Re: question on page-migration code

[Ray Bryant <raybry@engr.sgi.com>]

Marcelo Tosatti wrote:
> On Thu, Apr 07, 2005 at 05:16:30PM -0500, Ray Bryant wrote:
> 
>>Hirokazu (and Marcelo),
>>
>>In testing my manual page migration code, I've run up against a situation
>>where the migrations are occasionally very slow.  They work ok, but they
>>can take minutes to migrate a few megabytes of memory.
>>
>>Dropping into kdb shows that the migration code is waiting in msleep() in
>>migrate_page_common() due to an -EAGAIN return from page_migratable().
>>A little further digging shows that the specific return in page_migratable()
>>is the very last one there at the bottom of the routine.
>>
>>I'm puzzled as to why the page is still busy in this case.  Previous code
>>in page_migratable() has unmapped the page, its not in PageWriteback()
>>because we would have taken a different return statement in that case.
>>
>>According to /proc/meminfo, there are no pages in either SwapCache or
>>Dirty state, and the system has been sync'd before the migrate_pages()
>>call was issued.
> 
> 
> Who is using the page? 
> 
> A little debugging might help similar to what bad_page does can help: 
> 
>         printk(KERN_EMERG "flags:0x%0*lx mapping:%p mapcount:%d count:%d\n",
>                 (int)(2*sizeof(page_flags_t)), (unsigned long)page->flags,
>                 page->mapping, page_mapcount(page), page_count(page));
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> 
A little further digging shows that when we go into -EAGAIN case in
migrate_page_common(), we have flag bits 0x104D set, and when we finally
exit the routine, we have flags bits 0x004D set.  The 1 bit there is
PG_private, as near as I can tell (not PG_arch_1, I guess I can't count).

PagePrivate() is cleared by truncation specific code in migrate_onepage(),
but it doesn't appear to be cleared (directly) by code on the
generic_migrate_page() patch.  I wonder if this has something to do with
the problem I am seeing.

-- 
Best Regards,
Ray
-----------------------------------------------
                   Ray Bryant
512-453-9679 (work)         512-507-7807 (cell)
raybry@sgi.com             raybry@austin.rr.com
The box said: "Requires Windows 98 or better",
            so I installed Linux.
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