Hi Benjamin, Mel,
On 05/15/2013 10:09 AM, Tang Chen wrote:
Hi Benjamin, Mel,
Please see below.
On 05/14/2013 09:58 PM, Benjamin LaHaise wrote:
On Tue, May 14, 2013 at 09:24:58AM +0800, Tang Chen wrote:
Hi Mel, Benjamin, Jeff,
On 05/13/2013 11:01 PM, Benjamin LaHaise wrote:
On Mon, May 13, 2013 at 10:54:03AM -0400, Jeff Moyer wrote:
How do you propose to move the ring pages?
It's the same problem as doing a TLB shootdown: flush the old pages
from
userspace's mapping, copy any existing data to the new pages, then
repopulate the page tables. It will likely require the addition of
address_space_operations for the mapping, but that's not too hard to
do.
I think we add migrate_unpin() callback to decrease page->count if
necessary,
and migrate the page to a new page, and add migrate_pin() callback to
pin
the new page again.
You can't just decrease the page count for this to work. The pages are
pinned because aio_complete() can occur at any time and needs to have a
place to write the completion events. When changing pages, aio has to
take the appropriate lock when changing one page for another.
In aio_complete(),
aio_complete() {
......
spin_lock_irqsave(&ctx->completion_lock, flags);
//write the completion event.
spin_unlock_irqrestore(&ctx->completion_lock, flags);
......
}
So for this problem, I think we can hold kioctx->completion_lock in the aio
callbacks to prevent aio subsystem accessing pages who are being migrated.
Another problem here is:
We intend to call these callbacks in the page migrate path, and we need to
know which lock to hold. But there is no way for migrate path to know this
info.
The migrate path is common for all kinds of pages, so we cannot pass any
specific parameter to the callbacks in migrate path.
When we get a page, we cannot get any kioctx info from the page. So how can
the callback know which lock to require without any parameter ? Or do we
have
any other way to do so ?
Would you please give some more advice about this ?
BTW, we also need to update kioctx->ring_pages.
Thanks. :)
The migrate procedure will work just as before. We use callbacks to
decrease
the page->count before migration starts, and increase it when the
migration
is done.
And migrate_pin() and migrate_unpin() callbacks will be added to
struct address_space_operations.
I think the existing migratepage operation in address_space_operations
can
be used. Does it get called when hot unplug occurs? That is: is testing
with the migrate_pages syscall similar enough to the memory removal case?
But as I said, for anonymous pages such as aio ring buffer, they don't have
address_space_operations. So where should we put the callbacks' pointers ?
Add something like address_space_operations to struct anon_vma ?
Thanks. :)
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