linux-kernel - Re: mlockall(MCL_CURRENT) blocking infinitely

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

Message-ID: <20191106143855.GJ16085@quack2.suse.cz>
Date:   Wed, 6 Nov 2019 15:38:55 +0100
From:   Jan Kara <jack@...e.cz>
To:     snazy@...zy.de
Cc:     Jan Kara <jack@...e.cz>, Johannes Weiner <hannes@...xchg.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        Josef Bacik <josef@...icpanda.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-kernel@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Potyra, Stefan" <Stefan.Potyra@...ktrobit.com>
Subject: Re: mlockall(MCL_CURRENT) blocking infinitely

On Wed 06-11-19 14:45:43, Robert Stupp wrote:
> On Wed, 2019-11-06 at 13:03 +0100, Jan Kara wrote:
> > On Tue 05-11-19 13:22:11, Johannes Weiner wrote:
> > > What I don't quite understand yet is why the fault path doesn't
> > > make
> > > progress eventually. We must drop the mmap_sem without changing the
> > > state in any way. How can we keep looping on the same page?
> >
> > That may be a slight suboptimality with Josef's patches. If the page
> > is marked as PageReadahead, we always drop mmap_sem if we can and
> > start
> > readahead without checking whether that makes sense or not in
> > do_async_mmap_readahead(). OTOH page_cache_async_readahead() then
> > clears
> > PageReadahead so the only way how I can see we could loop like this
> > is when
> > file->ra->ra_pages is 0. Not sure if that's what's happening through.
> > We'd
> > need to find which of the paths in filemap_fault() calls
> > maybe_unlock_mmap_for_io() to tell more.
> 
> Yes, ra_pages==0

BTW, attached patch should workaround your problem as well. But that's just
a performance optimization that happens to paper over your problem. Real
fix is the proper handling of fault retry as you did it.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

View attachment "0001-mm-Don-t-bother-dropping-mmap_sem-for-zero-size-read.patch" of type "text/x-patch" (1081 bytes)