Re: [PATCH 00/10] AXFS: Advanced XIP filesystem

[Greg Ungerer <gerg@snapgear.com>]

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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