linux-kernel - Re: [PATCH RESEND 1/2] mm: swap: clean up swap readahead

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Date:   Mon, 26 Feb 2018 13:56:17 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        lkml <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, Hugh Dickins <hughd@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH RESEND 1/2] mm: swap: clean up swap readahead

On Fri, Feb 23, 2018 at 04:02:27PM +0800, Huang, Ying wrote:
> <minchan@...nel.org> writes:
> [snip]
> 
> > diff --git a/mm/swap_state.c b/mm/swap_state.c
> > index 39ae7cfad90f..c56cce64b2c3 100644
> > --- a/mm/swap_state.c
> > +++ b/mm/swap_state.c
> > @@ -332,32 +332,38 @@ struct page *lookup_swap_cache(swp_entry_t entry, struct vm_area_struct *vma,
> >  			       unsigned long addr)
> >  {
> >  	struct page *page;
> > -	unsigned long ra_info;
> > -	int win, hits, readahead;
> >  
> >  	page = find_get_page(swap_address_space(entry), swp_offset(entry));
> >  
> >  	INC_CACHE_INFO(find_total);
> >  	if (page) {
> > +		bool vma_ra = swap_use_vma_readahead();
> > +		bool readahead = TestClearPageReadahead(page);
> > +
> 
> TestClearPageReadahead() cannot be called for compound page.  As in
> 
> PAGEFLAG(Readahead, reclaim, PF_NO_COMPOUND)
> 	TESTCLEARFLAG(Readahead, reclaim, PF_NO_COMPOUND)
> 
> >  		INC_CACHE_INFO(find_success);
> >  		if (unlikely(PageTransCompound(page)))
> >  			return page;
> > -		readahead = TestClearPageReadahead(page);
> 
> So we can only call it here after checking whether page is compound.

Hi Huang,

Thanks for cathing this.
However, I don't see the reason we should rule out THP page for
readahead marker. Could't we relax the rule?

I hope we can do so that we could remove PageTransCompound check
for readahead marker, which makes code ugly.

>From 748b084d5c3960ec2418d8c51a678aada30f1072 Mon Sep 17 00:00:00 2001
From: Minchan Kim <minchan@...nel.org>
Date: Mon, 26 Feb 2018 13:46:43 +0900
Subject: [PATCH] mm: relax policy for PG_readahead

This flag is in use for anon THP page so let's relax it.

Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
Signed-off-by: Minchan Kim <minchan@...nel.org>
---
 include/linux/page-flags.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index e34a27727b9a..f12d4dfae580 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -318,8 +318,8 @@ PAGEFLAG(MappedToDisk, mappedtodisk, PF_NO_TAIL)
 /* PG_readahead is only used for reads; PG_reclaim is only for writes */
 PAGEFLAG(Reclaim, reclaim, PF_NO_TAIL)
 	TESTCLEARFLAG(Reclaim, reclaim, PF_NO_TAIL)
-PAGEFLAG(Readahead, reclaim, PF_NO_COMPOUND)
-	TESTCLEARFLAG(Readahead, reclaim, PF_NO_COMPOUND)
+PAGEFLAG(Readahead, reclaim, PF_NO_TAIL)
+	TESTCLEARFLAG(Readahead, reclaim, PF_NO_TAIL)
 
 #ifdef CONFIG_HIGHMEM
 /*
-- 
2.16.1.291.g4437f3f132-goog