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Message-ID: <20170825130011.GA30072@infradead.org>
Date: Fri, 25 Aug 2017 06:00:11 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Dan Williams <dan.j.williams@...el.com>
Cc: Christoph Hellwig <hch@...radead.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Jan Kara <jack@...e.cz>, Arnd Bergmann <arnd@...db.de>,
"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
Linux API <linux-api@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
linux-xfs@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
Andy Lutomirski <luto@...nel.org>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>,
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v6 3/5] mm: introduce mmap3 for safely defining new mmap
flags
On Thu, Aug 24, 2017 at 10:36:02AM -0700, Dan Williams wrote:
> I'll let Andy and Kirill restate their concerns, but one of the
> arguments that swayed me is that any new mmap flag with this hack must
> be documented to only work with MAP_SHARED and that MAP_PRIVATE is
> silently ignored. I agree with the mess and delays it causes for other
> archs and libc, but at the same time this is for new applications and
> libraries that know to look for the new flag, so they need to do the
> extra work to check for the new syscall.
True. That is for the original hack, but I spent some more time
looking at the mmap code, and there is one thing I noticed:
include/uapi/asm-generic/mman-common.h:
#define MAP_SHARED 0x01 /* Share changes */
#define MAP_PRIVATE 0x02 /* Changes are private */
#define MAP_TYPE 0x0f /* Mask for type of mapping */
mm/mmap.c:
if (file) {
struct inode *inode = file_inode(file);
switch (flags & MAP_TYPE) {
case MAP_SHARED:
...
case MAP_PRIVATE:
...
default:
return -EINVAL;
}
and very similar for the anonymous and nommu cases.
So if we pick e.g. 0x4 as the valid bit we don't even need to overload
the MAP_SHARED and MAP_PRIVATE meaning.
>
> However, if the fcntl lease approach works for the DMA cases then we
> only have the one mmap flag to add for now, so maybe the weird
> MAP_{SHARED|PRIVATE} semantics are tolerable.
---end quoted text---
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