From: Linus Torvalds <torvalds@linux-foundation.org>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Jens Axboe <axboe@kernel.dk>, Boaz Harrosh <boaz@plexistor.com>,
Dave Chinner <david@fromorbit.com>,
"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
Arnd Bergmann <arnd@arndb.de>,
Ross Zwisler <ross.zwisler@linux.intel.com>,
"linux-nvdimm@lists.01.org" <linux-nvdimm@ml01.01.org>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
Heiko Carstens <heiko.carstens@de.ibm.com>,
Christoph Hellwig <hch@lst.de>,
Martin Schwidefsky <schwidefsky@de.ibm.com>,
Paul Mackerras <paulus@samba.org>, Peter Anvin <hpa@zytor.com>,
Tejun Heo <tj@kernel.org>, Matthew Wilcox <willy@linux.intel.com>,
Andrew Morton <akpm@linux-foundation.org>,
Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH v4 1/9] introduce __pfn_t for scatterlists and pmem
Date: Fri, 5 Jun 2015 14:37:13 -0700 [thread overview]
Message-ID: <CA+55aFzPUY2jReg2NXYUpTB2oDYttO5+qw4oy9G1eg+BCm2aDA@mail.gmail.com> (raw)
In-Reply-To: <20150605211906.20751.59875.stgit@dwillia2-desk3.amr.corp.intel.com>
On Fri, Jun 5, 2015 at 2:19 PM, Dan Williams <dan.j.williams@intel.com> wrote:
> +enum {
> +#if BITS_PER_LONG == 64
> + PFN_SHIFT = 3,
> + /* device-pfn not covered by memmap */
> + PFN_DEV = (1UL << 2),
> +#else
> + PFN_SHIFT = 2,
> +#endif
> + PFN_MASK = (1UL << PFN_SHIFT) - 1,
> + PFN_SG_CHAIN = (1UL << 0),
> + PFN_SG_LAST = (1UL << 1),
> +};
Ugh. Just make PFN_SHIFT unconditional. Make it 2, unconditionally.
Or, if you want to have more bits, make it three unconditionally, and
make 'struct page' just be at least 8-byte aligned even on 32-bit.
Even on 32-bit architectures, there's plenty of bits. There's no
reason to "pack" this optimally. Remember: it's a page frame number,
so there's the page size shifting going on in physical memory, and
even if you shift the PFN by 3 - or four of five - bits
unconditionally (rather than try to shift it by some minimal number),
you're covering a *lot* of physical memory.
Say you're a 32-bit architecture with a 4k page size, and you lose
three bits to "type" bits. You still have 32+12-3=41 bits of physical
address space. Which is way more than realistic for a 32-bit
architecture anyway, even with PAE (or PXE or whatever ARM calls it).
Not that I see persistent memory being all that relevant on 32-bit
hardware anyway.
So I think if you actually do want that third bit, you're better off
just marking "struct page" as being __aligned__((8)) and getting the
three bits unconditionally. Just make the rule be that mem_map[] has
to be 8-byte aligned.
Even 16-byte alignment would probably be fine. No?
Linus
next prev parent reply other threads:[~2015-06-05 21:37 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-06-05 21:19 [PATCH v4 0/9] introduce __pfn_t, evacuate struct page from sgls Dan Williams
2015-06-05 21:19 ` [PATCH v4 1/9] introduce __pfn_t for scatterlists and pmem Dan Williams
2015-06-05 21:37 ` Linus Torvalds [this message]
2015-06-05 22:12 ` Dan Williams
2015-06-05 21:19 ` [PATCH v4 2/9] x86: support kmap_atomic_pfn_t() for persistent memory Dan Williams
2015-06-09 6:50 ` Christoph Hellwig
2015-06-10 12:12 ` Christoph Hellwig
2015-06-10 15:03 ` Matthew Wilcox
2015-06-10 15:11 ` Christoph Hellwig
2015-06-10 15:36 ` Dan Williams
2015-06-10 16:11 ` Christoph Hellwig
2015-06-10 16:17 ` Dan Williams
2015-06-05 21:19 ` [PATCH v4 3/9] dax: drop size parameter to ->direct_access() Dan Williams
2015-06-06 11:37 ` Matthew Wilcox
2015-06-09 6:51 ` Christoph Hellwig
2015-06-05 21:19 ` [PATCH v4 4/9] dax: fix mapping lifetime handling, convert to __pfn_t + kmap_atomic_pfn_t() Dan Williams
2015-06-06 11:58 ` Matthew Wilcox
2015-08-07 23:54 ` Dan Williams
2015-06-08 16:29 ` Elliott, Robert (Server Storage)
2015-06-08 16:36 ` Dan Williams
2015-06-09 6:55 ` Christoph Hellwig
2015-06-05 21:19 ` [PATCH v4 5/9] dma-mapping: allow archs to optionally specify a ->map_pfn() operation Dan Williams
2015-06-05 21:19 ` [PATCH v4 6/9] scatterlist: use sg_phys() Dan Williams
2015-06-09 6:59 ` Christoph Hellwig
2015-06-05 21:19 ` [PATCH v4 7/9] scatterlist: cleanup sg_chain() and sg_unmark_end() Dan Williams
2015-06-05 21:19 ` [PATCH v4 8/9] scatterlist: convert to __pfn_t Dan Williams
2015-06-05 21:19 ` [PATCH v4 9/9] x86: convert dma_map_ops to support mapping a __pfn_t Dan Williams
2015-06-09 6:58 ` Christoph Hellwig
2015-06-09 13:47 ` Konrad Rzeszutek Wilk
2015-06-05 21:23 ` [PATCH v4 0/9] introduce __pfn_t, evacuate struct page from sgls Dan Williams
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