list", which is rather confusing. Please let people know when > > this sort of thing is done. > > > > The earlier changelog mentioned a possible race condition. This > > changelog does not. In fact this changelog fails to provide any > > description of any userspace-visible runtime effects of the bug. > > Please send along such a description for inclusion, as always. > > > > The locking concern that Wei was originally looking at is no longer an > issue because we determined that the code in question could simply be > removed. > > I think the following can be added to the changelog: > > ----->o----- > > When migrating memcg charges of thp memory, there are two possibilities: > > (1) The underlying compound page is mapped by a pmd and thus does is not > on a deferred split queue (it's mapped), or > > (2) The compound page is not mapped by a pmd and is awaiting split on a > deferred split queue. > > The current charge migration implementation does *not* migrate charges for > thp memory on the deferred split queue, it only migrates charges for pages > that are mapped by a pmd. > > Thus, to migrate charges, the underlying compound page cannot be on a > deferred split queue; no list manipulation needs to be done in > mem_cgroup_move_account(). > > With the current code, the underlying compound page is moved to the > deferred split queue of the memcg its memory is not charged to, so > susbequent reclaim will consider these pages for the wrong memcg. Remove > the deferred split queue handling in mem_cgroup_move_account() entirely. I believe this still doesn't describe the underlying problem to the full extent. What happens with the page on the deferred list when it shouldn't be there in fact? Unless I am missing something deferred_split_scan will simply split that huge page. Which is a bit unfortunate but nothing really critical. This should be mentioned in the changelog. With that clarified, feel free to add Acked-by: Michal Hocko <mhocko@...e.com> -- Michal Hocko SUSE Labs
Powered by blists - more mailing lists