linux-kernel - Re: [PATCH V4 4/6] mm: reclaim MADV

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Date:   Fri, 24 Feb 2017 10:36:55 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Shaohua Li <shli@...com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Kernel-team@...com, mhocko@...e.com,
        hughd@...gle.com, riel@...hat.com, mgorman@...hsingularity.net,
        akpm@...ux-foundation.org
Subject: Re: [PATCH V4 4/6] mm: reclaim MADV_FREE pages

On Fri, Feb 24, 2017 at 11:12:18AM +0900, Minchan Kim wrote:
> > @@ -1525,8 +1531,8 @@ int try_to_unmap(struct page *page, enum ttu_flags flags)
> >  
> >  	if (ret != SWAP_MLOCK && !page_mapcount(page)) {
> >  		ret = SWAP_SUCCESS;
> > -		if (rp.lazyfreed && !PageDirty(page))
> > -			ret = SWAP_LZFREE;
> > +		if (rp.lazyfreed && PageDirty(page))
> > +			ret = SWAP_DIRTY;
> 
> Hmm, I don't understand why we need to introduce new return value.
> Can't we set SetPageSwapBacked and return SWAP_FAIL in try_to_unmap_one?

I think that's a bad idea. A function called "try_to_unmap" shouldn't
have as a side effect that it changes the page's LRU type in an error
case. Let try_to_unmap be about unmapping the page. If it fails, make
it report why and let the caller deal with the fallout. Any LRU fixups
are much better placed in vmscan.c.