linux-kernel - Re: [PATCH v2 00/10] evacuate struct page from the block layer, introduce __pfn

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]

Date:	Thu, 7 May 2015 16:42:25 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Boaz Harrosh <boaz@...xistor.com>, Jan Kara <jack@...e.cz>,
	Mike Snitzer <snitzer@...hat.com>, Neil Brown <neilb@...e.de>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Chris Mason <clm@...com>, Paul Mackerras <paulus@...ba.org>,
	"H. Peter Anvin" <hpa@...or.com>, Christoph Hellwig <hch@....de>,
	Alasdair Kergon <agk@...hat.com>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	Mel Gorman <mgorman@...e.de>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Jens Axboe <axboe@...nel.dk>, Theodore Ts'o <tytso@....edu>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Julia Lawall <Julia.Lawall@...6.fr>, Tejun Heo <tj@...nel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 00/10] evacuate struct page from the block layer,
 introduce __pfn_t

* Ingo Molnar <mingo@...nel.org> wrote:

> [...]
>
> For anything more complex, that maps any of this storage to 
> user-space, or exposes it to higher level struct page based APIs, 
> etc., where references matter and it's more of a cache with 
> potentially multiple users, not an IO space, the natural API is 
> struct page.

Let me walk back on this:

> I'd say that this particular series mostly addresses the 'pfn as 
> sector_t' side of the equation, where persistent memory is IO space, 
> not memory space, and as such it is the more natural and thus also 
> the cheaper/faster approach.

... but that does not appear to be the case: this series replaces a 
'struct page' interface with a pure pfn interface for the express 
purpose of being able to DMA to/from 'memory areas' that are not 
struct page backed.

> Linus probably disagrees? :-)

[ and he'd disagree rightfully ;-) ]

So what this patch set tries to achieve is (sector_t -> sector_t) IO 
between storage devices (i.e. a rare and somewhat weird usecase), and 
does it by squeezing one device's storage address into our formerly 
struct page backed descriptor, via a pfn.

That looks like a layering violation and a mistake to me. If we want 
to do direct (sector_t -> sector_t) IO, with no serialization worries, 
it should have its own (simple) API - which things like hierarchical 
RAID or RDMA APIs could use.

If what we want to do is to support say an mmap() of a file on 
persistent storage, and then read() into that file from another device 
via DMA, then I think we should have allocated struct page backing at 
mmap() time already, and all regular syscall APIs would 'just work' 
from that point on - far above what page-less, pfn-based APIs can do.

The temporary struct page backing can then be freed at munmap() time.

And if the usage is pure fd based, we don't really have fd-to-fd APIs 
beyond the rarely used splice variants (and even those don't do pure 
cross-IO, they use a pipe as an intermediary), so there's no problem 
to solve I suspect.

Thanks,

	Ingo
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/