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Re: Does HIGHMEM work on 32-bit MIPS ports?

To: linux-mips@linux-mips.org
Subject: Re: Does HIGHMEM work on 32-bit MIPS ports?
From: David VomLehn <dvomlehn@cisco.com>
Date: Wed, 05 Mar 2008 14:55:48 -0800
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We've made significant progress in getting HIGHMEM to work on our 24K processor, but things do not completely work yet. Since I don't yet have confidence that we know everything that's going on, I"m not ready to submit a full-blown patch, but here's what we've done so far. Please send comments/suggestions...

The function __flush_dcache_page (in arch/mips/mm/cache.c) was simply returning if the struct page* argument it was given indicated we had a page in high memory, so the dcache was never being flushed. This is an obvious Bad Thing.

Our first modification was to expand the check for high memory. If the page had a temporary mapping, i.e. it was mapped through kmap_atomic(), we call flush_data_cache_page(). We then immediately return:

   if (PageHighMem(page)) {
       addr = (unsigned long)kmap_atomic_to_vaddr(page);
       if (addr != 0) {
           flush_data_cache_page(addr);
       }
       return;
   }

(kmap_atomic_to_vaddr() returns the virtual address if the page is mapped with kmap_atomic(), otherwise it returns NULL). This change by itself is enough to be able to boot with NFS most of the time. I think it is not sufficient for permanently mapped kernel pages (those mapped with kmap_high()). So, I made two other modifications.

Additional Modification #1: To me, it looks like the return should be moved to right after the call to flush_data_cache_page() so that we only return immediately for temporary kernel mappings.

The next section of code, which I think already works correctly with high memory, is:

   if (mapping && !mapping_mapped(mapping)) {
       SetPageDcacheDirty(page);
       return;
   }

We then have the following:

   addr = (unsigned long) page_address(page);
   flush_data_cache_page(addr);

Additional Modification #2: If the page is in high memory, it may not have a kernel mapping, in which case page_address() will return NULL. So, I've modified the code to only call flush_data_cache_page() if the page_address() doesn't return NULL.

With the two additional modifications above, thing are still not completely reliable. So, two questions:

  1. Does what we've done so far make sense?
  2. Since the behavior is still somewhat flaky, I'm still missing
     something. Any suggestions?

--
David VomLehn, dvomlehn@cisco.com
The opinions expressed herein are likely mine, but might not be my employer's...



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