Re: [PATCH 00/10] AXFS: Advanced XIP filesystem
From: Greg Ungerer
Date: Mon Aug 25 2008 - 10:02:48 EST
Hi Jamie,
Jamie Lokier wrote:
Greg Ungerer wrote:
Sort of. It actually just uses a single ->read to bring in
the entire file contents. There is a few limitations on the use
of mmap() for non-mmu. Documentation/nommu-mmap.txt gives
more details. With no MMU it does rely on being able to kmalloc()
a single RAM region big enough to hold the entire file.
That's unfortunate, if you're using FDPIC-ELF or BFLT-XIP, you really want
to kmalloc() one region for code (i.e. mmap not the whole file), and
one separate for data.
That is what the BFLT loader does. For the XIP case it mmap()s
the text directly from the file, and then mmap()s a second region
for the data/bss (reading the data into that region).
I was referring to general mmap() of a file case above, not
the exec path.
Asking for a single larger region sometimes
creates much higher memory pressure while kmalloc() attempts to
defragment by evicting everything.
Sure.
But that's fiddly to do right in general.
The natural thing for AXFS to do to support no-MMU FDPIC-ELF or
BFLT-XIP is store the code segment uncompressed and contiguous, and
the data segment however the filesystem prefers, and the profiling
information to work out where these are is readily available from the
mmap() calls, which are always the same when an executable is run.
Yep.
Regards
Greg
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Greg Ungerer -- Chief Software Dude EMAIL: gerg@xxxxxxxxxxxx
SnapGear -- a Secure Computing Company PHONE: +61 7 3435 2888
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