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Re: [PATCHv6 05/37] thp: try to free page's buffers before attempt split

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  Re: [PATCHv6 05/37] thp: try to free page's buffers before attempt  split Matthew Wilcox <willy@infradead.org> - 2017-02-09 21:40 +0100
    Re: [PATCHv6 05/37] thp: try to free page's buffers before attempt  split "Kirill A. Shutemov" <kirill@shutemov.name> - 2017-02-13 15:40 +0100

#1577941 — Re: [PATCHv6 05/37] thp: try to free page's buffers before attempt split

On Thu, Jan 26, 2017 at 02:57:47PM +0300, Kirill A. Shutemov wrote:
> @@ -2146,6 +2146,23 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>  			goto out;
>  		}
>  
> +		/* Try to free buffers before attempt split */
> +		if (!PageSwapBacked(head) && PagePrivate(page)) {
> +			/*
> +			 * We cannot trigger writeback from here due possible
> +			 * recursion if triggered from vmscan, only wait.
> +			 *
> +			 * Caller can trigger writeback it on its own, if safe.
> +			 */

It took me a few reads to get this.  May I suggest:

		/*
		 * Cannot split a page with buffers.  If the caller has
		 * already started writeback, we can wait for it to finish,
		 * but we cannot start writeback if we were called from vmscan
		 */
> +		if (!PageSwapBacked(head) && PagePrivate(page)) {

Also, it looks weird to test PageSwapBacked of *head* and PagePrivate
of *page*.  I think it's correct, but it still looks weird.

Reviewed-by: Matthew Wilcox <mawilcox@microsoft.com>

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#1579823

On Thu, Feb 09, 2017 at 12:14:16PM -0800, Matthew Wilcox wrote:
> On Thu, Jan 26, 2017 at 02:57:47PM +0300, Kirill A. Shutemov wrote:
> > @@ -2146,6 +2146,23 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
> >  			goto out;
> >  		}
> >  
> > +		/* Try to free buffers before attempt split */
> > +		if (!PageSwapBacked(head) && PagePrivate(page)) {
> > +			/*
> > +			 * We cannot trigger writeback from here due possible
> > +			 * recursion if triggered from vmscan, only wait.
> > +			 *
> > +			 * Caller can trigger writeback it on its own, if safe.
> > +			 */
> 
> It took me a few reads to get this.  May I suggest:
> 
> 		/*
> 		 * Cannot split a page with buffers.  If the caller has
> 		 * already started writeback, we can wait for it to finish,
> 		 * but we cannot start writeback if we were called from vmscan
> 		 */

Yeah, that's better.

> > +		if (!PageSwapBacked(head) && PagePrivate(page)) {
> 
> Also, it looks weird to test PageSwapBacked of *head* and PagePrivate
> of *page*.  I think it's correct, but it still looks weird.

I'll change this.

> Reviewed-by: Matthew Wilcox <mawilcox@microsoft.com>

Thanks!

-- 
 Kirill A. Shutemov

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