Re: [PATCH v4 2/7] deactivate invalidated pages

[Minchan Kim]

On Tue, Dec 07, 2010 at 04:19:39PM +0100, Johannes Weiner wrote:
> On Wed, Dec 08, 2010 at 12:07:10AM +0900, Minchan Kim wrote:
> > On Tue, Dec 07, 2010 at 03:49:24PM +0100, Johannes Weiner wrote:
> > > On Mon, Dec 06, 2010 at 02:29:10AM +0900, Minchan Kim wrote:
> > > > Changelog since v3:
> > > >  - Change function comments - suggested by Johannes
> > > >  - Change function name - suggested by Johannes
> > > >  - add only dirty/writeback pages to deactive pagevec
> > > 
> > > Why the extra check?
> > > 
> > > > @@ -359,8 +360,16 @@ unsigned long invalidate_mapping_pages(struct address_space *mapping,
> > > >  			if (lock_failed)
> > > >  				continue;
> > > >  
> > > > -			ret += invalidate_inode_page(page);
> > > > -
> > > > +			ret = invalidate_inode_page(page);
> > > > +			/*
> > > > +			 * If the page is dirty or under writeback, we can not
> > > > +			 * invalidate it now.  But we assume that attempted
> > > > +			 * invalidation is a hint that the page is no longer
> > > > +			 * of interest and try to speed up its reclaim.
> > > > +			 */
> > > > +			if (!ret && (PageDirty(page) || PageWriteback(page)))
> > > > +				deactivate_page(page);
> > > 
> > > The writeback completion handler does not take the page lock, so you
> > > can still miss pages that finish writeback before this test, no?
> > 
> > Yes. but I think it's rare and even though it happens, it's not critical.
> > > 
> > > Can you explain why you felt the need to add these checks?
> > 
> > invalidate_inode_page can return 0 although the pages is !{dirty|writeback}.
> > Look invalidate_complete_page. As easiest example, if the page has buffer and
> > try_to_release_page can't release the buffer, it could return 0.
> 
> Ok, but somebody still tried to truncate the page, so why shouldn't we
> try to reclaim it?  The reason for deactivating at this location is
> that truncation is a strong hint for reclaim, not that it failed due
> to dirty/writeback pages.
> 
> What's the problem with deactivating pages where try_to_release_page()
> failed?

If try_to_release_page fails and the such pages stay long time in pagevec,
pagevec drain often happens. I think such pages are rare so skip such pages doesn't
hurt goal of this patch.

> 
> I don't think we should add more logic than necessary.  If there is a
> good reason for it, it needs to get a code comment at least.

Above my comment is enough to justify it? If you agree, I can add the comment.

Thanks for careful review, Hannes.

-- 
Kind regards,
Minchan Kim
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