linux-kernel - RE: [PATCH v4 4/5] cramfs: add mmap support

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Date:   Thu, 5 Oct 2017 20:00:07 +0000
From:   Chris Brandt <Chris.Brandt@...esas.com>
To:     Nicolas Pitre <nicolas.pitre@...aro.org>,
        Christoph Hellwig <hch@...radead.org>
CC:     Richard Weinberger <richard.weinberger@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "linux-embedded@...r.kernel.org" <linux-embedded@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v4 4/5] cramfs: add mmap support

On Wednesday, October 04, 2017, Nicolas Pitre wrote:
> On Wed, 4 Oct 2017, Christoph Hellwig wrote:
> 
> > As said in my last mail: look at the VM_MIXEDMAP flag and how it is
> > used by DAX, and you'll get out of the vma splitting business in the
> > fault path.
> 
> Alright, it appears to work.
> 
> The only downside so far is the lack of visibility from user space to
> confirm it actually works as intended. With the vma splitting approach
> you clearly see what gets directly mapped in /proc/*/maps thanks to
> remap_pfn_range() storing the actual physical address in vma->vm_pgoff.
> With VM_MIXEDMAP things are no longer visible. Any opinion for the best
> way to overcome this?
> 
> Anyway, here's a replacement for patch 4/5 below:
> 
> ----- >8
> Subject: cramfs: add mmap support
> 
> When cramfs_physmem is used then we have the opportunity to map files
> directly from ROM, directly into user space, saving on RAM usage.
> This gives us Execute-In-Place (XIP) support.


Tested on my setup:
 * Cortex A9 (with MMU)
 * CONFIG_XIP_KERNEL=y
 * booted with XIP CRAMFS as my rootfs 
 * all apps and libraries marked as XIP in my cramfs image



So far, functionally it seems to work the same as [PATCH v4 4/5].

As Nicolas said, before you could easily see that all my apps and 
libraries were XIP from Flash:

$ cat /proc/self/maps
00008000-000a1000 r-xp 1b005000 00:0c 18192      /bin/busybox
000a9000-000aa000 rw-p 00099000 00:0c 18192      /bin/busybox
000aa000-000ac000 rw-p 00000000 00:00 0          [heap]
b6e69000-b6f42000 r-xp 1b0bc000 00:0c 766540     /lib/libc-2.18-2013.10.so
b6f42000-b6f4a000 ---p 1b195000 00:0c 766540     /lib/libc-2.18-2013.10.so
b6f4a000-b6f4c000 r--p 000d9000 00:0c 766540     /lib/libc-2.18-2013.10.so
b6f4c000-b6f4d000 rw-p 000db000 00:0c 766540     /lib/libc-2.18-2013.10.so
b6f4d000-b6f50000 rw-p 00000000 00:00 0
b6f50000-b6f67000 r-xp 1b0a4000 00:0c 670372     /lib/ld-2.18-2013.10.so
b6f6a000-b6f6b000 rw-p 00000000 00:00 0
b6f6c000-b6f6e000 rw-p 00000000 00:00 0
b6f6e000-b6f6f000 r--p 00016000 00:0c 670372     /lib/ld-2.18-2013.10.so
b6f6f000-b6f70000 rw-p 00017000 00:0c 670372     /lib/ld-2.18-2013.10.so
beac0000-beae1000 rw-p 00000000 00:00 0          [stack]
bebc9000-bebca000 r-xp 00000000 00:00 0          [sigpage]
ffff0000-ffff1000 r-xp 00000000 00:00 0          [vectors]



But now just busybox looks like it's XIP:

$ cat /proc/self/maps
00008000-000a1000 r-xp 1b005000 00:0c 18192      /bin/busybox
000a9000-000aa000 rw-p 00099000 00:0c 18192      /bin/busybox
000aa000-000ac000 rw-p 00000000 00:00 0          [heap]
b6e4d000-b6f26000 r-xp 00000000 00:0c 766540     /lib/libc-2.18-2013.10.so
b6f26000-b6f2e000 ---p 000d9000 00:0c 766540     /lib/libc-2.18-2013.10.so
b6f2e000-b6f30000 r--p 000d9000 00:0c 766540     /lib/libc-2.18-2013.10.so
b6f30000-b6f31000 rw-p 000db000 00:0c 766540     /lib/libc-2.18-2013.10.so
b6f31000-b6f34000 rw-p 00000000 00:00 0
b6f34000-b6f4b000 r-xp 00000000 00:0c 670372     /lib/ld-2.18-2013.10.so
b6f4e000-b6f4f000 rw-p 00000000 00:00 0
b6f50000-b6f52000 rw-p 00000000 00:00 0
b6f52000-b6f53000 r--p 00016000 00:0c 670372     /lib/ld-2.18-2013.10.so
b6f53000-b6f54000 rw-p 00017000 00:0c 670372     /lib/ld-2.18-2013.10.so
bec93000-becb4000 rw-p 00000000 00:00 0          [stack]
befad000-befae000 r-xp 00000000 00:00 0          [sigpage]
ffff0000-ffff1000 r-xp 00000000 00:00 0          [vectors]


Regardless, from a functional standpoint:

Tested-by: Chris Brandt <chris.brandt@...esas.com>




Just FYI, the previous [PATCH v4 4/5] also included this (which was the 
only real difference between v3 and v4):


diff --git a/fs/cramfs/Kconfig b/fs/cramfs/Kconfig
index 5b4e0b7e13..306549be25 100644
--- a/fs/cramfs/Kconfig
+++ b/fs/cramfs/Kconfig
@@ -30,7 +30,7 @@ config CRAMFS_BLOCKDEV
 
 config CRAMFS_PHYSMEM
 	bool "Support CramFs image directly mapped in physical memory"
-	depends on CRAMFS
+	depends on CRAMFS = y
 	default y if !CRAMFS_BLOCKDEV
 	help
 	  This option allows the CramFs driver to load data directly from


Chris