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
| ||
|
Message-ID: <CAPcyv4j0tTD+rENqFExA68aw=-MmtCBaOe1qJovyrmJC=yBg-Q@mail.gmail.com> Date: Tue, 2 Oct 2018 13:18:54 -0700 From: Dan Williams <dan.j.williams@...el.com> To: Jan Kara <jack@...e.cz> Cc: Christoph Hellwig <hch@...radead.org>, Johannes Thumshirn <jthumshirn@...e.de>, Dave Jiang <dave.jiang@...el.com>, linux-nvdimm <linux-nvdimm@...ts.01.org>, Linux MM <linux-mm@...ck.org>, linux-fsdevel <linux-fsdevel@...r.kernel.org>, linux-ext4 <linux-ext4@...r.kernel.org>, linux-xfs <linux-xfs@...r.kernel.org>, Linux API <linux-api@...r.kernel.org> Subject: Re: Problems with VM_MIXEDMAP removal from /proc/<pid>/smaps On Tue, Oct 2, 2018 at 8:32 AM Jan Kara <jack@...e.cz> wrote: > > On Tue 02-10-18 07:52:06, Christoph Hellwig wrote: > > On Tue, Oct 02, 2018 at 04:44:13PM +0200, Johannes Thumshirn wrote: > > > On Tue, Oct 02, 2018 at 07:37:13AM -0700, Christoph Hellwig wrote: > > > > No, it should not. DAX is an implementation detail thay may change > > > > or go away at any time. > > > > > > Well we had an issue with an application checking for dax, this is how > > > we landed here in the first place. > > > > So what exacty is that "DAX" they are querying about (and no, I'm not > > joking, nor being philosophical). > > I believe the application we are speaking about is mostly concerned about > the memory overhead of the page cache. Think of a machine that has ~ 1TB of > DRAM, the database running on it is about that size as well and they want > database state stored somewhere persistently - which they may want to do by > modifying mmaped database files if they do small updates... So they really > want to be able to use close to all DRAM for the DB and not leave slack > space for the kernel page cache to cache 1TB of database files. VM_MIXEDMAP was never a reliable indication of DAX because it could be set for random other device-drivers that use vm_insert_mixed(). The MAP_SYNC flag positively indicates that page cache is disabled for a given mapping, although whether that property is due to "dax" or some other kernel mechanics is purely an internal detail. I'm not opposed to faking out VM_MIXEDMAP if this broken check has made it into production, but again, it's unreliable.
Powered by blists - more mailing lists