Re: [RFC][PATCH] tracking dirty pages in shared mappings

[Nick Piggin <piggin@cyberone.com.au>]

Peter Zijlstra wrote:

>On Sat, 2006-05-06 at 23:18 +1000, Nick Piggin wrote:
>
>
>>Looks pretty good. Christoph and I were looking at ways to improve
>>performance impact of this, and skipping the extra work for particular
>>(eg. shmem) mappings might be a good idea?
>>
>>Attached is a patch with a couple of things I've currently got.
>>
>
>will merge with mine and post a new version shortly.
>

Thanks.

>>In the long run, I'd like to be able to set_page_dirty and
>>balance_dirty_pages outside of both ptl and mmap_sem, for performance
>>reasons. That will require a reworking of arch code though :(
>>
>
>That would indeed be very nice if possible.
>

Yep. Let's not distract from getting the basic mechanism working though.
balance_dirty_pages would be patch 2..n ;)

>>Index: linux-2.6/include/linux/mm.h
>>===================================================================
>>--- linux-2.6.orig/include/linux/mm.h	2006-05-06 23:05:10.000000000 +1000
>>+++ linux-2.6/include/linux/mm.h	2006-05-06 23:06:17.000000000 +1000
>>@@ -183,8 +183,7 @@ extern unsigned int kobjsize(const void 
>> #define VM_SequentialReadHint(v)	((v)->vm_flags & VM_SEQ_READ)
>> #define VM_RandomReadHint(v)		((v)->vm_flags & VM_RAND_READ)
>> 
>>-#define VM_SharedWritable(v)		(((v)->vm_flags & (VM_SHARED | VM_MAYSHARE)) && \
>>-					 ((v)->vm_flags & VM_WRITE))
>>+#define VM_SharedWritable(v)		((v)->vm_flags & (VM_SHARED|VM_WRITE))
>> 
>> /*
>>  * mapping from the currently active vm_flags protection bits (the
>>
>
>That doesn't look right, your version is true even for unwritable
>shared, and writable non-shared VMAs.
>

Of course, thanks. I guess that should be

#define VM_SharedWritable(v) ((v)->vm_flags & (VM_SHARED|VM_WRITE) == (VM_SHARED|VM_WRITE))

BTW. It is unconventional (outside the read hints stuff) to use macros like
this. I guess real VM hackers have to know what is intended by any given esoteric
combination of flags in any given context.

Not that I hate it.

But if we're going to start using it, we should work out a sane convention and
stick to it. "StudlyCaps" seem to be out of favour, and using a vma_ prefix would
be more sensible.

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