Jie Zhang wrote:
> On 11/25/2009 02:16 PM, Jamie Lokier wrote:
> >Mike Frysinger wrote:
> >>From: Jie Zhang<jie.zh...@analog.com>
> >>
> >>The mmu code uses the copy_*_user_page() variants in access_process_vm()
> >>rather than copy_*_user() as the former includes an icache flush. This is
> >>important when doing things like setting software breakpoints with gdb.
> >>So switch the nommu code over to do the same.
> >
> >Reasonable, but it's a bit subtle don't you think?
> >How about a one-line comment saying why it's using copy_*_user_page()?
> >
> >(If it was called copy_*_user_flush_icache() I wouldn't say anything,
> >but it isn't).
> >
> But I think it's well known in Linux kernel developers that
> copy_to_user_page and copy_from_user_page should do cache flushing. It's
> documented in Documentation/cachetlb.txt. I don't think it's necessary
> to replicate it here.
You're right, however I now think the commit message is misleading.
Since this is the *only place in the entire kernel* where these
functions are used (plus the mmu equivalent), I'm not sure I'd agree
about well known, and the name could be better (copy_*_user_ptrace()),
but I agree now, it doesn't need a comment.
It was the talk of icache flush which bothered me, as I (wrongly)
assumed copy_*_user_page() was used elsewhere, without knowledge of
icache vs non-icache differences - which are often the responsibility
of userspace to get right, so often the kernel does not care.
In fact, it's not just icache. copy_*_user_page() has to do some
*data* cache flushing too, on some architecures. For example, see
arch/sparc/include/asm/cacheflush_64.h:
#define copy_to_user_page(vma, page, vaddr, dst, src, len) \
do { \
flush_cache_page(vma, vaddr, page_to_pfn(page)); \
memcpy(dst, src, len); \
flush_ptrace_access(vma, page, vaddr, src, len, 0); \
} while (0)
#define copy_from_user_page(vma, page, vaddr, dst, src, len) \
do { \
flush_cache_page(vma, vaddr, page_to_pfn(page)); \
memcpy(dst, src, len); \
flush_ptrace_access(vma, page, vaddr, dst, len, 1); \
} while (0)
I'm not sure why I don't see the same dcache flushing on ARM, so I
wonder if the ARM implementation of these buggy.
Anyway, that means the commit message is a little misleading, saying
it's for the icache flush. It is for whatever icache and dcache
flushes are needed for a ptrace access.
Which is why, given they are only used for ptrace (have just grepped),
I'm inclined to think it'd be clearer to rename the functions to
copy_*_user_ptrace(). And make your no-mmu change of course :-)
Any thoughts on the rename?
-- Jamie
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