Re: Warning on memory offline (and possible in usual migration?)

[Nick Piggin <npiggin@suse.de>]

On Fri, May 02, 2008 at 02:16:20PM -0700, Christoph Lameter wrote:
> I guess we need the following patch to handle !uptodate pages. Wish we had 
> a better solution that would allow the skipping of pages with buffers 
> under read I/O.

 
AFAIK, any filesystem which may not lock the page under read IO should
have PG_private set. In which case, if they don't have buffers they
should have a releasepage method. Otherwise, how would we ever reclaim
!uptodate && !buffers pages?

So I don't think we need this patch.

> 
> Subject: Page migration: Do not migrate page that is not uptodate
> 
> If we are migrating pages that are not mapped into a processes address
> space then we may encounter !Uptodate pages. Page migration is now used
> for offlining memory which scans unmapped pages.
> 
> If a page is not uptodate then read I/O may be in progress against it.
> So do not migrate it. On the other hand if the page has buffers then
> read I/O will lock a buffer. In that case we can migrate an !Uptodate
> page but then migration will stall in buffer_migrate_page() until the
> read is complete.
> 
> Signed-off-by: Christoph Lameter <clameter@sgi.com>
> 
> ---
>  mm/migrate.c |   17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> Index: linux-2.6/mm/migrate.c
> ===================================================================
> --- linux-2.6.orig/mm/migrate.c	2008-05-02 13:47:45.113707645 -0700
> +++ linux-2.6/mm/migrate.c	2008-05-02 14:08:32.203644985 -0700
> @@ -652,6 +652,23 @@ static int unmap_and_move(new_page_t get
>  			goto unlock;
>  		wait_on_page_writeback(page);
>  	}
> +
> +	/*
> +	 * Page may be under read I/O if its not uptodate and has no buffers.
> +	 * In that case the page contents are not stable and should not be
> +	 * migrated. So we just pass on that page and return -EAGAIN.
> +	 *
> +	 * If a page has buffers then the locks taken on the buffers
> +	 * will indicate that read I/O is in progress. Then PageUptodate does
> +	 * not matter because buffer_migrate_page() will stall until I/O is
> +	 * complete. It would be better if we could catch that here and delay
> +	 * migrating the page because we could migrate a the other pages on the
> +	 * migrate list instead of waiting for I/O to complete on this page
> +	 * (like done for writes in progress).
> +	 */
> +	if (!PageUptodate(page) && !page_has_buffers(page))
> +		goto unlock;
> +
>  	/*
>  	 * By try_to_unmap(), page->mapcount goes down to 0 here. In this case,
>  	 * we cannot notice that anon_vma is freed while we migrates a page.

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